From 21652a135b6bd1cb87dbcc2480aaf0886e056d63 Mon Sep 17 00:00:00 2001
From: Joseph Savona <josephsavona@users.noreply.github.com>
Date: Thu, 8 Dec 2022 07:35:14 -0800
Subject: [PATCH] Improved LeaveSSA pass
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## Problem

The previous version of LeaveSSA used a very simple approach in which
identifiers stored their pre-ssa id, and LeaveSSA restored this id back. The
upside of this approach is that it's very simple and trivially correct (assuming
no reordering of code). The downside is that after running LeaveSSA we lose all
information about which versions of variable declarations are distinct, and
which might merge together in a phi. That information is really useful for scope
analysis! Consider this input (variables are numbered as they would be in SSA
form):

```javascript

function foo(a, b, c) {

let x$1 = null;

if (a) {

x$2 = b;

} else {

x$3 = c;

}

x$4 = phi(x$2, x$3);

return x$4;

```

The current LeaveSSA assigns all 4 variables back to `x$1`, with a single let
declaration at `let x$1 = null`. However, from a reactive scopes perspective,
there are really just 2 versions of x: the initial x$1 (defined and never used)
and then x$2, x$3, and x$4, which have to be merged into a single scope because
they are part of a phi. In other words, we can't independently compute x$2, x$3,
or x$4 - if any of their inputs changes, we have to redo all the computation.
However, the existing structure makes it difficult to figure out the correct
starting point for this scope — there is no initial `let` declaration that we
can refer to.

Instead, we can represent the program as follows after LeaveSSA, and then use
this form for scope analysis:

```javascript

function foo(a, b, c) {

const x$1 = null; // NOTE: rewritten to const

let x$2; // synthesized declaration to allow later reassignment

if (a) {

x$2 = b;

} else {

x$2 = c;

}

return x$2;

```

Note that there are only 2 versions of x, and we have synthesized a variable
declaration for x$2 at the appropriate scope. Our scope analysis can then
determine that the range of x$2 is from the declaration to the end of the if.

## Approach

This pass does two main rewrites:

* For variables that do *not* appear as a phi id or operand, it rewrites the
declaration to be `const`. You can see this above for x$1.

* For variables that *do* appear as a phi or operand, it synthesizes a new `let`
binding at the appropriate scope (ie, in the appropriate block), and updates all
other operands from the phi to use the same id for the variable.  You can see
this above for x$2, x$3, and x$4.

Note that the let binding is generated at the narrowest scope possible. In this
example, we generate distinct let bindings for the other if and else branches:

```javascript

function foo(a, b, c) {

let x = null;

if (a) {

// we generate a `let x$2` here

if (b) {

x = 0; // becomes x$2

} else {

x = 1;  // becomes x$2

}

x // becomes x$2

} else {

// we generate a `let x$3` here

if (c) {

x = 2; // becomes x$3

} else {

x = 3; // becomes x$3

}

x; // becomes x$3

}

}

```

Because the different x values from the outer if/else can never join in a phi,
we can treat them as independent variables and (re)compute them independently.

The algorithm works by iterating in reverse-postorder, and looking ahead at
fallthrough blocks to find phi nodes that may need a let declaration (see above
example of where these are generated). It also tracks variables which _don't_
participate in a phi so that it can rewrite their declarations to `const`.

## TODO

This PR does *not* yet work for cases where there is unconditional assignment
within a `while` test condition. That would technically create a distinct
version of the variable that shadows the value for the loop, and you can't have
variable declarations in a while test condition.

That case already doesn't work, though, so i'm punting on it for now until we
figure out a bit more around

"value" blocks. We have some good options, like desugaring to a `for(;;)` and
manually implementing the while semantics in that case.
---
 compiler/forget/src/HIR/EnterSSA.ts           |   1 -
 compiler/forget/src/HIR/HIR.ts                |   2 -
 compiler/forget/src/HIR/HIRBuilder.ts         |   2 -
 compiler/forget/src/HIR/LeaveSSA.ts           | 161 +++++++++++++--
 compiler/forget/src/HIR/Pipeline.ts           |   2 +-
 compiler/forget/src/HIR/PrintHIR.ts           |   7 +-
 .../_bug_expression-with-assignment.expect.md |   6 +-
 .../hir/_bug_inverted-if-else.expect.md       |  91 ++++++++
 .../fixtures/hir/_bug_inverted-if-else.js     |  11 +
 .../hir/_bug_while-with-assignment.expect.md  | 106 ----------
 .../fixtures/hir/alias-while.expect.md        |  32 +--
 .../hir/assignment-variations.expect.md       |  14 +-
 .../src/__tests__/fixtures/hir/call.expect.md |  14 +-
 .../fixtures/hir/complex-while.expect.md      |  14 +-
 .../fixtures/hir/component.expect.md          |  20 +-
 .../fixtures/hir/conditional-break.expect.md  |  80 +++----
 .../hir/conditional-on-mutable.expect.md      |  40 ++--
 .../fixtures/hir/constructor.expect.md        |  14 +-
 .../hir/dependencies-outputs.expect.md        |  22 +-
 .../fixtures/hir/dependencies.expect.md       |  22 +-
 ...or.while-with-assignment-in-test.expect.md |  25 +++
 ...=> error.while-with-assignment-in-test.js} |   0
 .../fixtures/hir/extend-scopes-if.expect.md   |  24 +--
 .../fixtures/hir/frozen-after-alias.expect.md |  10 +-
 .../fixtures/hir/hook-call.expect.md          |  12 +-
 .../hir/hooks-freeze-arguments.expect.md      |  14 +-
 ...reeze-possibly-mutable-arguments.expect.md |  29 +--
 .../hir/independent-across-if.expect.md       |  16 +-
 .../fixtures/hir/independent.expect.md        |   8 +-
 .../hir/interdependent-across-if.expect.md    |  14 +-
 .../fixtures/hir/interdependent.expect.md     |  10 +-
 .../fixtures/hir/inverted-if.expect.md        |  14 +-
 .../fixtures/hir/jsx-fragment.expect.md       |   4 +-
 .../fixtures/hir/logical-expression.expect.md |   8 +-
 .../hir/mutable-lifetime-loops.expect.md      |  50 ++---
 .../mutable-lifetime-with-aliasing.expect.md  |  42 ++--
 .../hir/mutable-liverange-loop.expect.md      |  36 ++--
 ...g-scopes-interleaved-by-terminal.expect.md |  14 +-
 .../overlapping-scopes-interleaved.expect.md  |  10 +-
 .../hir/overlapping-scopes-shadowed.expect.md |  10 +-
 ...ng-scopes-shadowing-within-block.expect.md |  20 +-
 .../hir/overlapping-scopes-while.expect.md    |  14 +-
 .../overlapping-scopes-within-block.expect.md |  20 +-
 .../hir/property-assignment.expect.md         |  14 +-
 .../hir/reactive-scope-grouping.expect.md     |  12 +-
 .../fixtures/hir/reactive-scopes.expect.md    |  16 +-
 .../hir/reassignment-conditional.expect.md    |  20 +-
 .../reassignment-separate-scopes.expect.md    |  28 +--
 .../fixtures/hir/reassignment.expect.md       |  16 +-
 .../fixtures/hir/reverse-postorder.expect.md  |  28 +--
 .../fixtures/hir/simple-alias.expect.md       |  16 +-
 .../__tests__/fixtures/hir/simple.expect.md   |  10 +-
 .../hir/ssa-arrayexpression.expect.md         |  10 +-
 .../fixtures/hir/ssa-call-jsx-2.expect.md     |  16 +-
 .../fixtures/hir/ssa-call-jsx.expect.md       |  14 +-
 .../hir/ssa-complex-multiple-if.expect.md     |  20 +-
 .../hir/ssa-complex-single-if.expect.md       |  12 +-
 .../fixtures/hir/ssa-for-of.expect.md         |   8 +-
 .../__tests__/fixtures/hir/ssa-for.expect.md  |  10 +-
 .../fixtures/hir/ssa-if-else.expect.md        |  12 +-
 .../fixtures/hir/ssa-multiple-phis.expect.md  | 195 ++++++++++++++++++
 .../fixtures/hir/ssa-multiple-phis.js         |  19 ++
 .../ssa-nested-loops-no-reassign.expect.md    |  20 +-
 .../ssa-nested-partial-reassignment.expect.md | 105 ++++++++++
 .../hir/ssa-nested-partial-reassignment.js    |  11 +
 .../fixtures/hir/ssa-newexpression.expect.md  |  10 +-
 .../hir/ssa-objectexpression-phi.expect.md    |  20 +-
 .../hir/ssa-objectexpression.expect.md        |  14 +-
 .../fixtures/hir/ssa-property-call.expect.md  |  10 +-
 .../fixtures/hir/ssa-property.expect.md       |   8 +-
 .../fixtures/hir/ssa-reassign.expect.md       |  63 ++++++
 .../__tests__/fixtures/hir/ssa-reassign.js    |   7 +
 .../fixtures/hir/ssa-return.expect.md         |  10 +-
 .../fixtures/hir/ssa-shadowing.expect.md      |  16 +-
 .../fixtures/hir/ssa-sibling-phis.expect.md   | 191 +++++++++++++++++
 .../fixtures/hir/ssa-sibling-phis.js          |  19 ++
 .../fixtures/hir/ssa-simple-phi.expect.md     |  13 +-
 .../fixtures/hir/ssa-simple.expect.md         |   4 +-
 .../fixtures/hir/ssa-single-if.expect.md      |  10 +-
 .../fixtures/hir/ssa-switch.expect.md         |  18 +-
 .../fixtures/hir/ssa-throw.expect.md          |  10 +-
 .../hir/ssa-while-no-reassign.expect.md       |  10 +-
 .../fixtures/hir/ssa-while.expect.md          |  10 +-
 .../hir/switch-non-final-default.expect.md    |  21 +-
 .../hir/switch-with-fallthrough.expect.md     |  19 +-
 .../__tests__/fixtures/hir/switch.expect.md   |  23 ++-
 .../fixtures/hir/while-break.expect.md        |   8 +-
 .../hir/while-conditional-continue.expect.md  |  14 +-
 compiler/forget/src/__tests__/hir-test.ts     | 160 ++++++++------
 compiler/forget/src/index.ts                  |   2 +-
 90 files changed, 1589 insertions(+), 778 deletions(-)
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/_bug_inverted-if-else.expect.md
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/_bug_inverted-if-else.js
 delete mode 100644 compiler/forget/src/__tests__/fixtures/hir/_bug_while-with-assignment.expect.md
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/error.while-with-assignment-in-test.expect.md
 rename compiler/forget/src/__tests__/fixtures/hir/{_bug_while-with-assignment.js => error.while-with-assignment-in-test.js} (100%)
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/ssa-multiple-phis.expect.md
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/ssa-multiple-phis.js
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-reassignment.expect.md
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-reassignment.js
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.expect.md
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.js
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/ssa-sibling-phis.expect.md
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/ssa-sibling-phis.js

diff --git a/compiler/forget/src/HIR/EnterSSA.ts b/compiler/forget/src/HIR/EnterSSA.ts
index 2d1c0f795c..30a0891606 100644
--- a/compiler/forget/src/HIR/EnterSSA.ts
+++ b/compiler/forget/src/HIR/EnterSSA.ts
@@ -51,7 +51,6 @@ class SSABuilder {
   makeId(oldId: Identifier): Identifier {
     return {
       id: this.nextSsaId,
-      preSsaId: oldId.id,
       name: oldId.name,
       mutableRange: {
         start: makeInstructionId(0),
diff --git a/compiler/forget/src/HIR/HIR.ts b/compiler/forget/src/HIR/HIR.ts
index 01692cd791..eab9a26334 100644
--- a/compiler/forget/src/HIR/HIR.ts
+++ b/compiler/forget/src/HIR/HIR.ts
@@ -298,8 +298,6 @@ export type MutableRange = {
  * Represents a user-defined variable (has a name) or a temporary variable (no name).
  */
 export type Identifier = {
-  // the original `id` value prior to entering SSA form
-  preSsaId: IdentifierId | null;
   // unique value to distinguish a variable, since name is not guaranteed to exist or be unique
   id: IdentifierId;
   // null for temporaries. name is primarily used for debugging.
diff --git a/compiler/forget/src/HIR/HIRBuilder.ts b/compiler/forget/src/HIR/HIRBuilder.ts
index 15d1308eea..49a11189b3 100644
--- a/compiler/forget/src/HIR/HIRBuilder.ts
+++ b/compiler/forget/src/HIR/HIRBuilder.ts
@@ -111,7 +111,6 @@ export default class HIRBuilder {
   makeTemporary(): Identifier {
     const id = this.nextIdentifierId;
     return {
-      preSsaId: null,
       id,
       name: null,
       mutableRange: { start: makeInstructionId(0), end: makeInstructionId(0) },
@@ -124,7 +123,6 @@ export default class HIRBuilder {
     if (identifier == null) {
       const id = this.nextIdentifierId;
       identifier = {
-        preSsaId: null,
         id,
         name: node.name,
         mutableRange: {
diff --git a/compiler/forget/src/HIR/LeaveSSA.ts b/compiler/forget/src/HIR/LeaveSSA.ts
index f8a0df78f2..19f62f21bf 100644
--- a/compiler/forget/src/HIR/LeaveSSA.ts
+++ b/compiler/forget/src/HIR/LeaveSSA.ts
@@ -1,24 +1,153 @@
-import { HIRFunction } from "./HIR";
-import { eachBlockOperand } from "./visitors";
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {
+  Effect,
+  GeneratedSource,
+  HIRFunction,
+  Identifier,
+  Instruction,
+  InstructionKind,
+  Phi,
+} from "./HIR";
+import { eachInstructionValueOperand, eachTerminalOperand } from "./visitors";
 
 /**
- * Removes SSA form by restoring each `Identifier.id` value to its pre-SSA value,
- * and removing all phi nodes.
+ * Removes SSA form by creating unique variable declarations for the versions of each variables.
+ * - Versions of a variable that do not flow into a phi node are each assigned their own const variable
+ * declaration.
+ * - If multiple versions of a variable flow into some phi node, then all those versions are reassigned to
+ * the phi version. A single declaration is created at the same block scope as the phi, prior to any of the
+ * assignments.
  */
-export default function leaveSSA(fn: HIRFunction) {
-  const ir = fn.body;
+export function leaveSSA(fn: HIRFunction) {
+  // Maps identifiers that appear as a phi or phi operand to a single canonical identifier
+  // for all instances.
+  const variableMapping: Map<Identifier, Identifier> = new Map();
 
-  for (const param of fn.params) {
-    param.identifier.id = param.identifier.preSsaId ?? param.identifier.id;
-  }
+  for (const [, block] of fn.body.blocks) {
+    // Identifiers (from phis) that *may* need a new `let` declaration created. If the original
+    // variable declaration flows into the phi, then we can reuse its declaration - this is
+    // discovered during iteration of instructions.
+    const needsDeclaration: Set<Identifier> = new Set();
+
+    // Find any phi nodes which need a variable declaration in the current block
+    // This includes phis in fallthrough nodes, or blocks that form part of control flow
+    // such as for or while (and later if/switch).
+    const phis: Array<Phi> = [];
+    const terminal = block.terminal;
+    if (
+      (terminal.kind === "if" ||
+        terminal.kind === "switch" ||
+        terminal.kind === "while" ||
+        terminal.kind === "for") &&
+      terminal.fallthrough !== null
+    ) {
+      const fallthrough = fn.body.blocks.get(terminal.fallthrough)!;
+      phis.push(...fallthrough.phis);
+    }
+    if (terminal.kind === "while" || terminal.kind === "for") {
+      const test = fn.body.blocks.get(terminal.test)!;
+      phis.push(...test.phis);
+      const loop = fn.body.blocks.get(terminal.loop)!;
+      phis.push(...loop.phis);
+    }
+    if (terminal.kind === "for") {
+      const update = fn.body.blocks.get(terminal.update)!;
+      phis.push(...update.phis);
+    }
+
+    // For each phi, determine a canonical identifier to use for versions of the variable
+    // that appear in the phi (as its output id and operands). If this is the first time
+    // we're seeing the phi id, then we may need to generate a new variable declaration
+    // Note that there can be multiple phi nodes for the same variable, we capture the
+    // outermost scope by visiting predecessor blocks first.
+    for (const phi of phis) {
+      let canonicalId = variableMapping.get(phi.id);
+      if (canonicalId === undefined) {
+        // Determine a new canonical id. We use the id/operand whose id is lowest,
+        // which ensures that _if_ the original variable declaration is one of the
+        // options we'll choose it and can reuse the declaration.
+        canonicalId = phi.id;
+        for (const [, operand] of phi.operands) {
+          if (operand.id < canonicalId.id) {
+            canonicalId = operand;
+          }
+        }
+        variableMapping.set(phi.id, canonicalId);
+        needsDeclaration.add(canonicalId);
+      }
+      // all versions of the variable need to be remapped to the canonical id
+      for (const [, operand] of phi.operands) {
+        variableMapping.set(operand, canonicalId);
+      }
+    }
+
+    // Visit instructions and rewrite identifiers based on the variable mapping
+    // updated above.
+    for (const instr of block.instructions) {
+      const { lvalue, value } = instr;
+      if (lvalue !== null) {
+        lvalue.place.identifier =
+          variableMapping.get(lvalue.place.identifier) ??
+          lvalue.place.identifier;
+        if (lvalue.place.memberPath === null) {
+          if (!variableMapping.has(lvalue.place.identifier)) {
+            // This variable does not flow into a phi, therefore there
+            // is no reassignment. Convert the declaration to a const.
+            lvalue.kind = InstructionKind.Const;
+          } else if (
+            variableMapping.get(lvalue.place.identifier) ===
+            lvalue.place.identifier
+          ) {
+            // This is an existing declaration we can reuse as the canonical declaration for its
+            // phi. Note, the declaration must already be a `let` or else it would be invalid to
+            // reassign the variable in the first place.
+            needsDeclaration.delete(lvalue.place.identifier);
+          }
+        }
+      }
+      for (const operand of eachInstructionValueOperand(value)) {
+        operand.identifier =
+          variableMapping.get(operand.identifier) ?? operand.identifier;
+      }
+    }
+
+    for (const operand of eachTerminalOperand(terminal)) {
+      operand.identifier =
+        variableMapping.get(operand.identifier) ?? operand.identifier;
+    }
+
+    // Generate new let declarations for any remaining phi variables
+    for (const identifier of needsDeclaration) {
+      const instr: Instruction = {
+        // NOTE: reuse the terminal id since these lets must be scoped with the terminal anyway
+        // the only reason they exist is that there is a scope that will span the control flow.
+        id: block.terminal.id,
+        lvalue: {
+          place: {
+            kind: "Identifier",
+            memberPath: null,
+            identifier,
+            effect: Effect.Mutate,
+            loc: GeneratedSource,
+          },
+          kind: InstructionKind.Let,
+        },
+        value: {
+          kind: "Primitive",
+          value: undefined,
+          loc: GeneratedSource,
+        },
+        loc: GeneratedSource,
+      };
+      block.instructions.push(instr);
+    }
 
-  for (const [, block] of ir.blocks) {
     block.phis.clear();
   }
-
-  for (const [, block] of ir.blocks) {
-    for (const place of eachBlockOperand(block)) {
-      place.identifier.id = place.identifier.preSsaId ?? place.identifier.id;
-    }
-  }
 }
diff --git a/compiler/forget/src/HIR/Pipeline.ts b/compiler/forget/src/HIR/Pipeline.ts
index 306cb5206d..ba745275b8 100644
--- a/compiler/forget/src/HIR/Pipeline.ts
+++ b/compiler/forget/src/HIR/Pipeline.ts
@@ -12,7 +12,7 @@ import enterSSA from "../HIR/EnterSSA";
 import { Environment } from "../HIR/HIRBuilder";
 import { inferMutableRanges } from "../HIR/InferMutableLifetimes";
 import inferReferenceEffects from "../HIR/InferReferenceEffects";
-import leaveSSA from "../HIR/LeaveSSA";
+import { leaveSSA } from "../HIR/LeaveSSA";
 import codegen from "./Codegen";
 import { HIRFunction } from "./HIR";
 import { inferReactiveScopeDependencies } from "./InferReactiveScopeDependencies";
diff --git a/compiler/forget/src/HIR/PrintHIR.ts b/compiler/forget/src/HIR/PrintHIR.ts
index 337c535c1c..a450ff0d0f 100644
--- a/compiler/forget/src/HIR/PrintHIR.ts
+++ b/compiler/forget/src/HIR/PrintHIR.ts
@@ -128,7 +128,9 @@ export function printTerminal(terminal: Terminal): Array<string> | string {
     case "if": {
       value = `[${terminal.id}] If (${printPlace(terminal.test)}) then:bb${
         terminal.consequent
-      } else:bb${terminal.alternate}`;
+      } else:bb${terminal.alternate}${
+        terminal.fallthrough ? ` fallthrough=bb${terminal.fallthrough}` : ""
+      }`;
       break;
     }
     case "throw": {
@@ -157,6 +159,9 @@ export function printTerminal(terminal: Terminal): Array<string> | string {
           output.push(`  Default: bb${case_.block}`);
         }
       });
+      if (terminal.fallthrough) {
+        output.push(`  Fallthrough: bb${terminal.fallthrough}`);
+      }
       value = output;
       break;
     }
diff --git a/compiler/forget/src/__tests__/fixtures/hir/_bug_expression-with-assignment.expect.md b/compiler/forget/src/__tests__/fixtures/hir/_bug_expression-with-assignment.expect.md
index 2e6274626d..a703b6e9bf 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/_bug_expression-with-assignment.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/_bug_expression-with-assignment.expect.md
@@ -51,9 +51,9 @@ flowchart TB
 
 ```javascript
 function f$0() {
-  let x$1 = 1;
-  x$1 = 2;
-  return x$1 + x$1 + x$1;
+  const x$4 = 1;
+  const x$5 = 2;
+  return x$5 + x$5 + x$5;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/_bug_inverted-if-else.expect.md b/compiler/forget/src/__tests__/fixtures/hir/_bug_inverted-if-else.expect.md
new file mode 100644
index 0000000000..89989d051e
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/_bug_inverted-if-else.expect.md
@@ -0,0 +1,91 @@
+
+## Input
+
+```javascript
+function foo(a, b, c) {
+  let x = null;
+  label: {
+    if (a) {
+      x = b;
+      break label;
+    }
+    x = c;
+  }
+  return x;
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Let mutate x$8_@0 = null
+  [2] If (read a$5) then:bb3 else:bb2 fallthrough=bb2
+bb3:
+  predecessor blocks: bb0
+  [3] Reassign mutate x$9_@1[3:6] = read b$6
+  [4] Goto bb1
+bb2:
+  predecessor blocks: bb0
+  [5] Reassign mutate x$10_@1[3:6] = read c$7
+  [6] Goto bb1
+bb1:
+  predecessor blocks: bb3 bb2
+  x$11_@1[3:6]: phi(bb3: x$9_@1, bb2: x$10_@1)
+  [7] Return read x$11_@1
+scope1 [3:6]:
+ - read b$6
+ - read a$5
+ - read c$7
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Let mutate x$8_@0 = null
+    "]
+    bb0_instrs --> bb0_terminal(["If (read a$5)"])
+  end
+  subgraph bb3
+    bb3_instrs["
+      [3] Reassign mutate x$9_@1[3:6] = read b$6
+    "]
+    bb3_instrs --> bb3_terminal(["Goto"])
+  end
+  subgraph bb2
+    bb2_instrs["
+      [5] Reassign mutate x$10_@1[3:6] = read c$7
+    "]
+    bb2_instrs --> bb2_terminal(["Goto"])
+  end
+  subgraph bb1
+    bb1_terminal(["Return read x$11_@1"])
+  end
+
+  %% Jumps
+  bb0_terminal -- "then" --> bb3
+  bb0_terminal -- "else" --> bb2
+  bb3_terminal --> bb1
+  bb2_terminal --> bb1
+
+```
+
+## Code
+
+```javascript
+function foo$0(a$5, b$6, c$7) {
+  const x$8 = null;
+  bb2: if (a$5) {
+    const x$9 = b$6;
+  }
+
+  const x$10 = c$7;
+}
+
+```
+      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/_bug_inverted-if-else.js b/compiler/forget/src/__tests__/fixtures/hir/_bug_inverted-if-else.js
new file mode 100644
index 0000000000..574de55c64
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/_bug_inverted-if-else.js
@@ -0,0 +1,11 @@
+function foo(a, b, c) {
+  let x = null;
+  label: {
+    if (a) {
+      x = b;
+      break label;
+    }
+    x = c;
+  }
+  return x;
+}
diff --git a/compiler/forget/src/__tests__/fixtures/hir/_bug_while-with-assignment.expect.md b/compiler/forget/src/__tests__/fixtures/hir/_bug_while-with-assignment.expect.md
deleted file mode 100644
index 5a5062c6ed..0000000000
--- a/compiler/forget/src/__tests__/fixtures/hir/_bug_while-with-assignment.expect.md
+++ /dev/null
@@ -1,106 +0,0 @@
-
-## Input
-
-```javascript
-function f(reader) {
-  const queue = [1, 2, 3];
-  let value = 0;
-  let sum = 0;
-  // BUG: we need to codegen the complex test expression
-  while ((value = queue.pop()) != null) {
-    sum += value;
-  }
-  return sum;
-}
-
-```
-
-## HIR
-
-```
-bb0:
-  [1] Const mutate $11_@0 = 1
-  [2] Const mutate $12_@1 = 2
-  [3] Const mutate $13_@2 = 3
-  [4] Const mutate queue$14_@3[0:14] = Array [read $11_@0, read $12_@1, read $13_@2]
-  [5] Let mutate value$15_@4 = 0
-  [6] Let mutate sum$16_@3[0:14] = 0
-  [7] While test=bb1 loop=bb3 fallthrough=bb2
-bb1:
-  predecessor blocks: bb0 bb3
-  sum$21_@3[0:14]: phi(bb0: sum$16_@3, bb3: sum$22_@3)
-  [8] Reassign mutate value$18_@3[0:14] = Call mutate queue$14_@3.pop()
-  [9] Const mutate $19_@6 = null
-  [10] Const mutate $20_@7[10:12] = Binary read value$18_@3 != read $19_@6
-  [11] If (read $20_@7) then:bb3 else:bb2
-bb3:
-  predecessor blocks: bb1
-  [12] Reassign mutate sum$22_@3[0:14] = Binary read sum$21_@3 + read value$18_@3
-  [13] Goto(Continue) bb1
-bb2:
-  predecessor blocks: bb1
-  [14] Return read sum$21_@3
-scope7 [10:12]:
- - read $19_@6
-```
-
-### CFG
-
-```mermaid
-flowchart TB
-  %% Basic Blocks
-  subgraph bb0
-    bb0_instrs["
-      [1] Const mutate $11_@0 = 1
-      [2] Const mutate $12_@1 = 2
-      [3] Const mutate $13_@2 = 3
-      [4] Const mutate queue$14_@3[0:14] = Array [read $11_@0, read $12_@1, read $13_@2]
-      [5] Let mutate value$15_@4 = 0
-      [6] Let mutate sum$16_@3[0:14] = 0
-    "]
-    bb0_instrs --> bb0_terminal(["While"])
-  end
-  subgraph bb1
-    bb1_instrs["
-      [8] Reassign mutate value$18_@3[0:14] = Call mutate queue$14_@3.pop()
-      [9] Const mutate $19_@6 = null
-      [10] Const mutate $20_@7[10:12] = Binary read value$18_@3 != read $19_@6
-    "]
-    bb1_instrs --> bb1_terminal(["If (read $20_@7)"])
-  end
-  subgraph bb3
-    bb3_instrs["
-      [12] Reassign mutate sum$22_@3[0:14] = Binary read sum$21_@3 + read value$18_@3
-    "]
-    bb3_instrs --> bb3_terminal(["Goto"])
-  end
-  subgraph bb2
-    bb2_terminal(["Return read sum$21_@3"])
-  end
-
-  %% Jumps
-  bb0_terminal -- "test" --> bb1
-  bb0_terminal -- "loop" --> bb3
-  bb0_terminal -- "fallthrough" --> bb2
-  bb1_terminal -- "then" --> bb3
-  bb1_terminal -- "else" --> bb2
-  bb3_terminal --> bb1
-
-```
-
-## Code
-
-```javascript
-function f$0(reader$1) {
-  const queue$2 = [1, 2, 3];
-  let value$6 = 0;
-  let sum$7 = 0;
-  bb2: while (((value$6 = queue$2.pop()), value$6 != null)) {
-    sum$7 = sum$7 + value$6;
-  }
-
-  return sum$7;
-}
-
-```
-      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/alias-while.expect.md b/compiler/forget/src/__tests__/fixtures/hir/alias-while.expect.md
index c7746aa18f..b2cb6cb033 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/alias-while.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/alias-while.expect.md
@@ -36,7 +36,7 @@ bb1:
   a$12_@0[0:12]: phi(bb0: a$8_@0, bb3: a$15_@0)
   b$14_@0[0:12]: phi(bb0: b$9_@0, bb3: b$17_@0)
   c$16_@0[0:12]: phi(bb0: c$10_@0, bb3: c$18_@0)
-  [5] If (read cond$7) then:bb3 else:bb2
+  [5] If (read cond$7) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
   [6] Let mutate z$13_@0[0:12] = read a$12_@0
@@ -102,22 +102,22 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(cond$1) {
-  let a$2 = {};
-  let b$3 = {};
-  let c$4 = {};
-  bb2: while (cond$1) {
-    let z$5 = a$2;
-    a$2 = b$3;
-    b$3 = c$4;
-    c$4 = z$5;
-    mutate$6(a$2, b$3);
+function foo$0(cond$7) {
+  let a$8 = {};
+  let b$9 = {};
+  let c$10 = {};
+  bb2: while (cond$7) {
+    const z$13 = a$8;
+    a$8 = b$9;
+    b$9 = c$10;
+    c$10 = z$13;
+    mutate$6(a$8, b$9);
   }
 
-  a$2;
-  b$3;
-  c$4;
-  return a$2;
+  a$8;
+  b$9;
+  c$10;
+  return a$8;
 }
 
 ```
@@ -145,7 +145,7 @@ flowchart TB
 ## Code
 
 ```javascript
-function mutate$0(x$1, y$2) {}
+function mutate$0(x$3, y$4) {}
 
 ```
       
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/assignment-variations.expect.md b/compiler/forget/src/__tests__/fixtures/hir/assignment-variations.expect.md
index b9deb6ca80..c4208f4695 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/assignment-variations.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/assignment-variations.expect.md
@@ -65,10 +65,10 @@ flowchart TB
 
 ```javascript
 function f$0() {
-  let x$1 = 1;
-  x$1 = x$1 + 1;
-  x$1 = x$1 + 1;
-  x$1 = x$1 >>> 1;
+  const x$5 = 1;
+  const x$7 = x$5 + 1;
+  const x$9 = x$7 + 1;
+  const x$11 = x$9 >>> 1;
 }
 
 ```
@@ -107,9 +107,9 @@ flowchart TB
 ## Code
 
 ```javascript
-function g$0(a$1) {
-  a$1.c.b = a$1.b.c + 1;
-  a$1.c.b = a$1.b.c * 2;
+function g$0(a$4) {
+  a$4.c.b = a$4.b.c + 1;
+  a$4.c.b = a$4.b.c * 2;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/call.expect.md b/compiler/forget/src/__tests__/fixtures/hir/call.expect.md
index d943297060..e7eb75ae26 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/call.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/call.expect.md
@@ -89,14 +89,14 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  const b$3 = {};
-  foo$4(a$2, b$3);
-  let _$5 = <div a={a$2}></div>;
+function Component$0(props$9) {
+  const a$10 = [];
+  const b$11 = {};
+  foo$4(a$10, b$11);
+  const _$13 = <div a={a$10}></div>;
 
-  foo$4(b$3);
-  return <div a={a$2} b={b$3}></div>;
+  foo$4(b$11);
+  return <div a={a$10} b={b$11}></div>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/complex-while.expect.md b/compiler/forget/src/__tests__/fixtures/hir/complex-while.expect.md
index a132078235..3897cada95 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/complex-while.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/complex-while.expect.md
@@ -18,16 +18,16 @@ function foo(a, b, c) {
 
 ```
 bb0:
-  [1] If (read a$4) then:bb3 else:bb1
+  [1] If (read a$4) then:bb3 else:bb1 fallthrough=bb1
 bb3:
   predecessor blocks: bb0
   [2] While test=bb4 loop=bb6 fallthrough=bb1
 bb4:
   predecessor blocks: bb3 bb7
-  [3] If (read b$5) then:bb6 else:bb1
+  [3] If (read b$5) then:bb6 else:bb1 fallthrough=bb1
 bb6:
   predecessor blocks: bb4
-  [4] If (read c$6) then:bb1 else:bb7
+  [4] If (read c$6) then:bb1 else:bb7 fallthrough=bb7
 bb7:
   predecessor blocks: bb6
   [5] Goto(Continue) bb4
@@ -78,10 +78,10 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2, c$3) {
-  bb1: if (a$1) {
-    while (b$2) {
-      bb7: if (c$3) break;
+function foo$0(a$4, b$5, c$6) {
+  bb1: if (a$4) {
+    while (b$5) {
+      bb7: if (c$6) break;
     }
   }
 }
diff --git a/compiler/forget/src/__tests__/fixtures/hir/component.expect.md b/compiler/forget/src/__tests__/fixtures/hir/component.expect.md
index 1e4074ec71..e959a98117 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/component.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/component.expect.md
@@ -44,12 +44,12 @@ bb0:
   [7] Goto bb1
 bb1:
   predecessor blocks: bb0 bb5 bb10
-  [8] If (read items$27_@0) then:bb3 else:bb2
+  [8] If (read items$27_@0) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
   [9] Const mutate $34_@6 = null
   [10] Const mutate $36_@3[0:19] = Binary read item$10_@3 == read $34_@6
-  [11] If (read $36_@3) then:bb8 else:bb9
+  [11] If (read $36_@3) then:bb8 else:bb9 fallthrough=bb7
 bb8:
   predecessor blocks: bb3
   [12] Const mutate $37_@3[0:19] = read $36_@3
@@ -61,7 +61,7 @@ bb9:
 bb7:
   predecessor blocks: bb8 bb9
   $40_@3[0:19]: phi(bb8: $37_@3, bb9: $39_@3)
-  [16] If (read $40_@3) then:bb5 else:bb4
+  [16] If (read $40_@3) then:bb5 else:bb4 fallthrough=bb4
 bb5:
   predecessor blocks: bb7
   [17] Goto(Continue) bb1
@@ -72,7 +72,7 @@ bb4:
   [20] Const mutate $44_@8 = JSX <read $43_@7>{read item$10_@3}</read $43_@7>
   [21] Call mutate renderedItems$29_@2.push(read $44_@8)
   [22] Const mutate $49_@2[3:26] = Binary read renderedItems$29_@2.length >= read max$32_@5
-  [23] If (read $49_@2) then:bb2 else:bb10
+  [23] If (read $49_@2) then:bb2 else:bb10 fallthrough=bb10
 bb10:
   predecessor blocks: bb4
   [24] Goto(Continue) bb1
@@ -187,12 +187,12 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const items$2 = props$1.items;
-  const maxItems$3 = props$1.maxItems;
-  const renderedItems$4 = [];
-  const seen$5 = new Set$6();
-  const max$7 = Math$8.max(0, maxItems$3);
+function Component$0(props$26) {
+  const items$27 = props$26.items;
+  const maxItems$28 = props$26.maxItems;
+  const renderedItems$29 = [];
+  const seen$30 = new Set$6();
+  const max$32 = Math$8.max(0, maxItems$28);
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/conditional-break.expect.md b/compiler/forget/src/__tests__/fixtures/hir/conditional-break.expect.md
index a6f1237b15..bf49d3078d 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/conditional-break.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/conditional-break.expect.md
@@ -80,7 +80,7 @@ function Component(props) {
 bb0:
   [1] Const mutate a_DEBUG$5_@0[1:7] = Array []
   [2] Call mutate a_DEBUG$5_@0.push(read props$4.a)
-  [3] If (read props$4.b) then:bb2 else:bb1
+  [3] If (read props$4.b) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [4] Const mutate $6_@1 = null
@@ -129,15 +129,15 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a_DEBUG$2 = [];
-  a_DEBUG$2.push(props$1.a);
-  bb1: if (props$1.b) {
+function Component$0(props$4) {
+  const a_DEBUG$5 = [];
+  a_DEBUG$5.push(props$4.a);
+  bb1: if (props$4.b) {
     return null;
   }
 
-  a_DEBUG$2.push(props$1.d);
-  return a_DEBUG$2;
+  a_DEBUG$5.push(props$4.d);
+  return a_DEBUG$5;
 }
 
 ```
@@ -147,7 +147,7 @@ function Component$0(props$1) {
 bb0:
   [1] Const mutate a$4_@0[1:7] = Array []
   [2] Call mutate a$4_@0.push(read props$3.a)
-  [3] If (read props$3.b) then:bb2 else:bb1
+  [3] If (read props$3.b) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [4] Call mutate a$4_@0.push(read props$3.c)
@@ -198,15 +198,15 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  a$2.push(props$1.a);
-  bb1: if (props$1.b) {
-    a$2.push(props$1.c);
+function Component$0(props$3) {
+  const a$4 = [];
+  a$4.push(props$3.a);
+  bb1: if (props$3.b) {
+    a$4.push(props$3.c);
   }
 
-  a$2.push(props$1.d);
-  return a$2;
+  a$4.push(props$3.d);
+  return a$4;
 }
 
 ```
@@ -216,7 +216,7 @@ function Component$0(props$1) {
 bb0:
   [1] Const mutate a$5_@0[1:8] = Array []
   [2] Call mutate a$5_@0.push(read props$4.a)
-  [3] If (read props$4.b) then:bb2 else:bb1
+  [3] If (read props$4.b) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [4] Call mutate a$5_@0.push(read props$4.c)
@@ -268,16 +268,16 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  a$2.push(props$1.a);
-  bb1: if (props$1.b) {
-    a$2.push(props$1.c);
+function Component$0(props$4) {
+  const a$5 = [];
+  a$5.push(props$4.a);
+  bb1: if (props$4.b) {
+    a$5.push(props$4.c);
     return null;
   }
 
-  a$2.push(props$1.d);
-  return a$2;
+  a$5.push(props$4.d);
+  return a$5;
 }
 
 ```
@@ -287,7 +287,7 @@ function Component$0(props$1) {
 bb0:
   [1] Const mutate a$4_@0[1:7] = Array []
   [2] Call mutate a$4_@0.push(read props$3.a)
-  [3] If (read props$3.b) then:bb2 else:bb1
+  [3] If (read props$3.b) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [4] Call mutate a$4_@0.push(read props$3.c)
@@ -337,16 +337,16 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  a$2.push(props$1.a);
-  bb1: if (props$1.b) {
-    a$2.push(props$1.c);
-    return a$2;
+function Component$0(props$3) {
+  const a$4 = [];
+  a$4.push(props$3.a);
+  bb1: if (props$3.b) {
+    a$4.push(props$3.c);
+    return a$4;
   }
 
-  a$2.push(props$1.d);
-  return a$2;
+  a$4.push(props$3.d);
+  return a$4;
 }
 
 ```
@@ -356,7 +356,7 @@ function Component$0(props$1) {
 bb0:
   [1] Const mutate a$4_@0[1:7] = Array []
   [2] Call mutate a$4_@0.push(read props$3.a)
-  [3] If (read props$3.b) then:bb1 else:bb2
+  [3] If (read props$3.b) then:bb1 else:bb2 fallthrough=bb2
 bb2:
   predecessor blocks: bb0
   [4] Call mutate a$4_@0.push(read props$3.c)
@@ -406,15 +406,15 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  a$2.push(props$1.a);
-  bb2: if (props$1.b) {
-    a$2.push(props$1.d);
-    return a$2;
+function Component$0(props$3) {
+  const a$4 = [];
+  a$4.push(props$3.a);
+  bb2: if (props$3.b) {
+    a$4.push(props$3.d);
+    return a$4;
   }
 
-  a$2.push(props$1.c);
+  a$4.push(props$3.c);
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/conditional-on-mutable.expect.md b/compiler/forget/src/__tests__/fixtures/hir/conditional-on-mutable.expect.md
index a0be2fc143..b61fd849fd 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/conditional-on-mutable.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/conditional-on-mutable.expect.md
@@ -37,14 +37,14 @@ function mayMutate() {}
 bb0:
   [1] Const mutate a$7_@0[1:9] = Array []
   [2] Const mutate b$8_@0[1:9] = Array []
-  [3] If (read b$8_@0) then:bb2 else:bb1
+  [3] If (read b$8_@0) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [4] Call mutate a$7_@0.push(read props$6.p0)
   [5] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
-  [6] If (read props$6.p1) then:bb4 else:bb3
+  [6] If (read props$6.p1) then:bb4 else:bb3 fallthrough=bb3
 bb4:
   predecessor blocks: bb1
   [7] Call mutate b$8_@0.push(read props$6.p2)
@@ -109,18 +109,18 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  const b$3 = [];
-  bb1: if (b$3) {
-    a$2.push(props$1.p0);
+function Component$0(props$6) {
+  const a$7 = [];
+  const b$8 = [];
+  bb1: if (b$8) {
+    a$7.push(props$6.p0);
   }
 
-  bb3: if (props$1.p1) {
-    b$3.push(props$1.p2);
+  bb3: if (props$6.p1) {
+    b$8.push(props$6.p2);
   }
 
-  return <Foo$4 a={a$2} b={b$3}></Foo$4>;
+  return <Foo$4 a={a$7} b={b$8}></Foo$4>;
 }
 
 ```
@@ -131,14 +131,14 @@ bb0:
   [1] Const mutate a$9_@0[0:10] = Array []
   [2] Const mutate b$10_@0[0:10] = Array []
   [3] Const mutate $11_@0[0:10] = Call mutate mayMutate$4_@0(mutate b$10_@0)
-  [4] If (read $11_@0) then:bb2 else:bb1
+  [4] If (read $11_@0) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [5] Call mutate a$9_@0.push(read props$8.p0)
   [6] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
-  [7] If (read props$8.p1) then:bb4 else:bb3
+  [7] If (read props$8.p1) then:bb4 else:bb3 fallthrough=bb3
 bb4:
   predecessor blocks: bb1
   [8] Call mutate b$10_@0.push(read props$8.p2)
@@ -200,18 +200,18 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  const b$3 = [];
-  bb1: if (mayMutate$4(b$3)) {
-    a$2.push(props$1.p0);
+function Component$0(props$8) {
+  const a$9 = [];
+  const b$10 = [];
+  bb1: if (mayMutate$4(b$10)) {
+    a$9.push(props$8.p0);
   }
 
-  bb3: if (props$1.p1) {
-    b$3.push(props$1.p2);
+  bb3: if (props$8.p1) {
+    b$10.push(props$8.p2);
   }
 
-  return <Foo$6 a={a$2} b={b$3}></Foo$6>;
+  return <Foo$6 a={a$9} b={b$10}></Foo$6>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/constructor.expect.md b/compiler/forget/src/__tests__/fixtures/hir/constructor.expect.md
index 9c05ae4ac8..46b5471b1d 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/constructor.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/constructor.expect.md
@@ -89,14 +89,14 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  const b$3 = {};
-  new Foo$4(a$2, b$3);
-  let _$5 = <div a={a$2}></div>;
+function Component$0(props$9) {
+  const a$10 = [];
+  const b$11 = {};
+  new Foo$4(a$10, b$11);
+  const _$13 = <div a={a$10}></div>;
 
-  new Foo$4(b$3);
-  return <div a={a$2} b={b$3}></div>;
+  new Foo$4(b$11);
+  return <div a={a$10} b={b$11}></div>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/dependencies-outputs.expect.md b/compiler/forget/src/__tests__/fixtures/hir/dependencies-outputs.expect.md
index f8c609764b..5199b4ef6e 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/dependencies-outputs.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/dependencies-outputs.expect.md
@@ -27,14 +27,14 @@ bb0:
   [3] Const mutate $9_@1 = "div"
   [4] JSX <read $9_@1>{freeze x$8_@0}</read $9_@1>
   [5] Const mutate y$10_@2[5:12] = Array []
-  [6] If (read x$8_@0.length) then:bb2 else:bb1
+  [6] If (read x$8_@0.length) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [7] Call mutate y$10_@2.push(read x$8_@0)
   [8] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
-  [9] If (read b$7) then:bb4 else:bb3
+  [9] If (read b$7) then:bb4 else:bb3 fallthrough=bb3
 bb4:
   predecessor blocks: bb1
   [10] Call mutate y$10_@2.push(read b$7)
@@ -100,17 +100,17 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2) {
-  const x$3 = [];
-  x$3.push(a$1);
-  <div>{x$3}</div>;
-  const y$5 = [];
-  bb1: if (x$3.length) {
-    y$5.push(x$3);
+function foo$0(a$6, b$7) {
+  const x$8 = [];
+  x$8.push(a$6);
+  <div>{x$8}</div>;
+  const y$10 = [];
+  bb1: if (x$8.length) {
+    y$10.push(x$8);
   }
 
-  bb3: if (b$2) {
-    y$5.push(b$2);
+  bb3: if (b$7) {
+    y$10.push(b$7);
   }
 }
 
diff --git a/compiler/forget/src/__tests__/fixtures/hir/dependencies.expect.md b/compiler/forget/src/__tests__/fixtures/hir/dependencies.expect.md
index 739f36a5e7..1fc0a646bb 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/dependencies.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/dependencies.expect.md
@@ -27,14 +27,14 @@ bb0:
   [1] Const mutate items$9_@0[1:10] = Array [read z$8]
   [2] Call mutate items$9_@0.push(read x$6)
   [3] Const mutate items2$10_@1[3:7] = Array []
-  [4] If (read x$6) then:bb2 else:bb1
+  [4] If (read x$6) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [5] Call mutate items2$10_@1.push(read y$7)
   [6] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
-  [7] If (read y$7) then:bb4 else:bb3
+  [7] If (read y$7) then:bb4 else:bb3 fallthrough=bb3
 bb4:
   predecessor blocks: bb1
   [8] Call mutate items$9_@0.push(read x$6)
@@ -97,19 +97,19 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(x$1, y$2, z$3) {
-  const items$4 = [z$3];
-  items$4.push(x$1);
-  const items2$5 = [];
-  bb1: if (x$1) {
-    items2$5.push(y$2);
+function foo$0(x$6, y$7, z$8) {
+  const items$9 = [z$8];
+  items$9.push(x$6);
+  const items2$10 = [];
+  bb1: if (x$6) {
+    items2$10.push(y$7);
   }
 
-  bb3: if (y$2) {
-    items$4.push(x$1);
+  bb3: if (y$7) {
+    items$9.push(x$6);
   }
 
-  return items2$5;
+  return items2$10;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/error.while-with-assignment-in-test.expect.md b/compiler/forget/src/__tests__/fixtures/hir/error.while-with-assignment-in-test.expect.md
new file mode 100644
index 0000000000..5dc68ff63f
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/error.while-with-assignment-in-test.expect.md
@@ -0,0 +1,25 @@
+
+## Input
+
+```javascript
+function f(reader) {
+  const queue = [1, 2, 3];
+  let value = 0;
+  let sum = 0;
+  // BUG: we need to codegen the complex test expression
+  while ((value = queue.pop()) != null) {
+    sum += value;
+  }
+  return sum;
+}
+
+```
+
+
+## Error
+
+```
+TODO: Handle conversion of VariableDeclaration to expression
+```
+          
+      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/_bug_while-with-assignment.js b/compiler/forget/src/__tests__/fixtures/hir/error.while-with-assignment-in-test.js
similarity index 100%
rename from compiler/forget/src/__tests__/fixtures/hir/_bug_while-with-assignment.js
rename to compiler/forget/src/__tests__/fixtures/hir/error.while-with-assignment-in-test.js
diff --git a/compiler/forget/src/__tests__/fixtures/hir/extend-scopes-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/extend-scopes-if.expect.md
index b70dcde797..3e3438bab5 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/extend-scopes-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/extend-scopes-if.expect.md
@@ -24,13 +24,13 @@ function foo(a, b, c) {
 ```
 bb0:
   [1] Let mutate x$10_@0[1:8] = Array []
-  [2] If (read a$7) then:bb2 else:bb1
+  [2] If (read a$7) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [3] If (read b$8) then:bb4 else:bb1
+  [3] If (read b$8) then:bb4 else:bb1 fallthrough=bb1
 bb4:
   predecessor blocks: bb2
-  [4] If (read c$9) then:bb6 else:bb1
+  [4] If (read c$9) then:bb6 else:bb1 fallthrough=bb1
 bb6:
   predecessor blocks: bb4
   [5] Const mutate $11_@1 = 0
@@ -38,7 +38,7 @@ bb6:
   [7] Goto bb1
 bb1:
   predecessor blocks: bb6 bb4 bb2 bb0
-  [8] If (read a$7.length) then:bb8 else:bb7
+  [8] If (read a$7.length) then:bb8 else:bb7 fallthrough=bb7
 bb8:
   predecessor blocks: bb1
   [9] Return read a$7
@@ -105,18 +105,18 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2, c$3) {
-  let x$4 = [];
-  bb1: if (a$1) {
-    if (b$2) {
-      if (c$3) {
-        x$4.push(0);
+function foo$0(a$7, b$8, c$9) {
+  const x$10 = [];
+  bb1: if (a$7) {
+    if (b$8) {
+      if (c$9) {
+        x$10.push(0);
       }
     }
   }
 
-  bb7: if (a$1.length) {
-    return a$1;
+  bb7: if (a$7.length) {
+    return a$7;
   }
 
   return null;
diff --git a/compiler/forget/src/__tests__/fixtures/hir/frozen-after-alias.expect.md b/compiler/forget/src/__tests__/fixtures/hir/frozen-after-alias.expect.md
index 94852ad18c..7ceeea4df1 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/frozen-after-alias.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/frozen-after-alias.expect.md
@@ -51,10 +51,10 @@ flowchart TB
 
 ```javascript
 function Component$0() {
-  const a$1 = [];
-  const b$2 = a$1;
-  useFreeze$3(a$1);
-  foo$4(b$2);
+  const a$5 = [];
+  const b$6 = a$5;
+  useFreeze$3(a$5);
+  foo$4(b$6);
 }
 
 ```
@@ -109,7 +109,7 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(x$1) {}
+function foo$0(x$2) {}
 
 ```
       
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/hook-call.expect.md b/compiler/forget/src/__tests__/fixtures/hir/hook-call.expect.md
index 11a8b2bf95..a5151d72ed 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/hook-call.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/hook-call.expect.md
@@ -121,14 +121,14 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const x$2 = [];
-  const y$3 = useFreeze$4(x$2);
-  foo$5(y$3, x$2);
+function Component$0(props$10) {
+  const x$11 = [];
+  const y$12 = useFreeze$4(x$11);
+  foo$5(y$12, x$11);
   return (
     <Component$0>
-      {x$2}
-      {y$3}
+      {x$11}
+      {y$12}
     </Component$0>
   );
 }
diff --git a/compiler/forget/src/__tests__/fixtures/hir/hooks-freeze-arguments.expect.md b/compiler/forget/src/__tests__/fixtures/hir/hooks-freeze-arguments.expect.md
index a2a6c14e6d..ae4e7defa9 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/hooks-freeze-arguments.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/hooks-freeze-arguments.expect.md
@@ -50,11 +50,11 @@ flowchart TB
 
 ```javascript
 function Component$0() {
-  const a$1 = [];
-  useFreeze$2(a$1);
-  useFreeze$2(a$1);
-  call$3(a$1);
-  return a$1;
+  const a$4 = [];
+  useFreeze$2(a$4);
+  useFreeze$2(a$4);
+  call$3(a$4);
+  return a$4;
 }
 
 ```
@@ -82,7 +82,7 @@ flowchart TB
 ## Code
 
 ```javascript
-function useFreeze$0(x$1) {}
+function useFreeze$0(x$2) {}
 
 ```
 ## HIR
@@ -109,7 +109,7 @@ flowchart TB
 ## Code
 
 ```javascript
-function call$0(x$1) {}
+function call$0(x$2) {}
 
 ```
       
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/hooks-freeze-possibly-mutable-arguments.expect.md b/compiler/forget/src/__tests__/fixtures/hir/hooks-freeze-possibly-mutable-arguments.expect.md
index 4b570c3214..c662505eeb 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/hooks-freeze-possibly-mutable-arguments.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/hooks-freeze-possibly-mutable-arguments.expect.md
@@ -29,7 +29,7 @@ bb0:
   [1] Const mutate cond$8_@0 = read props$7.cond
   [2] Const mutate x$9_@1 = read props$7.x
   [3] Let mutate a$10_@2 = undefined
-  [4] If (read cond$8_@0) then:bb2 else:bb3
+  [4] If (read cond$8_@0) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [5] Reassign mutate a$11_@3[5:8] = read x$9_@1
@@ -99,20 +99,21 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const cond$2 = props$1.cond;
-  const x$3 = props$1.x;
-  let a$4 = undefined;
-  bb1: if (cond$2) {
-    a$4 = x$3;
+function Component$0(props$7) {
+  const cond$8 = props$7.cond;
+  const x$9 = props$7.x;
+  const a$10 = undefined;
+  let a$11 = undefined;
+  bb1: if (cond$8) {
+    a$11 = x$9;
   } else {
-    a$4 = [];
+    a$11 = [];
   }
 
-  useFreeze$5(a$4);
-  useFreeze$5(a$4);
-  call$6(a$4);
-  return a$4;
+  useFreeze$5(a$11);
+  useFreeze$5(a$11);
+  call$6(a$11);
+  return a$11;
 }
 
 ```
@@ -140,7 +141,7 @@ flowchart TB
 ## Code
 
 ```javascript
-function useFreeze$0(x$1) {}
+function useFreeze$0(x$2) {}
 
 ```
 ## HIR
@@ -167,7 +168,7 @@ flowchart TB
 ## Code
 
 ```javascript
-function call$0(x$1) {}
+function call$0(x$2) {}
 
 ```
       
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/independent-across-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/independent-across-if.expect.md
index 22e6b60d64..1da79760d0 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/independent-across-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/independent-across-if.expect.md
@@ -147,7 +147,7 @@ function Foo$0() {}
 bb0:
   [1] Const mutate a$9_@0[0:7] = Call mutate compute$3_@0(read props$8.a)
   [2] Const mutate b$10_@0[0:7] = Call mutate compute$3_@0(read props$8.b)
-  [3] If (read props$8.c) then:bb2 else:bb1
+  [3] If (read props$8.c) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [4] Call mutate mutate$5_@0(mutate a$9_@0)
@@ -198,15 +198,15 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = compute$3(props$1.a);
-  const b$4 = compute$3(props$1.b);
-  bb1: if (props$1.c) {
-    mutate$5(a$2);
-    mutate$5(b$4);
+function Component$0(props$8) {
+  const a$9 = compute$3(props$8.a);
+  const b$10 = compute$3(props$8.b);
+  bb1: if (props$8.c) {
+    mutate$5(a$9);
+    mutate$5(b$10);
   }
 
-  return <Foo$6 a={a$2} b={b$4}></Foo$6>;
+  return <Foo$6 a={a$9} b={b$10}></Foo$6>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/independent.expect.md b/compiler/forget/src/__tests__/fixtures/hir/independent.expect.md
index 796fd04a74..e2de083090 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/independent.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/independent.expect.md
@@ -58,10 +58,10 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = compute$3(props$1.a);
-  const b$4 = compute$3(props$1.b);
-  return <Foo$5 a={a$2} b={b$4}></Foo$5>;
+function Component$0(props$7) {
+  const a$8 = compute$3(props$7.a);
+  const b$9 = compute$3(props$7.b);
+  return <Foo$5 a={a$8} b={b$9}></Foo$5>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/interdependent-across-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/interdependent-across-if.expect.md
index 9faec8686e..45773e510c 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/interdependent-across-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/interdependent-across-if.expect.md
@@ -114,7 +114,7 @@ function Foo$0() {}
 bb0:
   [1] Const mutate a$9_@0[0:6] = Call mutate compute$3_@0(read props$8.a)
   [2] Const mutate b$10_@0[0:6] = Call mutate compute$3_@0(read props$8.b)
-  [3] If (read props$8.c) then:bb2 else:bb1
+  [3] If (read props$8.c) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [4] Call mutate foo$5_@0(mutate a$9_@0, mutate b$10_@0)
@@ -163,14 +163,14 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = compute$3(props$1.a);
-  const b$4 = compute$3(props$1.b);
-  bb1: if (props$1.c) {
-    foo$5(a$2, b$4);
+function Component$0(props$8) {
+  const a$9 = compute$3(props$8.a);
+  const b$10 = compute$3(props$8.b);
+  bb1: if (props$8.c) {
+    foo$5(a$9, b$10);
   }
 
-  return <Foo$6 a={a$2} b={b$4}></Foo$6>;
+  return <Foo$6 a={a$9} b={b$10}></Foo$6>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/interdependent.expect.md b/compiler/forget/src/__tests__/fixtures/hir/interdependent.expect.md
index 6e9508ee13..065a9d8099 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/interdependent.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/interdependent.expect.md
@@ -60,11 +60,11 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = compute$3(props$1.a);
-  const b$4 = compute$3(props$1.b);
-  foo$5(a$2, b$4);
-  return <Foo$6 a={a$2} b={b$4}></Foo$6>;
+function Component$0(props$8) {
+  const a$9 = compute$3(props$8.a);
+  const b$10 = compute$3(props$8.b);
+  foo$5(a$9, b$10);
+  return <Foo$6 a={a$9} b={b$10}></Foo$6>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/inverted-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/inverted-if.expect.md
index 9b20e7e8d2..da7e9ce49f 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/inverted-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/inverted-if.expect.md
@@ -19,10 +19,10 @@ function foo(a, b, c) {
 ```
 bb0:
   [1] Let mutate y$8_@0[1:6] = Array []
-  [2] If (read a$5) then:bb3 else:bb1
+  [2] If (read a$5) then:bb3 else:bb1 fallthrough=bb1
 bb3:
   predecessor blocks: bb0
-  [3] If (read b$6) then:bb5 else:bb1
+  [3] If (read b$6) then:bb5 else:bb1 fallthrough=bb1
 bb5:
   predecessor blocks: bb3
   [4] Call mutate y$8_@0.push(read c$7)
@@ -72,11 +72,11 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2, c$3) {
-  let y$4 = [];
-  bb1: if (a$1) {
-    if (b$2) {
-      y$4.push(c$3);
+function foo$0(a$5, b$6, c$7) {
+  const y$8 = [];
+  bb1: if (a$5) {
+    if (b$6) {
+      y$8.push(c$7);
     }
   }
 }
diff --git a/compiler/forget/src/__tests__/fixtures/hir/jsx-fragment.expect.md b/compiler/forget/src/__tests__/fixtures/hir/jsx-fragment.expect.md
index f47d28c59d..d98c2ec93b 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/jsx-fragment.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/jsx-fragment.expect.md
@@ -76,10 +76,10 @@ flowchart TB
 ## Code
 
 ```javascript
-function Foo$0(props$1) {
+function Foo$0(props$13) {
   return (
     <>
-      Hello {props$1.greeting}
+      Hello {props$13.greeting}
       {<div>{<>Text</>}</div>}
     </>
   );
diff --git a/compiler/forget/src/__tests__/fixtures/hir/logical-expression.expect.md b/compiler/forget/src/__tests__/fixtures/hir/logical-expression.expect.md
index c806987d47..c20f685f1f 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/logical-expression.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/logical-expression.expect.md
@@ -24,7 +24,7 @@ function g() {}
 ```
 bb0:
   [1] Const mutate $5_@0[0:2] = Call mutate f$1_@0()
-  [2] If (read $5_@0) then:bb2 else:bb3
+  [2] If (read $5_@0) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [3] Const mutate $6_@1[0:6] = Call mutate g$4_@1()
@@ -92,7 +92,7 @@ function And$0() {
 ```
 bb0:
   [1] Const mutate $5_@0[0:2] = Call mutate f$1_@0()
-  [2] If (read $5_@0) then:bb2 else:bb3
+  [2] If (read $5_@0) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [3] Const mutate $6_@1[0:6] = read $5_@0
@@ -162,7 +162,7 @@ bb0:
   [1] Const mutate $9_@0[0:2] = Call mutate f$2_@0()
   [2] Const mutate $10_@1 = null
   [3] Const mutate $11_@2 = Binary read $9_@0 != read $10_@1
-  [4] If (read $11_@2) then:bb2 else:bb3
+  [4] If (read $11_@2) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [5] Const mutate $12_@3[0:8] = read $9_@0
@@ -221,7 +221,7 @@ flowchart TB
 ## Code
 
 ```javascript
-function QuestionQuestion$0(props$1) {
+function QuestionQuestion$0(props$8) {
   bb1: if (f$2() != null) {
   } else {
   }
diff --git a/compiler/forget/src/__tests__/fixtures/hir/mutable-lifetime-loops.expect.md b/compiler/forget/src/__tests__/fixtures/hir/mutable-lifetime-loops.expect.md
index f10d9bb39d..1c3331937f 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/mutable-lifetime-loops.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/mutable-lifetime-loops.expect.md
@@ -63,7 +63,7 @@ flowchart TB
 ## Code
 
 ```javascript
-function mutate$0(x$1, y$2) {}
+function mutate$0(x$3, y$4) {}
 
 ```
 ## HIR
@@ -90,7 +90,7 @@ flowchart TB
 ## Code
 
 ```javascript
-function cond$0(x$1) {}
+function cond$0(x$2) {}
 
 ```
 ## HIR
@@ -109,7 +109,7 @@ bb1:
   c$22_@0[0:23]: phi(bb0: c$15_@0, bb4: c$25_@0)
   d$24_@0[0:23]: phi(bb0: d$16_@0, bb4: d$26_@0)
   [6] Const mutate $17_@1[6:8] = true
-  [7] If (read $17_@1) then:bb3 else:bb2
+  [7] If (read $17_@1) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
   [8] Let mutate z$19_@0[0:23] = read a$18_@0
@@ -119,7 +119,7 @@ bb3:
   [12] Reassign mutate d$26_@0[0:23] = read z$19_@0
   [13] Call mutate mutate$7_@0(mutate a$21_@0, mutate b$23_@0)
   [14] Const mutate $29_@0[0:23] = Call mutate cond$8_@0(mutate a$21_@0)
-  [15] If (read $29_@0) then:bb2 else:bb4
+  [15] If (read $29_@0) then:bb2 else:bb4 fallthrough=bb4
 bb4:
   predecessor blocks: bb3
   [16] Goto(Continue) bb1
@@ -129,16 +129,16 @@ bb2:
   b$31_@0[0:23]: phi(bb3: b$23_@0, bb1: b$20_@0)
   c$32_@0[0:23]: phi(bb3: c$25_@0, bb1: c$22_@0)
   d$33_@0[0:23]: phi(bb3: d$26_@0, bb1: d$24_@0)
-  [17] If (read a$30_@0) then:bb7 else:bb7
+  [17] If (read a$30_@0) then:bb7 else:bb7 fallthrough=bb7
 bb7:
   predecessor blocks: bb2
-  [18] If (read b$31_@0) then:bb9 else:bb9
+  [18] If (read b$31_@0) then:bb9 else:bb9 fallthrough=bb9
 bb9:
   predecessor blocks: bb7
-  [19] If (read c$32_@0) then:bb11 else:bb11
+  [19] If (read c$32_@0) then:bb11 else:bb11 fallthrough=bb11
 bb11:
   predecessor blocks: bb9
-  [20] If (read d$33_@0) then:bb13 else:bb13
+  [20] If (read d$33_@0) then:bb13 else:bb13 fallthrough=bb13
 bb13:
   predecessor blocks: bb11
   [21] Const mutate $34_@2 = null
@@ -225,35 +225,35 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  let a$2 = {};
-  let b$3 = {};
-  let c$4 = {};
-  let d$5 = {};
+function Component$0(props$12) {
+  let a$18 = {};
+  let b$20 = {};
+  let c$22 = {};
+  let d$24 = {};
   bb2: while (true) {
-    let z$6 = a$2;
-    a$2 = b$3;
-    b$3 = c$4;
-    c$4 = d$5;
-    d$5 = z$6;
-    mutate$7(a$2, b$3);
+    const z$19 = a$18;
+    a$18 = b$20;
+    b$20 = c$22;
+    c$22 = d$24;
+    d$24 = z$19;
+    mutate$7(a$18, b$20);
 
-    bb4: if (cond$8(a$2)) break;
+    bb4: if (cond$8(a$18)) break;
   }
 
-  bb7: if (a$2) {
+  bb7: if (a$18) {
   }
 
-  bb9: if (b$3) {
+  bb9: if (b$20) {
   }
 
-  bb11: if (c$4) {
+  bb11: if (c$22) {
   }
 
-  bb13: if (d$5) {
+  bb13: if (d$24) {
   }
 
-  mutate$7(d$5, null);
+  mutate$7(d$24, null);
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/mutable-lifetime-with-aliasing.expect.md b/compiler/forget/src/__tests__/fixtures/hir/mutable-lifetime-with-aliasing.expect.md
index b1dfa768c4..eb555749f2 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/mutable-lifetime-with-aliasing.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/mutable-lifetime-with-aliasing.expect.md
@@ -59,7 +59,7 @@ flowchart TB
 ## Code
 
 ```javascript
-function mutate$0(x$1, y$2) {}
+function mutate$0(x$3, y$4) {}
 
 ```
 ## HIR
@@ -73,19 +73,19 @@ bb0:
   [5] Const mutate x$15_@1[0:15] = Object {  }
   [6] Reassign mutate x$15_@1.b[0:15] = read b$12_@1
   [7] Const mutate y$16_@1[0:15] = Call mutate mutate$8_@1(mutate x$15_@1, mutate d$14_@1)
-  [8] If (read a$11_@0) then:bb1 else:bb1
+  [8] If (read a$11_@0) then:bb1 else:bb1 fallthrough=bb1
 bb1:
   predecessor blocks: bb0
-  [9] If (read b$12_@1) then:bb3 else:bb3
+  [9] If (read b$12_@1) then:bb3 else:bb3 fallthrough=bb3
 bb3:
   predecessor blocks: bb1
-  [10] If (read c$13_@2) then:bb5 else:bb5
+  [10] If (read c$13_@2) then:bb5 else:bb5 fallthrough=bb5
 bb5:
   predecessor blocks: bb3
-  [11] If (read d$14_@1) then:bb7 else:bb7
+  [11] If (read d$14_@1) then:bb7 else:bb7 fallthrough=bb7
 bb7:
   predecessor blocks: bb5
-  [12] If (read y$16_@1) then:bb9 else:bb9
+  [12] If (read y$16_@1) then:bb9 else:bb9 fallthrough=bb9
 bb9:
   predecessor blocks: bb7
   [13] Const mutate $17_@3 = null
@@ -148,32 +148,32 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = {};
-  const b$3 = [a$2];
-  const c$4 = {};
-  const d$5 = {
-    c: c$4,
+function Component$0(props$10) {
+  const a$11 = {};
+  const b$12 = [a$11];
+  const c$13 = {};
+  const d$14 = {
+    c: c$13,
   };
-  const x$6 = {};
-  x$6.b = b$3;
-  const y$7 = mutate$8(x$6, d$5);
-  bb1: if (a$2) {
+  const x$15 = {};
+  x$15.b = b$12;
+  const y$16 = mutate$8(x$15, d$14);
+  bb1: if (a$11) {
   }
 
-  bb3: if (b$3) {
+  bb3: if (b$12) {
   }
 
-  bb5: if (c$4) {
+  bb5: if (c$13) {
   }
 
-  bb7: if (d$5) {
+  bb7: if (d$14) {
   }
 
-  bb9: if (y$7) {
+  bb9: if (y$16) {
   }
 
-  mutate$8(x$6, null);
+  mutate$8(x$15, null);
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/mutable-liverange-loop.expect.md b/compiler/forget/src/__tests__/fixtures/hir/mutable-liverange-loop.expect.md
index 63374ae199..58b79e8e30 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/mutable-liverange-loop.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/mutable-liverange-loop.expect.md
@@ -100,27 +100,27 @@ bb0:
 bb1:
   predecessor blocks: bb0 bb4
   [6] Const mutate $16_@2[6:8] = true
-  [7] If (read $16_@2) then:bb3 else:bb2
+  [7] If (read $16_@2) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
   [8] Call mutate mutate$6_@0(mutate a$12_@0, mutate b$13_@0)
   [9] Const mutate $21_@0[0:18] = Call mutate cond$7_@0(mutate a$12_@0)
-  [10] If (read $21_@0) then:bb2 else:bb4
+  [10] If (read $21_@0) then:bb2 else:bb4 fallthrough=bb4
 bb4:
   predecessor blocks: bb3
   [11] Goto(Continue) bb1
 bb2:
   predecessor blocks: bb3 bb1
-  [12] If (read a$12_@0) then:bb7 else:bb7
+  [12] If (read a$12_@0) then:bb7 else:bb7 fallthrough=bb7
 bb7:
   predecessor blocks: bb2
-  [13] If (read b$13_@0) then:bb9 else:bb9
+  [13] If (read b$13_@0) then:bb9 else:bb9 fallthrough=bb9
 bb9:
   predecessor blocks: bb7
-  [14] If (read c$14_@1) then:bb11 else:bb11
+  [14] If (read c$14_@1) then:bb11 else:bb11 fallthrough=bb11
 bb11:
   predecessor blocks: bb9
-  [15] If (read d$15_@0) then:bb13 else:bb13
+  [15] If (read d$15_@0) then:bb13 else:bb13 fallthrough=bb13
 bb13:
   predecessor blocks: bb11
   [16] Const mutate $28_@3 = null
@@ -202,30 +202,30 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  let a$2 = {};
-  let b$3 = {};
-  let c$4 = {};
-  let d$5 = {};
+function Component$0(props$11) {
+  const a$12 = {};
+  const b$13 = {};
+  const c$14 = {};
+  const d$15 = {};
   bb2: while (true) {
-    mutate$6(a$2, b$3);
+    mutate$6(a$12, b$13);
 
-    bb4: if (cond$7(a$2)) break;
+    bb4: if (cond$7(a$12)) break;
   }
 
-  bb7: if (a$2) {
+  bb7: if (a$12) {
   }
 
-  bb9: if (b$3) {
+  bb9: if (b$13) {
   }
 
-  bb11: if (c$4) {
+  bb11: if (c$14) {
   }
 
-  bb13: if (d$5) {
+  bb13: if (d$15) {
   }
 
-  mutate$6(d$5, null);
+  mutate$6(d$15, null);
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-interleaved-by-terminal.expect.md b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-interleaved-by-terminal.expect.md
index a1cb864a20..f78fcdc0a9 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-interleaved-by-terminal.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-interleaved-by-terminal.expect.md
@@ -21,7 +21,7 @@ function foo(a, b, c) {
 bb0:
   [1] Const mutate x$9_@0[1:6] = Array []
   [2] Const mutate y$10_@0[1:6] = Array []
-  [3] If (read x$9_@0) then:bb1 else:bb1
+  [3] If (read x$9_@0) then:bb1 else:bb1 fallthrough=bb1
 bb1:
   predecessor blocks: bb0
   [4] Call mutate y$10_@0.push(read a$6)
@@ -61,14 +61,14 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2, c$3) {
-  const x$4 = [];
-  const y$5 = [];
-  bb1: if (x$4) {
+function foo$0(a$6, b$7, c$8) {
+  const x$9 = [];
+  const y$10 = [];
+  bb1: if (x$9) {
   }
 
-  y$5.push(a$1);
-  x$4.push(b$2);
+  y$10.push(a$6);
+  x$9.push(b$7);
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-interleaved.expect.md b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-interleaved.expect.md
index 6803ea0884..97cc7a29ec 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-interleaved.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-interleaved.expect.md
@@ -47,11 +47,11 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2) {
-  let x$3 = [];
-  let y$4 = [];
-  x$3.push(a$1);
-  y$4.push(b$2);
+function foo$0(a$5, b$6) {
+  const x$7 = [];
+  const y$8 = [];
+  x$7.push(a$5);
+  y$8.push(b$6);
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-shadowed.expect.md b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-shadowed.expect.md
index 774fc10534..e31eebbaa1 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-shadowed.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-shadowed.expect.md
@@ -47,11 +47,11 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2) {
-  let x$3 = [];
-  let y$4 = [];
-  y$4.push(b$2);
-  x$3.push(a$1);
+function foo$0(a$5, b$6) {
+  const x$7 = [];
+  const y$8 = [];
+  y$8.push(b$6);
+  x$7.push(a$5);
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-shadowing-within-block.expect.md b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-shadowing-within-block.expect.md
index 78046cb56c..43f7316eb8 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-shadowing-within-block.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-shadowing-within-block.expect.md
@@ -22,11 +22,11 @@ function foo(a, b, c) {
 ```
 bb0:
   [1] Let mutate x$11_@0[1:11] = Array []
-  [2] If (read a$8) then:bb2 else:bb1
+  [2] If (read a$8) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [3] Let mutate y$12_@1[3:7] = Array []
-  [4] If (read b$9) then:bb4 else:bb3
+  [4] If (read b$9) then:bb4 else:bb3 fallthrough=bb3
 bb4:
   predecessor blocks: bb2
   [5] Call mutate y$12_@1.push(read c$10)
@@ -98,19 +98,19 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2, c$3) {
-  let x$4 = [];
-  bb1: if (a$1) {
-    let y$5 = [];
+function foo$0(a$8, b$9, c$10) {
+  const x$11 = [];
+  bb1: if (a$8) {
+    const y$12 = [];
 
-    bb3: if (b$2) {
-      y$5.push(c$3);
+    bb3: if (b$9) {
+      y$12.push(c$10);
     }
 
-    x$4.push(<div>{y$5}</div>);
+    x$11.push(<div>{y$12}</div>);
   }
 
-  return x$4;
+  return x$11;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-while.expect.md b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-while.expect.md
index 433734e79d..9bb61e3ccc 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-while.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-while.expect.md
@@ -22,7 +22,7 @@ bb0:
   [3] While test=bb1 loop=bb3 fallthrough=bb2
 bb1:
   predecessor blocks: bb0 bb3
-  [4] If (read c$8) then:bb3 else:bb2
+  [4] If (read c$8) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
   [5] Call mutate y$10_@0.push(read b$7)
@@ -76,12 +76,12 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2, c$3) {
-  let x$4 = [];
-  let y$5 = [];
-  bb2: while (c$3) {
-    y$5.push(b$2);
-    x$4.push(a$1);
+function foo$0(a$6, b$7, c$8) {
+  const x$9 = [];
+  const y$10 = [];
+  bb2: while (c$8) {
+    y$10.push(b$7);
+    x$9.push(a$6);
   }
 }
 
diff --git a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-within-block.expect.md b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-within-block.expect.md
index ab02513d1b..fdf3a1010d 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-within-block.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/overlapping-scopes-within-block.expect.md
@@ -22,11 +22,11 @@ function foo(a, b, c) {
 ```
 bb0:
   [1] Let mutate x$9_@0[1:9] = Array []
-  [2] If (read a$6) then:bb2 else:bb1
+  [2] If (read a$6) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [3] Let mutate y$10_@0[1:9] = Array []
-  [4] If (read b$7) then:bb4 else:bb3
+  [4] If (read b$7) then:bb4 else:bb3 fallthrough=bb3
 bb4:
   predecessor blocks: bb2
   [5] Call mutate y$10_@0.push(read c$8)
@@ -90,19 +90,19 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2, c$3) {
-  let x$4 = [];
-  bb1: if (a$1) {
-    let y$5 = [];
+function foo$0(a$6, b$7, c$8) {
+  const x$9 = [];
+  bb1: if (a$6) {
+    const y$10 = [];
 
-    bb3: if (b$2) {
-      y$5.push(c$3);
+    bb3: if (b$7) {
+      y$10.push(c$8);
     }
 
-    x$4.push(y$5);
+    x$9.push(y$10);
   }
 
-  return x$4;
+  return x$9;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/property-assignment.expect.md b/compiler/forget/src/__tests__/fixtures/hir/property-assignment.expect.md
index 74ea0ae523..f410249c7e 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/property-assignment.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/property-assignment.expect.md
@@ -57,13 +57,13 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const x$2 = {};
-  const y$3 = [];
-  x$2.y = y$3;
-  const child$4 = <Component$0 data={y$3}></Component$0>;
-  x$2.y.push(props$1.p0);
-  return <Component$0 data={x$2}>{child$4}</Component$0>;
+function Component$0(props$6) {
+  const x$7 = {};
+  const y$8 = [];
+  x$7.y = y$8;
+  const child$9 = <Component$0 data={y$8}></Component$0>;
+  x$7.y.push(props$6.p0);
+  return <Component$0 data={x$7}>{child$9}</Component$0>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/reactive-scope-grouping.expect.md b/compiler/forget/src/__tests__/fixtures/hir/reactive-scope-grouping.expect.md
index 8b413bb7ca..0b4516c790 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/reactive-scope-grouping.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/reactive-scope-grouping.expect.md
@@ -51,12 +51,12 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = {};
-  let y$2 = [];
-  let z$3 = {};
-  y$2.push(z$3);
-  x$1.y = y$2;
-  return x$1;
+  const x$4 = {};
+  const y$5 = [];
+  const z$6 = {};
+  y$5.push(z$6);
+  x$4.y = y$5;
+  return x$4;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/reactive-scopes.expect.md b/compiler/forget/src/__tests__/fixtures/hir/reactive-scopes.expect.md
index ef5a0a4dd9..6aa83da0dd 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/reactive-scopes.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/reactive-scopes.expect.md
@@ -22,10 +22,10 @@ bb0:
   [1] Let mutate x$10_@0[1:8] = Array []
   [2] Const mutate $11_@1 = 1
   [3] Const mutate $12_@2 = Binary read a$8.length === read $11_@1
-  [4] If (read $12_@2) then:bb2 else:bb1
+  [4] If (read $12_@2) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [5] If (read b$9) then:bb4 else:bb1
+  [5] If (read b$9) then:bb4 else:bb1 fallthrough=bb1
 bb4:
   predecessor blocks: bb2
   [6] Call mutate x$10_@0.push(read b$9)
@@ -88,15 +88,15 @@ flowchart TB
 ## Code
 
 ```javascript
-function f$0(a$1, b$2) {
-  let x$3 = [];
-  bb1: if (a$1.length === 1) {
-    if (b$2) {
-      x$3.push(b$2);
+function f$0(a$8, b$9) {
+  const x$10 = [];
+  bb1: if (a$8.length === 1) {
+    if (b$9) {
+      x$10.push(b$9);
     }
   }
 
-  return <div>{x$3}</div>;
+  return <div>{x$10}</div>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/reassignment-conditional.expect.md b/compiler/forget/src/__tests__/fixtures/hir/reassignment-conditional.expect.md
index 8c0260e471..fcefff2c63 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/reassignment-conditional.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/reassignment-conditional.expect.md
@@ -26,7 +26,7 @@ bb0:
   [1] Let mutate x$7_@0[1:7] = Array []
   [2] Call mutate x$7_@0.push(read props$6.p0)
   [3] Let mutate y$8_@0[1:7] = read x$7_@0
-  [4] If (read props$6.p1) then:bb2 else:bb1
+  [4] If (read props$6.p1) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [5] Reassign mutate x$9_@0[1:7] = Array []
@@ -88,18 +88,18 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  let x$2 = [];
-  x$2.push(props$1.p0);
-  let y$3 = x$2;
-  bb1: if (props$1.p1) {
-    x$2 = [];
+function Component$0(props$6) {
+  let x$7 = [];
+  x$7.push(props$6.p0);
+  const y$8 = x$7;
+  bb1: if (props$6.p1) {
+    x$7 = [];
   }
 
-  let _$4 = <Component$0 x={x$2}></Component$0>;
+  const _$12 = <Component$0 x={x$7}></Component$0>;
 
-  y$3.push(props$1.p2);
-  return <Component$0 x={x$2} y={y$3}></Component$0>;
+  y$8.push(props$6.p2);
+  return <Component$0 x={x$7} y={y$8}></Component$0>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/reassignment-separate-scopes.expect.md b/compiler/forget/src/__tests__/fixtures/hir/reassignment-separate-scopes.expect.md
index d978caf4b8..c15bd7a239 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/reassignment-separate-scopes.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/reassignment-separate-scopes.expect.md
@@ -35,7 +35,7 @@ function foo(a, b, c) {
 ```
 bb0:
   [1] Let mutate x$16_@0[1:5] = Array []
-  [2] If (read a$13) then:bb2 else:bb1
+  [2] If (read a$13) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [3] Call mutate x$16_@0.push(read a$13)
@@ -48,6 +48,7 @@ bb1:
   [8] Switch (read b$14)
     Case read $20_@3: bb5
     Default: bb4
+    Fallthrough: bb3
 bb5:
   predecessor blocks: bb1
   [9] Reassign mutate x$22_@4[9:14] = Array []
@@ -150,31 +151,32 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2, c$3) {
-  let x$4 = [];
-  bb1: if (a$1) {
-    x$4.push(a$1);
+function foo$0(a$13, b$14, c$15) {
+  const x$16 = [];
+  bb1: if (a$13) {
+    x$16.push(a$13);
   }
 
-  let y$5 = <div>{x$4}</div>;
+  const y$19 = <div>{x$16}</div>;
+  let x$22 = undefined;
 
-  bb3: switch (b$2) {
+  bb3: switch (b$14) {
     case 0: {
-      x$4 = [];
-      x$4.push(b$2);
+      x$22 = [];
+      x$22.push(b$14);
       break bb3;
     }
 
     default: {
-      x$4 = [];
-      x$4.push(c$3);
+      x$22 = [];
+      x$22.push(c$15);
     }
   }
 
   return (
     <div>
-      {y$5}
-      {x$4}
+      {y$19}
+      {x$22}
     </div>
   );
 }
diff --git a/compiler/forget/src/__tests__/fixtures/hir/reassignment.expect.md b/compiler/forget/src/__tests__/fixtures/hir/reassignment.expect.md
index 678e22ed47..2b5d7d45de 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/reassignment.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/reassignment.expect.md
@@ -66,15 +66,15 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  let x$2 = [];
-  x$2.push(props$1.p0);
-  let y$3 = x$2;
-  x$2 = [];
-  let _$4 = <Component$0 x={x$2}></Component$0>;
+function Component$0(props$6) {
+  const x$7 = [];
+  x$7.push(props$6.p0);
+  const y$8 = x$7;
+  const x$9 = [];
+  const _$10 = <Component$0 x={x$9}></Component$0>;
 
-  y$3.push(props$1.p1);
-  return <Component$0 x={x$2} y={y$3}></Component$0>;
+  y$8.push(props$6.p1);
+  return <Component$0 x={x$9} y={y$8}></Component$0>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/reverse-postorder.expect.md b/compiler/forget/src/__tests__/fixtures/hir/reverse-postorder.expect.md
index 243ee6ce67..4133c31866 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/reverse-postorder.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/reverse-postorder.expect.md
@@ -37,7 +37,7 @@ function Component(props) {
 ```
 bb0:
   [1] Let mutate x$7_@0 = undefined
-  [2] If (read props$6.cond) then:bb2 else:bb10
+  [2] If (read props$6.cond) then:bb2 else:bb10 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [3] Const mutate $8_@1 = 2
@@ -48,6 +48,7 @@ bb2:
     Case read $9_@2: bb6
     Case read $8_@1: bb4
     Default: bb4
+    Fallthrough: bb1
 bb8:
   predecessor blocks: bb2
   [7] Reassign mutate x$11_@4[7:17] = read props$6.v0
@@ -62,7 +63,7 @@ bb4:
   [12] Goto bb1
 bb10:
   predecessor blocks: bb0
-  [13] If (read props$6.cond2) then:bb12 else:bb13
+  [13] If (read props$6.cond2) then:bb12 else:bb13 fallthrough=bb1
 bb12:
   predecessor blocks: bb10
   [14] Reassign mutate x$14_@4[7:17] = read props$6.b
@@ -167,17 +168,18 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  let x$2 = undefined;
-  bb1: if (props$1.cond) {
-    switch (props$1.test) {
+function Component$0(props$6) {
+  const x$7 = undefined;
+  let x$11 = undefined;
+  bb1: if (props$6.cond) {
+    switch (props$6.test) {
       case 0: {
-        x$2 = props$1.v0;
+        x$11 = props$6.v0;
         break bb1;
       }
 
       case 1: {
-        x$2 = props$1.v1;
+        x$11 = props$6.v1;
         break bb1;
       }
 
@@ -185,18 +187,18 @@ function Component$0(props$1) {
       }
 
       default: {
-        x$2 = props$1.v2;
+        x$11 = props$6.v2;
       }
     }
   } else {
-    if (props$1.cond2) {
-      x$2 = props$1.b;
+    if (props$6.cond2) {
+      x$11 = props$6.b;
     } else {
-      x$2 = props$1.c;
+      x$11 = props$6.c;
     }
   }
 
-  x$2;
+  x$11;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/simple-alias.expect.md b/compiler/forget/src/__tests__/fixtures/hir/simple-alias.expect.md
index 907b8254f9..f2ad814ebd 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/simple-alias.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/simple-alias.expect.md
@@ -84,14 +84,14 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let a$1 = {};
-  let b$2 = {};
-  let c$3 = {};
-  a$1 = b$2;
-  b$2 = c$3;
-  c$3 = a$1;
-  mutate$4(a$1, b$2);
-  return c$3;
+  const a$5 = {};
+  const b$6 = {};
+  const c$7 = {};
+  const a$8 = b$6;
+  const b$9 = c$7;
+  const c$10 = a$8;
+  mutate$4(a$8, b$9);
+  return c$10;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/simple.expect.md b/compiler/forget/src/__tests__/fixtures/hir/simple.expect.md
index d925fc326d..3d6174d171 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/simple.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/simple.expect.md
@@ -15,7 +15,7 @@ function foo(x, y) {
 
 ```
 bb0:
-  [1] If (read x$8) then:bb2 else:bb1
+  [1] If (read x$8) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [2] Const mutate $10_@0 = false
@@ -71,11 +71,11 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(x$1, y$2) {
-  bb1: if (x$1) {
-    return foo$0(false, y$2);
+function foo$0(x$8, y$9) {
+  bb1: if (x$8) {
+    return foo$0(false, y$9);
   }
-  return [y$2 * 10];
+  return [y$9 * 10];
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-arrayexpression.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-arrayexpression.expect.md
index f78d6502e2..899cbf7728 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-arrayexpression.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-arrayexpression.expect.md
@@ -45,11 +45,11 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = 1;
-  const b$3 = 2;
-  const x$4 = [a$2, b$3];
-  return x$4;
+function Component$0(props$5) {
+  const a$6 = 1;
+  const b$7 = 2;
+  const x$8 = [a$6, b$7];
+  return x$8;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-call-jsx-2.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-call-jsx-2.expect.md
index 4c325410d3..1112def4d9 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-call-jsx-2.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-call-jsx-2.expect.md
@@ -53,7 +53,7 @@ bb0:
   [2] Const mutate b$12_@0[0:10] = Object {  }
   [3] Call mutate foo$4_@0(mutate a$11_@0, mutate b$12_@0)
   [4] Const mutate $13_@0[0:10] = Call mutate foo$4_@0()
-  [5] If (read $13_@0) then:bb2 else:bb1
+  [5] If (read $13_@0) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [6] Const mutate $14_@1 = "div"
@@ -113,16 +113,16 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  const b$3 = {};
-  foo$4(a$2, b$3);
+function Component$0(props$10) {
+  const a$11 = [];
+  const b$12 = {};
+  foo$4(a$11, b$12);
   bb1: if (foo$4()) {
-    let _$5 = <div a={a$2}></div>;
+    const _$15 = <div a={a$11}></div>;
   }
 
-  foo$4(a$2, b$3);
-  return <div a={a$2} b={b$3}></div>;
+  foo$4(a$11, b$12);
+  return <div a={a$11} b={b$12}></div>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-call-jsx.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-call-jsx.expect.md
index 33937f5de1..002fba21e2 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-call-jsx.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-call-jsx.expect.md
@@ -89,14 +89,14 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  const b$3 = {};
-  foo$4(a$2, b$3);
-  let _$5 = <div a={a$2}></div>;
+function Component$0(props$9) {
+  const a$10 = [];
+  const b$11 = {};
+  foo$4(a$10, b$11);
+  const _$13 = <div a={a$10}></div>;
 
-  foo$4(a$2, b$3);
-  return <div a={a$2} b={b$3}></div>;
+  foo$4(a$10, b$11);
+  return <div a={a$10} b={b$11}></div>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-complex-multiple-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-complex-multiple-if.expect.md
index 0b4da48098..9a5d73f716 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-complex-multiple-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-complex-multiple-if.expect.md
@@ -25,7 +25,7 @@ bb0:
   [2] Let mutate y$8_@1 = 2
   [3] Const mutate $9_@2 = 2
   [4] Const mutate $10_@3 = Binary read y$8_@1 === read $9_@2
-  [5] If (read $10_@3) then:bb2 else:bb1
+  [5] If (read $10_@3) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [6] Reassign mutate x$11_@0[1:14] = 3
@@ -35,7 +35,7 @@ bb1:
   x$17_@0[1:14]: phi(bb2: x$11_@0, bb0: x$7_@0)
   [8] Const mutate $12_@4 = 3
   [9] Const mutate $14_@5 = Binary read y$8_@1 === read $12_@4
-  [10] If (read $14_@5) then:bb4 else:bb3
+  [10] If (read $14_@5) then:bb4 else:bb3 fallthrough=bb3
 bb4:
   predecessor blocks: bb1
   [11] Reassign mutate x$15_@0[1:14] = 5
@@ -107,17 +107,19 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  let y$2 = 2;
-  bb1: if (y$2 === 2) {
-    x$1 = 3;
+  let x$7 = 1;
+  const y$8 = 2;
+  bb1: if (y$8 === 2) {
+    x$7 = 3;
   }
 
-  bb3: if (y$2 === 3) {
-    x$1 = 5;
+  let x$15 = undefined;
+
+  bb3: if (y$8 === 3) {
+    x$15 = 5;
   }
 
-  y$2 = x$1;
+  const y$18 = x$15;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-complex-single-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-complex-single-if.expect.md
index f6e9d49f66..8f2d5e5db9 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-complex-single-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-complex-single-if.expect.md
@@ -22,7 +22,7 @@ bb0:
   [2] Let mutate y$6_@1 = 2
   [3] Const mutate $7_@2 = 2
   [4] Const mutate $8_@3 = Binary read y$6_@1 === read $7_@2
-  [5] If (read $8_@3) then:bb2 else:bb1
+  [5] If (read $8_@3) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [6] Reassign mutate x$9_@0[1:9] = 3
@@ -75,13 +75,13 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  let y$2 = 2;
-  bb1: if (y$2 === 2) {
-    x$1 = 3;
+  let x$5 = 1;
+  const y$6 = 2;
+  bb1: if (y$6 === 2) {
+    x$5 = 3;
   }
 
-  y$2 = x$1;
+  const y$11 = x$5;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-for-of.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-for-of.expect.md
index cca1c44dbb..7aafc6d070 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-for-of.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-for-of.expect.md
@@ -23,11 +23,11 @@ bb0:
   [2] Goto bb1
 bb1:
   predecessor blocks: bb0 bb4
-  [3] If (read items$5_@0) then:bb3 else:bb2
+  [3] If (read items$5_@0) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
   [4] Let mutate y$7_@1 = 0
-  [5] If (read cond$4) then:bb5 else:bb4
+  [5] If (read cond$4) then:bb5 else:bb4 fallthrough=bb4
 bb5:
   predecessor blocks: bb3
   [6] Reassign mutate y$9_@2 = 1
@@ -88,8 +88,8 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(cond$1) {
-  let items$2 = [];
+function foo$0(cond$4) {
+  const items$5 = [];
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-for.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-for.expect.md
index ca49eecaa4..6572b52fbd 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-for.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-for.expect.md
@@ -27,7 +27,7 @@ bb1:
   x$13_@0[0:13]: phi(bb3: x$7_@0, bb4: x$14_@0)
   [5] Const mutate $9_@2 = 10
   [6] Const mutate $11_@3[6:8] = Binary read i$8_@1 < read $9_@2
-  [7] If (read $11_@3) then:bb5 else:bb2
+  [7] If (read $11_@3) then:bb5 else:bb2 fallthrough=bb2
 bb5:
   predecessor blocks: bb1
   [8] Const mutate $12_@4 = 1
@@ -103,12 +103,12 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  bb2: for (let i$2 = 0; i$2 < 10; update$3()) {
-    x$1 = x$1 + 1;
+  let x$7 = 1;
+  bb2: for (const i$8 = 0; i$8 < 10; update$3()) {
+    x$7 = x$7 + 1;
   }
 
-  return x$1;
+  return x$7;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-if-else.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-if-else.expect.md
index 97a31f325a..cc530e4c59 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-if-else.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-if-else.expect.md
@@ -21,7 +21,7 @@ function foo() {
 bb0:
   [1] Let mutate x$5_@0 = 1
   [2] Let mutate y$6_@1 = 2
-  [3] If (read y$6_@1) then:bb2 else:bb3
+  [3] If (read y$6_@1) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [4] Let mutate z$7_@2 = Binary read x$5_@0 + read y$6_@1
@@ -81,12 +81,12 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  let y$2 = 2;
-  bb1: if (y$2) {
-    let z$3 = x$1 + y$2;
+  const x$5 = 1;
+  const y$6 = 2;
+  bb1: if (y$6) {
+    const z$7 = x$5 + y$6;
   } else {
-    let z$4 = x$1;
+    const z$8 = x$5;
   }
 }
 
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-multiple-phis.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-multiple-phis.expect.md
new file mode 100644
index 0000000000..9cc8ded1f7
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-multiple-phis.expect.md
@@ -0,0 +1,195 @@
+
+## Input
+
+```javascript
+function foo(a, b, c, d) {
+  let x = 0;
+  if (true) {
+    if (true) {
+      x = a;
+    } else {
+      x = b;
+    }
+    x;
+  } else {
+    if (true) {
+      x = c;
+    } else {
+      x = d;
+    }
+    x;
+  }
+  x;
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Let mutate x$13_@0 = 0
+  [2] Const mutate $14_@1 = true
+  [3] If (read $14_@1) then:bb2 else:bb6 fallthrough=bb1
+bb2:
+  predecessor blocks: bb0
+  [4] Const mutate $15_@2 = true
+  [5] If (read $15_@2) then:bb4 else:bb5 fallthrough=bb3
+bb4:
+  predecessor blocks: bb2
+  [6] Reassign mutate x$16_@3[6:17] = read a$9
+  [7] Goto bb3
+bb5:
+  predecessor blocks: bb2
+  [8] Reassign mutate x$17_@3[6:17] = read b$10
+  [9] Goto bb3
+bb3:
+  predecessor blocks: bb4 bb5
+  x$18_@3[6:17]: phi(bb4: x$16_@3, bb5: x$17_@3)
+  [10] read x$18_@3
+  [11] Goto bb1
+bb6:
+  predecessor blocks: bb0
+  [12] Const mutate $19_@4 = true
+  [13] If (read $19_@4) then:bb8 else:bb9 fallthrough=bb7
+bb8:
+  predecessor blocks: bb6
+  [14] Reassign mutate x$20_@3[6:17] = read c$11
+  [15] Goto bb7
+bb9:
+  predecessor blocks: bb6
+  [16] Reassign mutate x$21_@3[6:17] = read d$12
+  [17] Goto bb7
+bb7:
+  predecessor blocks: bb8 bb9
+  x$22_@3[6:17]: phi(bb8: x$20_@3, bb9: x$21_@3)
+  [18] read x$22_@3
+  [19] Goto bb1
+bb1:
+  predecessor blocks: bb3 bb7
+  x$23_@3[6:17]: phi(bb3: x$18_@3, bb7: x$22_@3)
+  [20] read x$23_@3
+  [21] Return
+scope3 [6:17]:
+ - read a$9
+ - read b$10
+ - read $15_@2
+ - read c$11
+ - read d$12
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Let mutate x$13_@0 = 0
+      [2] Const mutate $14_@1 = true
+    "]
+    bb0_instrs --> bb0_terminal(["If (read $14_@1)"])
+  end
+  subgraph bb2
+    bb2_instrs["
+      [4] Const mutate $15_@2 = true
+    "]
+    bb2_instrs --> bb2_terminal(["If (read $15_@2)"])
+  end
+  subgraph bb4
+    bb4_instrs["
+      [6] Reassign mutate x$16_@3[6:17] = read a$9
+    "]
+    bb4_instrs --> bb4_terminal(["Goto"])
+  end
+  subgraph bb5
+    bb5_instrs["
+      [8] Reassign mutate x$17_@3[6:17] = read b$10
+    "]
+    bb5_instrs --> bb5_terminal(["Goto"])
+  end
+  subgraph bb3
+    bb3_instrs["
+      [10] read x$18_@3
+    "]
+    bb3_instrs --> bb3_terminal(["Goto"])
+  end
+  subgraph bb6
+    bb6_instrs["
+      [12] Const mutate $19_@4 = true
+    "]
+    bb6_instrs --> bb6_terminal(["If (read $19_@4)"])
+  end
+  subgraph bb8
+    bb8_instrs["
+      [14] Reassign mutate x$20_@3[6:17] = read c$11
+    "]
+    bb8_instrs --> bb8_terminal(["Goto"])
+  end
+  subgraph bb9
+    bb9_instrs["
+      [16] Reassign mutate x$21_@3[6:17] = read d$12
+    "]
+    bb9_instrs --> bb9_terminal(["Goto"])
+  end
+  subgraph bb7
+    bb7_instrs["
+      [18] read x$22_@3
+    "]
+    bb7_instrs --> bb7_terminal(["Goto"])
+  end
+  subgraph bb1
+    bb1_instrs["
+      [20] read x$23_@3
+    "]
+    bb1_instrs --> bb1_terminal(["Return"])
+  end
+
+  %% Jumps
+  bb0_terminal -- "then" --> bb2
+  bb0_terminal -- "else" --> bb6
+  bb0_terminal -- "fallthrough" --> bb1
+  bb2_terminal -- "then" --> bb4
+  bb2_terminal -- "else" --> bb5
+  bb2_terminal -- "fallthrough" --> bb3
+  bb4_terminal --> bb3
+  bb5_terminal --> bb3
+  bb3_terminal --> bb1
+  bb6_terminal -- "then" --> bb8
+  bb6_terminal -- "else" --> bb9
+  bb6_terminal -- "fallthrough" --> bb7
+  bb8_terminal --> bb7
+  bb9_terminal --> bb7
+  bb7_terminal --> bb1
+
+```
+
+## Code
+
+```javascript
+function foo$0(a$9, b$10, c$11, d$12) {
+  const x$13 = 0;
+  let x$18 = undefined;
+  bb1: if (true) {
+    bb3: if (true) {
+      x$18 = a$9;
+    } else {
+      x$18 = b$10;
+    }
+
+    x$18;
+  } else {
+    bb7: if (true) {
+      x$18 = c$11;
+    } else {
+      x$18 = d$12;
+    }
+
+    x$18;
+  }
+
+  x$18;
+}
+
+```
+      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-multiple-phis.js b/compiler/forget/src/__tests__/fixtures/hir/ssa-multiple-phis.js
new file mode 100644
index 0000000000..03ad0b030e
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-multiple-phis.js
@@ -0,0 +1,19 @@
+function foo(a, b, c, d) {
+  let x = 0;
+  if (true) {
+    if (true) {
+      x = a;
+    } else {
+      x = b;
+    }
+    x;
+  } else {
+    if (true) {
+      x = c;
+    } else {
+      x = d;
+    }
+    x;
+  }
+  x;
+}
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-loops-no-reassign.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-loops-no-reassign.expect.md
index ad442592f1..e0f0d190f6 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-loops-no-reassign.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-loops-no-reassign.expect.md
@@ -25,19 +25,19 @@ bb0:
   [2] While test=bb1 loop=bb3 fallthrough=bb2
 bb1:
   predecessor blocks: bb0 bb5
-  [3] If (read a$6) then:bb3 else:bb2
+  [3] If (read a$6) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
   [4] While test=bb4 loop=bb6 fallthrough=bb5
 bb4:
   predecessor blocks: bb3 bb8
-  [5] If (read b$7) then:bb6 else:bb5
+  [5] If (read b$7) then:bb6 else:bb5 fallthrough=bb5
 bb6:
   predecessor blocks: bb4
   [6] While test=bb7 loop=bb9 fallthrough=bb8
 bb7:
   predecessor blocks: bb6 bb9
-  [7] If (read c$8) then:bb9 else:bb8
+  [7] If (read c$8) then:bb9 else:bb8 fallthrough=bb8
 bb9:
   predecessor blocks: bb7
   [8] Const mutate $13_@1 = 1
@@ -124,17 +124,17 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2, c$3) {
-  let x$4 = 0;
-  bb2: while (a$1) {
-    bb5: while (b$2) {
-      bb8: while (c$3) {
-        x$4 + 1;
+function foo$0(a$6, b$7, c$8) {
+  const x$9 = 0;
+  bb2: while (a$6) {
+    bb5: while (b$7) {
+      bb8: while (c$8) {
+        x$9 + 1;
       }
     }
   }
 
-  return x$4;
+  return x$9;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-reassignment.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-reassignment.expect.md
new file mode 100644
index 0000000000..69b0c972f1
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-reassignment.expect.md
@@ -0,0 +1,105 @@
+
+## Input
+
+```javascript
+function foo(a, b, c, d, e) {
+  let x = null;
+  if (a) {
+    x = b;
+  } else {
+    if (c) {
+      x = d;
+    }
+  }
+  return x;
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Let mutate x$12_@0[1:8] = null
+  [2] If (read a$7) then:bb2 else:bb3 fallthrough=bb1
+bb2:
+  predecessor blocks: bb0
+  [3] Reassign mutate x$13_@0[1:8] = read b$8
+  [4] Goto bb1
+bb3:
+  predecessor blocks: bb0
+  [5] If (read c$9) then:bb5 else:bb1 fallthrough=bb1
+bb5:
+  predecessor blocks: bb3
+  [6] Reassign mutate x$14_@0[1:8] = read d$10
+  [7] Goto bb1
+bb1:
+  predecessor blocks: bb2 bb5 bb3
+  x$15_@0[1:8]: phi(bb2: x$13_@0, bb5: x$14_@0, bb3: x$12_@0)
+  [8] Return read x$15_@0
+scope0 [1:8]:
+ - read b$8
+ - read d$10
+ - read c$9
+ - read a$7
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Let mutate x$12_@0[1:8] = null
+    "]
+    bb0_instrs --> bb0_terminal(["If (read a$7)"])
+  end
+  subgraph bb2
+    bb2_instrs["
+      [3] Reassign mutate x$13_@0[1:8] = read b$8
+    "]
+    bb2_instrs --> bb2_terminal(["Goto"])
+  end
+  subgraph bb3
+    bb3_terminal(["If (read c$9)"])
+  end
+  subgraph bb5
+    bb5_instrs["
+      [6] Reassign mutate x$14_@0[1:8] = read d$10
+    "]
+    bb5_instrs --> bb5_terminal(["Goto"])
+  end
+  subgraph bb1
+    bb1_terminal(["Return read x$15_@0"])
+  end
+
+  %% Jumps
+  bb0_terminal -- "then" --> bb2
+  bb0_terminal -- "else" --> bb3
+  bb0_terminal -- "fallthrough" --> bb1
+  bb2_terminal --> bb1
+  bb3_terminal -- "then" --> bb5
+  bb3_terminal -- "else" --> bb1
+  bb5_terminal --> bb1
+
+```
+
+## Code
+
+```javascript
+function foo$0(a$7, b$8, c$9, d$10, e$11) {
+  let x$12 = null;
+  bb1: if (a$7) {
+    x$12 = b$8;
+  } else {
+    if (c$9) {
+      x$12 = d$10;
+    }
+  }
+
+  return x$12;
+}
+
+```
+      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-reassignment.js b/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-reassignment.js
new file mode 100644
index 0000000000..00574a537c
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-reassignment.js
@@ -0,0 +1,11 @@
+function foo(a, b, c, d, e) {
+  let x = null;
+  if (a) {
+    x = b;
+  } else {
+    if (c) {
+      x = d;
+    }
+  }
+  return x;
+}
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-newexpression.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-newexpression.expect.md
index 48104c33af..c9bdd07aad 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-newexpression.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-newexpression.expect.md
@@ -72,11 +72,11 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = [];
-  const b$3 = {};
-  let c$4 = new Foo$5(a$2, b$3);
-  return c$4;
+function Component$0(props$6) {
+  const a$7 = [];
+  const b$8 = {};
+  const c$9 = new Foo$5(a$7, b$8);
+  return c$9;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-objectexpression-phi.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-objectexpression-phi.expect.md
index 4d3e698300..65082830ad 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-objectexpression-phi.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-objectexpression-phi.expect.md
@@ -26,7 +26,7 @@ bb0:
   [2] Let mutate y$7_@0[1:10] = 2
   [3] Const mutate $8_@2 = 1
   [4] Const mutate $9_@3 = Binary read x$6_@0 > read $8_@2
-  [5] If (read $9_@3) then:bb2 else:bb3
+  [5] If (read $9_@3) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [6] Reassign mutate x$10_@0[1:10] = 2
@@ -91,19 +91,19 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  let y$2 = 2;
-  bb1: if (x$1 > 1) {
-    x$1 = 2;
+  let x$6 = 1;
+  let y$7 = 2;
+  bb1: if (x$6 > 1) {
+    x$6 = 2;
   } else {
-    y$2 = 3;
+    y$7 = 3;
   }
 
-  let t$5 = {
-    x: x$1,
-    y: y$2,
+  const t$14 = {
+    x: x$6,
+    y: y$7,
   };
-  return t$5;
+  return t$14;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-objectexpression.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-objectexpression.expect.md
index 69f7933223..4bc5e82a50 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-objectexpression.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-objectexpression.expect.md
@@ -45,14 +45,14 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  const a$2 = 1;
-  const b$3 = 2;
-  const x$4 = {
-    a: a$2,
-    b: b$3,
+function Component$0(props$5) {
+  const a$6 = 1;
+  const b$7 = 2;
+  const x$8 = {
+    a: a$6,
+    b: b$7,
   };
-  return x$4;
+  return x$8;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-property-call.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-property-call.expect.md
index dfc1a42c1b..342e1cbe70 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-property-call.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-property-call.expect.md
@@ -47,12 +47,12 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  const x$1 = [];
-  const y$2 = {
-    x: x$1,
+  const x$4 = [];
+  const y$5 = {
+    x: x$4,
   };
-  y$2.x.push([]);
-  return y$2;
+  y$5.x.push([]);
+  return y$5;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-property.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-property.expect.md
index f9e4208798..6a2845e3ef 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-property.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-property.expect.md
@@ -44,10 +44,10 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  const x$1 = [];
-  const y$2 = {};
-  y$2.x = x$1;
-  return y$2;
+  const x$3 = [];
+  const y$4 = {};
+  y$4.x = x$3;
+  return y$4;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.expect.md
new file mode 100644
index 0000000000..e5c957e562
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.expect.md
@@ -0,0 +1,63 @@
+
+## Input
+
+```javascript
+function foo(a, b, c) {
+  let x = 0;
+  x = a;
+  x = b;
+  x = c;
+  return x;
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Let mutate x$8_@0 = 0
+  [2] Reassign mutate x$9_@1 = read a$5
+  [3] Reassign mutate x$10_@2 = read b$6
+  [4] Reassign mutate x$11_@3 = read c$7
+  [5] Return read x$11_@3
+scope1 [2:3]:
+ - read a$5
+scope2 [3:4]:
+ - read b$6
+scope3 [4:5]:
+ - read c$7
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Let mutate x$8_@0 = 0
+      [2] Reassign mutate x$9_@1 = read a$5
+      [3] Reassign mutate x$10_@2 = read b$6
+      [4] Reassign mutate x$11_@3 = read c$7
+    "]
+    bb0_instrs --> bb0_terminal(["Return read x$11_@3"])
+  end
+
+  %% Jumps
+  %% empty
+```
+
+## Code
+
+```javascript
+function foo$0(a$5, b$6, c$7) {
+  const x$8 = 0;
+  const x$9 = a$5;
+  const x$10 = b$6;
+  const x$11 = c$7;
+  return x$11;
+}
+
+```
+      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.js b/compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.js
new file mode 100644
index 0000000000..ea96f4a745
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.js
@@ -0,0 +1,7 @@
+function foo(a, b, c) {
+  let x = 0;
+  x = a;
+  x = b;
+  x = c;
+  return x;
+}
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-return.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-return.expect.md
index a9ec87d3b5..33be65cfc8 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-return.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-return.expect.md
@@ -20,7 +20,7 @@ bb0:
   [1] Let mutate x$4_@0[1:7] = 1
   [2] Const mutate $5_@1 = 1
   [3] Const mutate $6_@2 = Binary read x$4_@0 === read $5_@1
-  [4] If (read $6_@2) then:bb2 else:bb1
+  [4] If (read $6_@2) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [5] Reassign mutate x$7_@0[1:7] = 2
@@ -67,12 +67,12 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  bb1: if (x$1 === 1) {
-    x$1 = 2;
+  let x$4 = 1;
+  bb1: if (x$4 === 1) {
+    x$4 = 2;
   }
 
-  return x$1;
+  return x$4;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-shadowing.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-shadowing.expect.md
index 455c1f41c0..3e6d8ad457 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-shadowing.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-shadowing.expect.md
@@ -49,7 +49,7 @@ function log$0() {}
 ```
 bb0:
   [1] Let mutate str$6_@0[1:8] = ""
-  [2] If (read cond$5) then:bb2 else:bb3
+  [2] If (read cond$5) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [3] Let mutate str$7_@1 = "other test"
@@ -110,16 +110,16 @@ flowchart TB
 ## Code
 
 ```javascript
-function Foo$0(cond$1) {
-  let str$2 = "";
-  bb1: if (cond$1) {
-    let str$3 = "other test";
-    log$4(str$3);
+function Foo$0(cond$5) {
+  let str$6 = "";
+  bb1: if (cond$5) {
+    const str$7 = "other test";
+    log$4(str$7);
   } else {
-    str$2 = "fallthrough test";
+    str$6 = "fallthrough test";
   }
 
-  log$4(str$2);
+  log$4(str$6);
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-sibling-phis.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-sibling-phis.expect.md
new file mode 100644
index 0000000000..4b02019526
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-sibling-phis.expect.md
@@ -0,0 +1,191 @@
+
+## Input
+
+```javascript
+function foo(a, b, c, d) {
+  let x = 0;
+  if (true) {
+    if (true) {
+      x = a;
+    } else {
+      x = b;
+    }
+    x;
+  } else {
+    if (true) {
+      x = c;
+    } else {
+      x = d;
+    }
+    x;
+  }
+  // note: intentionally no phi here so that there are two distinct phis above
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Let mutate x$13_@0 = 0
+  [2] Const mutate $14_@1 = true
+  [3] If (read $14_@1) then:bb2 else:bb6 fallthrough=bb1
+bb2:
+  predecessor blocks: bb0
+  [4] Const mutate $15_@2 = true
+  [5] If (read $15_@2) then:bb4 else:bb5 fallthrough=bb3
+bb4:
+  predecessor blocks: bb2
+  [6] Reassign mutate x$16_@3[6:9] = read a$9
+  [7] Goto bb3
+bb5:
+  predecessor blocks: bb2
+  [8] Reassign mutate x$17_@3[6:9] = read b$10
+  [9] Goto bb3
+bb3:
+  predecessor blocks: bb4 bb5
+  x$18_@3[6:9]: phi(bb4: x$16_@3, bb5: x$17_@3)
+  [10] read x$18_@3
+  [11] Goto bb1
+bb6:
+  predecessor blocks: bb0
+  [12] Const mutate $19_@4 = true
+  [13] If (read $19_@4) then:bb8 else:bb9 fallthrough=bb7
+bb8:
+  predecessor blocks: bb6
+  [14] Reassign mutate x$20_@5[14:17] = read c$11
+  [15] Goto bb7
+bb9:
+  predecessor blocks: bb6
+  [16] Reassign mutate x$21_@5[14:17] = read d$12
+  [17] Goto bb7
+bb7:
+  predecessor blocks: bb8 bb9
+  x$22_@5[14:17]: phi(bb8: x$20_@5, bb9: x$21_@5)
+  [18] read x$22_@5
+  [19] Goto bb1
+bb1:
+  predecessor blocks: bb3 bb7
+  [20] Return
+scope3 [6:9]:
+ - read a$9
+ - read b$10
+scope5 [14:17]:
+ - read c$11
+ - read d$12
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Let mutate x$13_@0 = 0
+      [2] Const mutate $14_@1 = true
+    "]
+    bb0_instrs --> bb0_terminal(["If (read $14_@1)"])
+  end
+  subgraph bb2
+    bb2_instrs["
+      [4] Const mutate $15_@2 = true
+    "]
+    bb2_instrs --> bb2_terminal(["If (read $15_@2)"])
+  end
+  subgraph bb4
+    bb4_instrs["
+      [6] Reassign mutate x$16_@3[6:9] = read a$9
+    "]
+    bb4_instrs --> bb4_terminal(["Goto"])
+  end
+  subgraph bb5
+    bb5_instrs["
+      [8] Reassign mutate x$17_@3[6:9] = read b$10
+    "]
+    bb5_instrs --> bb5_terminal(["Goto"])
+  end
+  subgraph bb3
+    bb3_instrs["
+      [10] read x$18_@3
+    "]
+    bb3_instrs --> bb3_terminal(["Goto"])
+  end
+  subgraph bb6
+    bb6_instrs["
+      [12] Const mutate $19_@4 = true
+    "]
+    bb6_instrs --> bb6_terminal(["If (read $19_@4)"])
+  end
+  subgraph bb8
+    bb8_instrs["
+      [14] Reassign mutate x$20_@5[14:17] = read c$11
+    "]
+    bb8_instrs --> bb8_terminal(["Goto"])
+  end
+  subgraph bb9
+    bb9_instrs["
+      [16] Reassign mutate x$21_@5[14:17] = read d$12
+    "]
+    bb9_instrs --> bb9_terminal(["Goto"])
+  end
+  subgraph bb7
+    bb7_instrs["
+      [18] read x$22_@5
+    "]
+    bb7_instrs --> bb7_terminal(["Goto"])
+  end
+  subgraph bb1
+    bb1_terminal(["Return"])
+  end
+
+  %% Jumps
+  bb0_terminal -- "then" --> bb2
+  bb0_terminal -- "else" --> bb6
+  bb0_terminal -- "fallthrough" --> bb1
+  bb2_terminal -- "then" --> bb4
+  bb2_terminal -- "else" --> bb5
+  bb2_terminal -- "fallthrough" --> bb3
+  bb4_terminal --> bb3
+  bb5_terminal --> bb3
+  bb3_terminal --> bb1
+  bb6_terminal -- "then" --> bb8
+  bb6_terminal -- "else" --> bb9
+  bb6_terminal -- "fallthrough" --> bb7
+  bb8_terminal --> bb7
+  bb9_terminal --> bb7
+  bb7_terminal --> bb1
+
+```
+
+## Code
+
+```javascript
+function foo$0(a$9, b$10, c$11, d$12) {
+  const x$13 = 0;
+  bb1: if (true) {
+    let x$16 = undefined;
+
+    bb3: if (true) {
+      x$16 = a$9;
+    } else {
+      x$16 = b$10;
+    }
+
+    x$16;
+  } else {
+    let x$20 = undefined;
+
+    bb7: if (true) {
+      x$20 = c$11;
+    } else {
+      x$20 = d$12;
+    }
+
+    x$20;
+  }
+}
+
+```
+      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-sibling-phis.js b/compiler/forget/src/__tests__/fixtures/hir/ssa-sibling-phis.js
new file mode 100644
index 0000000000..72d80d0ad1
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-sibling-phis.js
@@ -0,0 +1,19 @@
+function foo(a, b, c, d) {
+  let x = 0;
+  if (true) {
+    if (true) {
+      x = a;
+    } else {
+      x = b;
+    }
+    x;
+  } else {
+    if (true) {
+      x = c;
+    } else {
+      x = d;
+    }
+    x;
+  }
+  // note: intentionally no phi here so that there are two distinct phis above
+}
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-simple-phi.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-simple-phi.expect.md
index 081aec327f..90bdbee104 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-simple-phi.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-simple-phi.expect.md
@@ -23,7 +23,7 @@ bb0:
   [1] Let mutate y$5_@0 = 2
   [2] Const mutate $6_@1 = 1
   [3] Const mutate $7_@2 = Binary read y$5_@0 > read $6_@1
-  [4] If (read $7_@2) then:bb2 else:bb3
+  [4] If (read $7_@2) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [5] Reassign mutate y$8_@3[5:10] = 1
@@ -89,14 +89,15 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let y$1 = 2;
-  bb1: if (y$1 > 1) {
-    y$1 = 1;
+  const y$5 = 2;
+  let y$8 = undefined;
+  bb1: if (y$5 > 1) {
+    y$8 = 1;
   } else {
-    y$1 = 2;
+    y$8 = 2;
   }
 
-  let x$4 = y$1;
+  const x$11 = y$8;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-simple.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-simple.expect.md
index cce524f1ce..cc17b32e36 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-simple.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-simple.expect.md
@@ -40,8 +40,8 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  let y$2 = 2;
+  const x$3 = 1;
+  const y$4 = 2;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-single-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-single-if.expect.md
index 06b0a5f5b5..8d0a4b4338 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-single-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-single-if.expect.md
@@ -19,7 +19,7 @@ function foo() {
 bb0:
   [1] Let mutate x$4_@0 = 1
   [2] Let mutate y$5_@1 = 2
-  [3] If (read y$5_@1) then:bb2 else:bb1
+  [3] If (read y$5_@1) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [4] Let mutate z$6_@2 = Binary read x$4_@0 + read y$5_@1
@@ -65,10 +65,10 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  let y$2 = 2;
-  bb1: if (y$2) {
-    let z$3 = x$1 + y$2;
+  const x$4 = 1;
+  const y$5 = 2;
+  bb1: if (y$5) {
+    const z$6 = x$4 + y$5;
   }
 }
 
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-switch.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-switch.expect.md
index da88ebc5f8..a358ffe9e7 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-switch.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-switch.expect.md
@@ -37,6 +37,7 @@ bb0:
     Case read $14_@4: bb5
     Case read $12_@2: bb3
     Default: bb2
+    Fallthrough: bb1
 bb5:
   predecessor blocks: bb0
   [7] Const mutate $15_@5 = 1
@@ -130,24 +131,25 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  bb1: switch (x$1) {
-    case x$1 === 1: {
-      x$1 = x$1 + 1;
+  const x$10 = 1;
+  let x$16 = undefined;
+  bb1: switch (x$10) {
+    case x$10 === 1: {
+      x$16 = x$10 + 1;
       break bb1;
     }
 
-    case x$1 === 2: {
-      x$1 = x$1 + 2;
+    case x$10 === 2: {
+      x$16 = x$10 + 2;
       break bb1;
     }
 
     default: {
-      x$1 = x$1 + 3;
+      x$16 = x$10 + 3;
     }
   }
 
-  let y$9 = x$1;
+  const y$22 = x$16;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-throw.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-throw.expect.md
index 7e1e0bb0db..669e36b3a7 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-throw.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-throw.expect.md
@@ -19,7 +19,7 @@ bb0:
   [1] Let mutate x$4_@0[1:7] = 1
   [2] Const mutate $5_@1 = 1
   [3] Const mutate $6_@2 = Binary read x$4_@0 === read $5_@1
-  [4] If (read $6_@2) then:bb2 else:bb1
+  [4] If (read $6_@2) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
   [5] Reassign mutate x$7_@0[1:7] = 2
@@ -66,12 +66,12 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  bb1: if (x$1 === 1) {
-    x$1 = 2;
+  let x$4 = 1;
+  bb1: if (x$4 === 1) {
+    x$4 = 2;
   }
 
-  throw x$1;
+  throw x$4;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-while-no-reassign.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-while-no-reassign.expect.md
index 31d2037c47..e20b65de8f 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-while-no-reassign.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-while-no-reassign.expect.md
@@ -23,7 +23,7 @@ bb1:
   predecessor blocks: bb0 bb3
   [3] Const mutate $6_@1 = 10
   [4] Const mutate $8_@2[4:6] = Binary read x$5_@0 < read $6_@1
-  [5] If (read $8_@2) then:bb3 else:bb2
+  [5] If (read $8_@2) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
   [6] Const mutate $9_@3 = 1
@@ -82,12 +82,12 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  bb2: while (x$1 < 10) {
-    x$1 + 1;
+  const x$5 = 1;
+  bb2: while (x$5 < 10) {
+    x$5 + 1;
   }
 
-  return x$1;
+  return x$5;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-while.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-while.expect.md
index 8bb036c3d3..7af54f1cce 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-while.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-while.expect.md
@@ -24,7 +24,7 @@ bb1:
   x$7_@0[0:9]: phi(bb0: x$5_@0, bb3: x$10_@0)
   [3] Const mutate $6_@1 = 10
   [4] Const mutate $8_@2[4:6] = Binary read x$7_@0 < read $6_@1
-  [5] If (read $8_@2) then:bb3 else:bb2
+  [5] If (read $8_@2) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
   [6] Const mutate $9_@3 = 1
@@ -80,12 +80,12 @@ flowchart TB
 
 ```javascript
 function foo$0() {
-  let x$1 = 1;
-  bb2: while (x$1 < 10) {
-    x$1 = x$1 + 1;
+  let x$5 = 1;
+  bb2: while (x$5 < 10) {
+    x$5 = x$5 + 1;
   }
 
-  return x$1;
+  return x$5;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/switch-non-final-default.expect.md b/compiler/forget/src/__tests__/fixtures/hir/switch-non-final-default.expect.md
index 454453f975..fdf868c6cc 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/switch-non-final-default.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/switch-non-final-default.expect.md
@@ -42,6 +42,7 @@ bb0:
     Case read $13_@2: bb6
     Default: bb1
     Case read $12_@1: bb2
+    Fallthrough: bb1
 bb6:
   predecessor blocks: bb0
   [7] Call mutate x$10_@0.push(read props$9.p2)
@@ -121,17 +122,17 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  let x$2 = [];
-  let y$3 = undefined;
-  bb1: switch (props$1.p0) {
+function Component$0(props$9) {
+  const x$10 = [];
+  let y$11 = undefined;
+  bb1: switch (props$9.p0) {
     case 1: {
       break bb1;
     }
 
     case true: {
-      x$2.push(props$1.p2);
-      y$3 = [];
+      x$10.push(props$9.p2);
+      y$11 = [];
       break bb1;
     }
 
@@ -140,13 +141,13 @@ function Component$0(props$1) {
     }
 
     case false: {
-      y$3 = x$2;
+      y$11 = x$10;
     }
   }
 
-  const child$7 = <Component$0 data={x$2}></Component$0>;
-  y$3.push(props$1.p4);
-  return <Component$0 data={y$3}>{child$7}</Component$0>;
+  const child$19 = <Component$0 data={x$10}></Component$0>;
+  y$11.push(props$9.p4);
+  return <Component$0 data={y$11}>{child$19}</Component$0>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/switch-with-fallthrough.expect.md b/compiler/forget/src/__tests__/fixtures/hir/switch-with-fallthrough.expect.md
index 8186d5c74a..267fbbf5dc 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/switch-with-fallthrough.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/switch-with-fallthrough.expect.md
@@ -51,6 +51,7 @@ bb0:
     Case read $12_@2: bb4
     Case read $11_@1: bb3
     Default: bb2
+    Fallthrough: bb1
 bb10:
   predecessor blocks: bb0
   [9] Reassign mutate y$17_@7 = 0
@@ -159,15 +160,15 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(x$1) {
-  let y$2 = undefined;
-  bb1: switch (x$1) {
+function foo$0(x$9) {
+  const y$10 = undefined;
+  bb1: switch (x$9) {
     case 0: {
-      y$2 = 0;
+      const y$17 = 0;
     }
 
     case 1: {
-      y$2 = 1;
+      const y$18 = 1;
       break bb1;
     }
 
@@ -176,20 +177,20 @@ function foo$0(x$1) {
     }
 
     case 3: {
-      y$2 = 3;
+      const y$19 = 3;
       break bb1;
     }
 
     case 4: {
-      y$2 = 4;
+      const y$20 = 4;
     }
 
     case 5: {
-      y$2 = 5;
+      const y$21 = 5;
     }
 
     default: {
-      y$2 = 0;
+      const y$22 = 0;
     }
   }
 }
diff --git a/compiler/forget/src/__tests__/fixtures/hir/switch.expect.md b/compiler/forget/src/__tests__/fixtures/hir/switch.expect.md
index 8c9bfc2497..18676026ea 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/switch.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/switch.expect.md
@@ -36,6 +36,7 @@ bb0:
     Case read $12_@2: bb4
     Case read $11_@1: bb2
     Default: bb1
+    Fallthrough: bb1
 bb4:
   predecessor blocks: bb0
   [6] Call mutate x$9_@0.push(read props$8.p2)
@@ -116,24 +117,24 @@ flowchart TB
 ## Code
 
 ```javascript
-function Component$0(props$1) {
-  let x$2 = [];
-  let y$3 = undefined;
-  bb1: switch (props$1.p0) {
+function Component$0(props$8) {
+  const x$9 = [];
+  let y$10 = undefined;
+  bb1: switch (props$8.p0) {
     case true: {
-      x$2.push(props$1.p2);
-      x$2.push(props$1.p3);
-      y$3 = [];
+      x$9.push(props$8.p2);
+      x$9.push(props$8.p3);
+      const y$13 = [];
     }
 
     case false: {
-      y$3 = x$2;
+      y$10 = x$9;
     }
   }
 
-  const child$6 = <Component$0 data={x$2}></Component$0>;
-  y$3.push(props$1.p4);
-  return <Component$0 data={y$3}>{child$6}</Component$0>;
+  const child$19 = <Component$0 data={x$9}></Component$0>;
+  y$10.push(props$8.p4);
+  return <Component$0 data={y$10}>{child$19}</Component$0>;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/while-break.expect.md b/compiler/forget/src/__tests__/fixtures/hir/while-break.expect.md
index 4de197229b..bfcbbd99d8 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/while-break.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/while-break.expect.md
@@ -18,7 +18,7 @@ bb0:
   [1] While test=bb1 loop=bb2 fallthrough=bb2
 bb1:
   predecessor blocks: bb0
-  [2] If (read a$3) then:bb2 else:bb2
+  [2] If (read a$3) then:bb2 else:bb2 fallthrough=bb2
 bb2:
   predecessor blocks: bb1
   [3] Return read b$4
@@ -52,11 +52,11 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2) {
-  bb2: while (a$1) {
+function foo$0(a$3, b$4) {
+  bb2: while (a$3) {
     break;
   }
-  return b$2;
+  return b$4;
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/fixtures/hir/while-conditional-continue.expect.md b/compiler/forget/src/__tests__/fixtures/hir/while-conditional-continue.expect.md
index 016ff2f8de..8beb1687f4 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/while-conditional-continue.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/while-conditional-continue.expect.md
@@ -22,10 +22,10 @@ bb0:
   [1] While test=bb1 loop=bb3 fallthrough=bb2
 bb1:
   predecessor blocks: bb0 bb5 bb4
-  [2] If (read a$5) then:bb3 else:bb2
+  [2] If (read a$5) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
-  [3] If (read b$6) then:bb5 else:bb4
+  [3] If (read b$6) then:bb5 else:bb4 fallthrough=bb4
 bb5:
   predecessor blocks: bb3
   [4] Goto(Continue) bb1
@@ -86,15 +86,15 @@ flowchart TB
 ## Code
 
 ```javascript
-function foo$0(a$1, b$2, c$3, d$4) {
-  bb2: while (a$1) {
-    bb4: if (b$2) {
+function foo$0(a$5, b$6, c$7, d$8) {
+  bb2: while (a$5) {
+    bb4: if (b$6) {
       continue;
     }
-    c$3();
+    c$7();
   }
 
-  d$4();
+  d$8();
 }
 
 ```
diff --git a/compiler/forget/src/__tests__/hir-test.ts b/compiler/forget/src/__tests__/hir-test.ts
index ec77ac67ae..74f49383ed 100644
--- a/compiler/forget/src/__tests__/hir-test.ts
+++ b/compiler/forget/src/__tests__/hir-test.ts
@@ -48,61 +48,58 @@ describe("React Forget (HIR version)", () => {
         }
       }
 
-      const ast = parser.parse(input, {
-        sourceFilename: file,
-        plugins: ["typescript", "jsx"],
-      });
-      let items: Array<[string, string, string]> = [];
-      traverse(ast, {
-        FunctionDeclaration: {
-          enter(nodePath) {
-            const { ir } = run(nodePath, {
-              eliminateRedundantPhi: true,
-              inferReferenceEffects: true,
-              inferMutableRanges: true,
-              inferReactiveScopeVariables: true,
-              inferReactiveScopes: true,
-              inferReactiveScopeDependencies: true,
-              leaveSSA: false,
-              codegen: false,
-            });
+      let items: Array<[string, string, string]> | null = null;
+      let error: Error | null = null;
+      try {
+        items = transform(input, file);
+      } catch (e) {
+        error = e;
+      }
+      let outputs: Array<string>;
 
-            // Print the HIR before leaving SSA.
-            const textHIR = printFunction(ir);
-            const visualization = visualizeHIRMermaid(ir);
+      const expectError = file.startsWith("error.");
+      if (expectError) {
+        if (error === null) {
+          throw new Error(
+            `Expected an error to be thrown for fixture: '${file}', remove the 'error.' prefix if an error is not expected.`
+          );
+        } else {
+          outputs = [formatErrorOutput(error)];
+        }
+      } else {
+        if (error !== null) {
+          console.error(error);
+          throw new Error(
+            `Expected fixture '${file}' to succeed but it failed with error: '${error.message}'. See console output for details.`
+          );
+        }
+        if (items === null || items.length === 0) {
+          throw new Error(`Expected at least one output for file '${file}'.`);
+        }
+        outputs = formatOutput(items);
+      }
+      return `
+## Input
 
-            const { ast } = run(nodePath, {
-              eliminateRedundantPhi: true,
-              inferReferenceEffects: true,
-              inferMutableRanges: true,
-              inferReactiveScopeVariables: true,
-              inferReactiveScopes: true,
-              inferReactiveScopeDependencies: true,
-              leaveSSA: true,
-              codegen: true,
-            });
+${wrapWithTripleBackticks(input, "javascript")}
 
-            invariant(
-              ast !== null,
-              "ast is null when codegen option is enabled"
-            );
-            const text = prettier.format(
-              generate(ast).code.replace("\n\n", "\n"),
-              {
-                semi: true,
-                parser: "babel-ts",
-              }
-            );
-            items.push([textHIR, text, visualization]);
-          },
-        },
-      });
-      invariant(
-        items.length > 0,
-        "Visitor failed, check that the input has a function"
-      );
-      const outputs = items.map(([hir, text, visualization]) => {
-        return `
+${outputs.join("\n")}
+      `;
+    }
+  );
+});
+
+function formatErrorOutput(error: Error): string {
+  return `
+## Error
+
+${wrapWithTripleBackticks(error.message)}
+          `;
+}
+
+function formatOutput(items: Array<[string, string, string]>): Array<string> {
+  return items.map(([hir, text, visualization]) => {
+    return `
 ## HIR
 
 ${wrapWithTripleBackticks(hir)}
@@ -115,14 +112,55 @@ ${wrapWithTripleBackticks(visualization, "mermaid")}
 
 ${wrapWithTripleBackticks(text, "javascript")}
         `.trim();
-      });
-      return `
-## Input
+  });
+}
 
-${wrapWithTripleBackticks(input, "javascript")}
+function transform(
+  text: string,
+  file: string
+): Array<[string, string, string]> {
+  const items: Array<[string, string, string]> = [];
+  const ast = parser.parse(text, {
+    sourceFilename: file,
+    plugins: ["typescript", "jsx"],
+  });
+  traverse(ast, {
+    FunctionDeclaration: {
+      enter(nodePath) {
+        const { ir } = run(nodePath, {
+          eliminateRedundantPhi: true,
+          inferReferenceEffects: true,
+          inferMutableRanges: true,
+          inferReactiveScopeVariables: true,
+          inferReactiveScopes: true,
+          inferReactiveScopeDependencies: true,
+          leaveSSA: false,
+          codegen: false,
+        });
 
-${outputs.join("\n")}
-      `;
-    }
-  );
-});
+        // Print the HIR before leaving SSA.
+        const textHIR = printFunction(ir);
+        const visualization = visualizeHIRMermaid(ir);
+
+        const { ast } = run(nodePath, {
+          eliminateRedundantPhi: true,
+          inferReferenceEffects: true,
+          inferMutableRanges: true,
+          inferReactiveScopeVariables: true,
+          inferReactiveScopes: true,
+          inferReactiveScopeDependencies: true,
+          leaveSSA: true,
+          codegen: true,
+        });
+
+        invariant(ast !== null, "ast is null when codegen option is enabled");
+        const text = prettier.format(generate(ast).code.replace("\n\n", "\n"), {
+          semi: true,
+          parser: "babel-ts",
+        });
+        items.push([textHIR, text, visualization]);
+      },
+    },
+  });
+  return items;
+}
diff --git a/compiler/forget/src/index.ts b/compiler/forget/src/index.ts
index f143316c21..83cc7f9512 100644
--- a/compiler/forget/src/index.ts
+++ b/compiler/forget/src/index.ts
@@ -34,7 +34,7 @@ import { inferReactiveScopeDependencies } from "./HIR/InferReactiveScopeDependen
 import { inferReactiveScopes } from "./HIR/InferReactiveScopes";
 import { inferReactiveScopeVariables } from "./HIR/InferReactiveScopeVariables";
 import inferReferenceEffects from "./HIR/InferReferenceEffects";
-import leaveSSA from "./HIR/LeaveSSA";
+import { leaveSSA } from "./HIR/LeaveSSA";
 import printHIR from "./HIR/PrintHIR";
 
 function parseFunctions(