From d03c5bc0a0a3ffa16f6c9cf4834bbe2212c3d11c Mon Sep 17 00:00:00 2001
From: Joseph Savona <josephsavona@users.noreply.github.com>
Date: Fri, 9 Dec 2022 07:39:28 -0800
Subject: [PATCH] Run LeaveSSA prior to analyzing reactive scopes
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Reorders LeaveSSA so that it runs before we begin evaluating reactive scopes.
Note that reactive scopes must span the full construction of each variable — for
variables with a phi, this must span the declaration and all assignments of the
phi operands. And that's exactly what the new LeaveSSA does! LeaveSSA removes
phi nodes and ensure that all versions of a variable which flow into a phi have
been assigned a single canonical identifier (with an appropriate mutable range).

This PR includes this and some related changes:

* Reorders the pass

* Changes hir-test to print the final HIR, eg just prior to codegen

* Teaches LeaveSSA to update the mutable range of the canonical identifiers it
assigns, based on the min/max of the variables assigned.
---
 .../playground/components/Editor/Output.tsx   | 16 +--
 .../src/HIR/InferReactiveScopeVariables.ts    |  9 +-
 compiler/forget/src/HIR/LeaveSSA.ts           | 50 ++++++++--
 compiler/forget/src/HIR/Pipeline.ts           |  6 +-
 .../_bug_expression-with-assignment.expect.md |  8 +-
 .../hir/_bug_inverted-if-else.expect.md       | 21 ++--
 .../fixtures/hir/alias-while.expect.md        | 39 ++++----
 .../hir/assignment-variations.expect.md       | 16 +--
 .../src/__tests__/fixtures/hir/call.expect.md |  4 +-
 .../fixtures/hir/component.expect.md          | 11 ++-
 .../fixtures/hir/constructor.expect.md        |  4 +-
 .../fixtures/hir/extend-scopes-if.expect.md   |  4 +-
 ...reeze-possibly-mutable-arguments.expect.md | 34 ++++---
 .../fixtures/hir/inverted-if.expect.md        |  4 +-
 .../fixtures/hir/logical-expression.expect.md | 45 +++++----
 .../hir/mutable-lifetime-loops.expect.md      | 72 ++++++--------
 .../hir/mutable-liverange-loop.expect.md      | 16 +--
 .../overlapping-scopes-interleaved.expect.md  |  8 +-
 .../hir/overlapping-scopes-shadowed.expect.md |  8 +-
 ...ng-scopes-shadowing-within-block.expect.md |  8 +-
 .../hir/overlapping-scopes-while.expect.md    |  8 +-
 .../overlapping-scopes-within-block.expect.md |  8 +-
 .../hir/reactive-scope-grouping.expect.md     | 12 +--
 .../fixtures/hir/reactive-scopes.expect.md    |  4 +-
 .../hir/reassignment-conditional.expect.md    | 19 ++--
 .../reassignment-separate-scopes.expect.md    | 30 +++---
 .../fixtures/hir/reassignment.expect.md       | 16 +--
 .../fixtures/hir/reverse-postorder.expect.md  | 59 +++++------
 .../fixtures/hir/simple-alias.expect.md       | 24 ++---
 .../fixtures/hir/ssa-call-jsx-2.expect.md     |  4 +-
 .../fixtures/hir/ssa-call-jsx.expect.md       |  4 +-
 .../hir/ssa-complex-multiple-if.expect.md     | 24 ++---
 .../hir/ssa-complex-single-if.expect.md       | 13 ++-
 .../fixtures/hir/ssa-for-of.expect.md         | 12 +--
 .../__tests__/fixtures/hir/ssa-for.expect.md  | 13 ++-
 .../fixtures/hir/ssa-if-else.expect.md        | 16 +--
 .../fixtures/hir/ssa-multiple-phis.expect.md  | 49 +++++----
 .../ssa-nested-loops-no-reassign.expect.md    |  4 +-
 .../hir/ssa-nested-partial-phi.expect.md      | 99 +++++++++++++++++++
 .../fixtures/hir/ssa-nested-partial-phi.js    |  9 ++
 .../ssa-nested-partial-reassignment.expect.md | 13 ++-
 .../fixtures/hir/ssa-newexpression.expect.md  |  4 +-
 .../hir/ssa-objectexpression-phi.expect.md    | 31 +++---
 ...a-property-alias-alias-mutate-if.expect.md |  8 +-
 .../hir/ssa-property-alias-if.expect.md       |  8 +-
 .../ssa-property-alias-mutate-if.expect.md    |  8 +-
 ...-property-alias-mutate-inside-if.expect.md |  8 +-
 .../fixtures/hir/ssa-reassign.expect.md       | 16 +--
 .../fixtures/hir/ssa-return.expect.md         | 20 ++--
 .../fixtures/hir/ssa-shadowing.expect.md      | 13 ++-
 .../fixtures/hir/ssa-sibling-phis.expect.md   | 40 ++++----
 .../fixtures/hir/ssa-simple-phi.expect.md     | 21 ++--
 .../fixtures/hir/ssa-simple.expect.md         |  8 +-
 .../fixtures/hir/ssa-single-if.expect.md      | 12 +--
 .../fixtures/hir/ssa-switch.expect.md         | 32 +++---
 .../fixtures/hir/ssa-throw.expect.md          | 20 ++--
 .../hir/ssa-while-no-reassign.expect.md       |  4 +-
 .../fixtures/hir/ssa-while.expect.md          | 13 ++-
 .../hir/switch-non-final-default.expect.md    | 23 ++---
 .../hir/switch-with-fallthrough.expect.md     | 28 +++---
 .../__tests__/fixtures/hir/switch.expect.md   | 23 ++---
 compiler/forget/src/__tests__/hir-test.ts     | 20 +---
 62 files changed, 658 insertions(+), 525 deletions(-)
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-phi.expect.md
 create mode 100644 compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-phi.js

diff --git a/compiler/forget/packages/playground/components/Editor/Output.tsx b/compiler/forget/packages/playground/components/Editor/Output.tsx
index 42d7b622b6..0c9e7536d2 100644
--- a/compiler/forget/packages/playground/components/Editor/Output.tsx
+++ b/compiler/forget/packages/playground/components/Editor/Output.tsx
@@ -87,6 +87,9 @@ function compile(source: string): CompilerOutput | CompilerError {
     inferMutableRanges(ir);
     const inferMutableRangesOutput = printHIR(ir.body);
 
+    leaveSSA(ir);
+    const leaveSSAOutput = printHIR(ir.body);
+
     inferReactiveScopeVariables(ir);
     const inferReactiveScopeVariablesOutput = printHIR(ir.body);
 
@@ -96,9 +99,6 @@ function compile(source: string): CompilerOutput | CompilerError {
     inferReactiveScopeDependencies(ir);
     const inferReactiveScopeDependenciesOutput = printHIR(ir.body);
 
-    leaveSSA(ir);
-    const leaveSSAOutput = printHIR(ir.body);
-
     codegen(ir);
     const ast = codegen(ir);
     const generated = generate(
@@ -183,6 +183,11 @@ function Output({ store, setTabsOpen, tabsOpen }: Props) {
             output={compilerOutput.inferMutableRangesOutput}
           ></TextTabContent>
         ),
+        LeaveSSA: (
+          <TextTabContent
+            output={compilerOutput.leaveSSAOutput}
+          ></TextTabContent>
+        ),
         InferReactiveScopeVariables: (
           <TextTabContent
             output={compilerOutput.inferReactiveScopeVariablesOutput}
@@ -198,11 +203,6 @@ function Output({ store, setTabsOpen, tabsOpen }: Props) {
             output={compilerOutput.inferReactiveScopeDependenciesOutput}
           ></TextTabContent>
         ),
-        LeaveSSA: (
-          <TextTabContent
-            output={compilerOutput.leaveSSAOutput}
-          ></TextTabContent>
-        ),
         JS: <TextTabContent output={compilerOutput.codegenOutput} />,
         SourceMap: (
           <>
diff --git a/compiler/forget/src/HIR/InferReactiveScopeVariables.ts b/compiler/forget/src/HIR/InferReactiveScopeVariables.ts
index 512e3fa9e0..ddb5ff1917 100644
--- a/compiler/forget/src/HIR/InferReactiveScopeVariables.ts
+++ b/compiler/forget/src/HIR/InferReactiveScopeVariables.ts
@@ -5,6 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+import invariant from "invariant";
 import DisjointSet from "./DisjointSet";
 import {
   HIRFunction,
@@ -68,10 +69,10 @@ export function inferReactiveScopeVariables(fn: HIRFunction) {
   // that mutate together.
   const scopeIdentifiers = new DisjointSet<Identifier>();
   for (const [_, block] of fn.body.blocks) {
-    for (const phi of block.phis) {
-      const operands: Array<Identifier> = [phi.id, ...phi.operands.values()];
-      scopeIdentifiers.union(operands);
-    }
+    invariant(
+      block.phis.size === 0,
+      "Expected phis to be cleared by LeaveSSA pass"
+    );
 
     for (const instr of block.instructions) {
       const operands: Array<Identifier> = [];
diff --git a/compiler/forget/src/HIR/LeaveSSA.ts b/compiler/forget/src/HIR/LeaveSSA.ts
index 19f62f21bf..2ac8da4984 100644
--- a/compiler/forget/src/HIR/LeaveSSA.ts
+++ b/compiler/forget/src/HIR/LeaveSSA.ts
@@ -11,8 +11,11 @@ import {
   HIRFunction,
   Identifier,
   Instruction,
+  InstructionId,
   InstructionKind,
+  makeInstructionId,
   Phi,
+  Place,
 } from "./HIR";
 import { eachInstructionValueOperand, eachTerminalOperand } from "./visitors";
 
@@ -28,6 +31,7 @@ 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();
+  const hasDeclaration: Set<Identifier> = new Set();
 
   for (const [, block] of fn.body.blocks) {
     // Identifiers (from phis) that *may* need a new `let` declaration created. If the original
@@ -78,13 +82,28 @@ export function leaveSSA(fn: HIRFunction) {
             canonicalId = operand;
           }
         }
+        canonicalId.mutableRange.start = Math.min(
+          canonicalId.mutableRange.start,
+          terminal.id
+        ) as InstructionId;
         variableMapping.set(phi.id, canonicalId);
-        needsDeclaration.add(canonicalId);
+        if (!hasDeclaration.has(canonicalId)) {
+          needsDeclaration.add(canonicalId);
+        }
       }
+
       // all versions of the variable need to be remapped to the canonical id
+      // also extend the mutable range of the canonical id based on the min/max
+      // of the ranges of its operands
+      let start = canonicalId.mutableRange.start as number;
+      let end = canonicalId.mutableRange.end as number;
       for (const [, operand] of phi.operands) {
+        start = Math.min(start, operand.mutableRange.start);
+        end = Math.max(end, operand.mutableRange.end);
         variableMapping.set(operand, canonicalId);
       }
+      canonicalId.mutableRange.start = makeInstructionId(start);
+      canonicalId.mutableRange.end = makeInstructionId(end);
     }
 
     // Visit instructions and rewrite identifiers based on the variable mapping
@@ -92,9 +111,7 @@ export function leaveSSA(fn: HIRFunction) {
     for (const instr of block.instructions) {
       const { lvalue, value } = instr;
       if (lvalue !== null) {
-        lvalue.place.identifier =
-          variableMapping.get(lvalue.place.identifier) ??
-          lvalue.place.identifier;
+        updatePlace(lvalue.place, variableMapping);
         if (lvalue.place.memberPath === null) {
           if (!variableMapping.has(lvalue.place.identifier)) {
             // This variable does not flow into a phi, therefore there
@@ -109,11 +126,11 @@ export function leaveSSA(fn: HIRFunction) {
             // reassign the variable in the first place.
             needsDeclaration.delete(lvalue.place.identifier);
           }
+          hasDeclaration.add(lvalue.place.identifier);
         }
       }
       for (const operand of eachInstructionValueOperand(value)) {
-        operand.identifier =
-          variableMapping.get(operand.identifier) ?? operand.identifier;
+        updatePlace(operand, variableMapping);
       }
     }
 
@@ -151,3 +168,24 @@ export function leaveSSA(fn: HIRFunction) {
     block.phis.clear();
   }
 }
+
+function updatePlace(
+  place: Place,
+  variableMapping: Map<Identifier, Identifier>
+) {
+  const prevIdentifier = place.identifier;
+  const nextIdentifier = variableMapping.get(prevIdentifier);
+  if (nextIdentifier === undefined || nextIdentifier === prevIdentifier) {
+    return;
+  }
+  nextIdentifier.mutableRange.start = makeInstructionId(
+    Math.min(
+      nextIdentifier.mutableRange.start,
+      prevIdentifier.mutableRange.start
+    )
+  );
+  nextIdentifier.mutableRange.end = makeInstructionId(
+    Math.max(nextIdentifier.mutableRange.end, prevIdentifier.mutableRange.end)
+  );
+  place.identifier = nextIdentifier;
+}
diff --git a/compiler/forget/src/HIR/Pipeline.ts b/compiler/forget/src/HIR/Pipeline.ts
index 2663a1e4f4..c447c36e8e 100644
--- a/compiler/forget/src/HIR/Pipeline.ts
+++ b/compiler/forget/src/HIR/Pipeline.ts
@@ -51,6 +51,9 @@ export default function (
   if (flags.inferMutableRanges) {
     inferMutableRanges(ir);
   }
+  if (flags.leaveSSA) {
+    leaveSSA(ir);
+  }
   if (flags.inferReactiveScopeVariables) {
     inferReactiveScopeVariables(ir);
   }
@@ -60,9 +63,6 @@ export default function (
   if (flags.inferReactiveScopeDependencies) {
     inferReactiveScopeDependencies(ir);
   }
-  if (flags.leaveSSA) {
-    leaveSSA(ir);
-  }
   if (flags.codegen) {
     return {
       ast: codegen(ir),
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 a703b6e9bf..6968aebf9f 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
@@ -15,8 +15,8 @@ function f() {
 
 ```
 bb0:
-  [1] Let mutate x$4_@0 = 1
-  [2] Reassign mutate x$5_@1 = 2
+  [1] Const mutate x$4_@0 = 1
+  [2] Const mutate x$5_@1 = 2
   [3] Const mutate $6_@2 = Binary read x$5_@1 + read x$5_@1
   [4] Const mutate $7_@3 = Binary read $6_@2 + read x$5_@1
   [5] Return read $7_@3
@@ -35,8 +35,8 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$4_@0 = 1
-      [2] Reassign mutate x$5_@1 = 2
+      [1] Const mutate x$4_@0 = 1
+      [2] Const mutate x$5_@1 = 2
       [3] Const mutate $6_@2 = Binary read x$5_@1 + read x$5_@1
       [4] Const mutate $7_@3 = Binary read $6_@2 + read x$5_@1
     "]
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
index 89989d051e..9a6334a713 100644
--- 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
@@ -20,23 +20,22 @@ function foo(a, b, c) {
 
 ```
 bb0:
-  [1] Let mutate x$8_@0 = null
+  [1] Const 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
+  [3] Const mutate x$9_@1 = read b$6
   [4] Goto bb1
 bb2:
   predecessor blocks: bb0
-  [5] Reassign mutate x$10_@1[3:6] = read c$7
+  [5] Const mutate x$10_@2 = 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]:
+  [7] Return read x$11
+scope1 [3:4]:
  - read b$6
- - read a$5
+scope2 [5:6]:
  - read c$7
 ```
 
@@ -47,24 +46,24 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$8_@0 = null
+      [1] Const 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
+      [3] Const mutate x$9_@1 = read b$6
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb2
     bb2_instrs["
-      [5] Reassign mutate x$10_@1[3:6] = read c$7
+      [5] Const mutate x$10_@2 = read c$7
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb1
-    bb1_terminal(["Return read x$11_@1"])
+    bb1_terminal(["Return read x$11"])
   end
 
   %% Jumps
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 b2cb6cb033..70ddeda17b 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/alias-while.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/alias-while.expect.md
@@ -33,24 +33,21 @@ bb0:
   [4] While test=bb1 loop=bb3 fallthrough=bb2
 bb1:
   predecessor blocks: bb0 bb3
-  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 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
-  [6] Let mutate z$13_@0[0:12] = read a$12_@0
-  [7] Reassign mutate a$15_@0[0:12] = read b$14_@0
-  [8] Reassign mutate b$17_@0[0:12] = read c$16_@0
-  [9] Reassign mutate c$18_@0[0:12] = read z$13_@0
-  [10] Call mutate mutate$6_@0(mutate a$15_@0, mutate b$17_@0)
+  [6] Const mutate z$13_@0[0:12] = read a$8_@0
+  [7] Reassign mutate a$8_@0[0:12] = read b$9_@0
+  [8] Reassign mutate b$9_@0[0:12] = read c$10_@0
+  [9] Reassign mutate c$10_@0[0:12] = read z$13_@0
+  [10] Call mutate mutate$6_@0(mutate a$8_@0, mutate b$9_@0)
   [11] Goto(Continue) bb1
 bb2:
   predecessor blocks: bb1
-  [12] read a$12_@0
-  [13] read b$14_@0
-  [14] read c$16_@0
-  [15] Return freeze a$12_@0
+  [12] read a$8_@0
+  [13] read b$9_@0
+  [14] read c$10_@0
+  [15] Return freeze a$8_@0
 
 ```
 
@@ -72,21 +69,21 @@ flowchart TB
   end
   subgraph bb3
     bb3_instrs["
-      [6] Let mutate z$13_@0[0:12] = read a$12_@0
-      [7] Reassign mutate a$15_@0[0:12] = read b$14_@0
-      [8] Reassign mutate b$17_@0[0:12] = read c$16_@0
-      [9] Reassign mutate c$18_@0[0:12] = read z$13_@0
-      [10] Call mutate mutate$6_@0(mutate a$15_@0, mutate b$17_@0)
+      [6] Const mutate z$13_@0[0:12] = read a$8_@0
+      [7] Reassign mutate a$8_@0[0:12] = read b$9_@0
+      [8] Reassign mutate b$9_@0[0:12] = read c$10_@0
+      [9] Reassign mutate c$10_@0[0:12] = read z$13_@0
+      [10] Call mutate mutate$6_@0(mutate a$8_@0, mutate b$9_@0)
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb2
     bb2_instrs["
-      [12] read a$12_@0
-      [13] read b$14_@0
-      [14] read c$16_@0
+      [12] read a$8_@0
+      [13] read b$9_@0
+      [14] read c$10_@0
     "]
-    bb2_instrs --> bb2_terminal(["Return freeze a$12_@0"])
+    bb2_instrs --> bb2_terminal(["Return freeze a$8_@0"])
   end
 
   %% Jumps
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 c4208f4695..7dfa8c02dd 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/assignment-variations.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/assignment-variations.expect.md
@@ -20,13 +20,13 @@ function g(a) {
 
 ```
 bb0:
-  [1] Let mutate x$5_@0 = 1
+  [1] Const mutate x$5_@0 = 1
   [2] Const mutate $6_@1 = 1
-  [3] Reassign mutate x$7_@2 = Binary read x$5_@0 + read $6_@1
+  [3] Const mutate x$7_@2 = Binary read x$5_@0 + read $6_@1
   [4] Const mutate $8_@3 = 1
-  [5] Reassign mutate x$9_@4 = Binary read x$7_@2 + read $8_@3
+  [5] Const mutate x$9_@4 = Binary read x$7_@2 + read $8_@3
   [6] Const mutate $10_@5 = 1
-  [7] Reassign mutate x$11_@6 = Binary read x$9_@4 >>> read $10_@5
+  [7] Const mutate x$11_@6 = Binary read x$9_@4 >>> read $10_@5
   [8] Return
 scope2 [3:4]:
  - read x$5_@0
@@ -46,13 +46,13 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$5_@0 = 1
+      [1] Const mutate x$5_@0 = 1
       [2] Const mutate $6_@1 = 1
-      [3] Reassign mutate x$7_@2 = Binary read x$5_@0 + read $6_@1
+      [3] Const mutate x$7_@2 = Binary read x$5_@0 + read $6_@1
       [4] Const mutate $8_@3 = 1
-      [5] Reassign mutate x$9_@4 = Binary read x$7_@2 + read $8_@3
+      [5] Const mutate x$9_@4 = Binary read x$7_@2 + read $8_@3
       [6] Const mutate $10_@5 = 1
-      [7] Reassign mutate x$11_@6 = Binary read x$9_@4 >>> read $10_@5
+      [7] Const mutate x$11_@6 = Binary read x$9_@4 >>> read $10_@5
     "]
     bb0_instrs --> bb0_terminal(["Return"])
   end
diff --git a/compiler/forget/src/__tests__/fixtures/hir/call.expect.md b/compiler/forget/src/__tests__/fixtures/hir/call.expect.md
index e7eb75ae26..aa50a5f7a8 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/call.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/call.expect.md
@@ -50,7 +50,7 @@ bb0:
   [2] Const mutate b$11_@0[0:7] = Object {  }
   [3] Call mutate foo$4_@0(mutate a$10_@0, mutate b$11_@0)
   [4] Const mutate $12_@1 = "div"
-  [5] Let mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
+  [5] Const mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
   [6] Call mutate foo$4_@0(mutate b$11_@0)
   [7] Const mutate $14_@3 = "div"
   [8] Const mutate $15_@4 = JSX <read $14_@3 a={read a$10_@0} b={freeze b$11_@0} ></read $14_@3>
@@ -74,7 +74,7 @@ flowchart TB
       [2] Const mutate b$11_@0[0:7] = Object {  }
       [3] Call mutate foo$4_@0(mutate a$10_@0, mutate b$11_@0)
       [4] Const mutate $12_@1 = 'div'
-      [5] Let mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
+      [5] Const mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
       [6] Call mutate foo$4_@0(mutate b$11_@0)
       [7] Const mutate $14_@3 = 'div'
       [8] Const mutate $15_@4 = JSX <read $14_@3 a={read a$10_@0} b={freeze b$11_@0} ></read $14_@3>
diff --git a/compiler/forget/src/__tests__/fixtures/hir/component.expect.md b/compiler/forget/src/__tests__/fixtures/hir/component.expect.md
index e959a98117..f7c39ff2fe 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/component.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/component.expect.md
@@ -49,6 +49,7 @@ 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] Let mutate $37_@3[0:19] = undefined
   [11] If (read $36_@3) then:bb8 else:bb9 fallthrough=bb7
 bb8:
   predecessor blocks: bb3
@@ -56,12 +57,11 @@ bb8:
   [13] Goto bb7
 bb9:
   predecessor blocks: bb3
-  [14] Const mutate $39_@3[0:19] = Call mutate seen$30_@3.has(mutate item$10_@3)
+  [14] Const mutate $37_@3[0:19] = Call mutate seen$30_@3.has(mutate item$10_@3)
   [15] Goto bb7
 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 fallthrough=bb4
+  [16] If (read $37_@3) then:bb5 else:bb4 fallthrough=bb4
 bb5:
   predecessor blocks: bb7
   [17] Goto(Continue) bb1
@@ -117,6 +117,7 @@ flowchart TB
     bb3_instrs["
       [9] Const mutate $34_@6 = null
       [10] Const mutate $36_@3[0:19] = Binary read item$10_@3 == read $34_@6
+      [11] Let mutate $37_@3[0:19] = undefined
     "]
     bb3_instrs --> bb3_terminal(["If (read $36_@3)"])
   end
@@ -128,12 +129,12 @@ flowchart TB
   end
   subgraph bb9
     bb9_instrs["
-      [14] Const mutate $39_@3[0:19] = Call mutate seen$30_@3.has(mutate item$10_@3)
+      [14] Const mutate $37_@3[0:19] = Call mutate seen$30_@3.has(mutate item$10_@3)
     "]
     bb9_instrs --> bb9_terminal(["Goto"])
   end
   subgraph bb7
-    bb7_terminal(["If (read $40_@3)"])
+    bb7_terminal(["If (read $37_@3)"])
   end
   subgraph bb5
     bb5_terminal(["Goto"])
diff --git a/compiler/forget/src/__tests__/fixtures/hir/constructor.expect.md b/compiler/forget/src/__tests__/fixtures/hir/constructor.expect.md
index 46b5471b1d..06aee4b185 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/constructor.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/constructor.expect.md
@@ -50,7 +50,7 @@ bb0:
   [2] Const mutate b$11_@0[0:7] = Object {  }
   [3] New mutate Foo$4_@0(mutate a$10_@0, mutate b$11_@0)
   [4] Const mutate $12_@1 = "div"
-  [5] Let mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
+  [5] Const mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
   [6] New mutate Foo$4_@0(mutate b$11_@0)
   [7] Const mutate $14_@3 = "div"
   [8] Const mutate $15_@4 = JSX <read $14_@3 a={read a$10_@0} b={freeze b$11_@0} ></read $14_@3>
@@ -74,7 +74,7 @@ flowchart TB
       [2] Const mutate b$11_@0[0:7] = Object {  }
       [3] New mutate Foo$4_@0(mutate a$10_@0, mutate b$11_@0)
       [4] Const mutate $12_@1 = 'div'
-      [5] Let mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
+      [5] Const mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
       [6] New mutate Foo$4_@0(mutate b$11_@0)
       [7] Const mutate $14_@3 = 'div'
       [8] Const mutate $15_@4 = JSX <read $14_@3 a={read a$10_@0} b={freeze b$11_@0} ></read $14_@3>
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 3e3438bab5..81d880bcdc 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
@@ -23,7 +23,7 @@ function foo(a, b, c) {
 
 ```
 bb0:
-  [1] Let mutate x$10_@0[1:8] = Array []
+  [1] Const mutate x$10_@0[1:8] = Array []
   [2] If (read a$7) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
@@ -59,7 +59,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$10_@0[1:8] = Array []
+      [1] Const mutate x$10_@0[1:8] = Array []
     "]
     bb0_instrs --> bb0_terminal(["If (read a$7)"])
   end
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 c662505eeb..9cd53a4082 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
@@ -28,29 +28,30 @@ function call(x) {}
 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
+  [3] Const mutate a$10_@2 = undefined
+  [4] Let mutate a$11_@3[4:9] = undefined
   [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
+  [5] Reassign mutate a$11_@3[4:9] = read x$9_@1
   [6] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [7] Reassign mutate a$12_@3[5:8] = Array []
+  [7] Reassign mutate a$11_@3[4:9] = Array []
   [8] Goto bb1
 bb1:
   predecessor blocks: bb2 bb3
-  a$14_@3[5:8]: phi(bb2: a$11_@3, bb3: a$12_@3)
-  [9] Call read useFreeze$5(freeze a$14_@3)
-  [10] Call read useFreeze$5(read a$14_@3)
-  [11] Call mutate call$6_@4(read a$14_@3)
-  [12] Return read a$14_@3
+  [9] Call read useFreeze$5(freeze a$11_@3)
+  [10] Call read useFreeze$5(read a$11_@3)
+  [11] Call mutate call$6_@4(read a$11_@3)
+  [12] Return read a$11_@3
 scope0 [1:2]:
  - read props$7.cond
 scope1 [2:3]:
  - read props$7.x
-scope3 [5:8]:
+scope3 [4:9]:
  - read x$9_@1
+ - read cond$8_@0
 ```
 
 ### CFG
@@ -62,29 +63,30 @@ flowchart TB
     bb0_instrs["
       [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
+      [3] Const mutate a$10_@2 = undefined
+      [4] Let mutate a$11_@3[4:9] = undefined
     "]
     bb0_instrs --> bb0_terminal(["If (read cond$8_@0)"])
   end
   subgraph bb2
     bb2_instrs["
-      [5] Reassign mutate a$11_@3[5:8] = read x$9_@1
+      [5] Reassign mutate a$11_@3[4:9] = read x$9_@1
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [7] Reassign mutate a$12_@3[5:8] = Array []
+      [7] Reassign mutate a$11_@3[4:9] = Array []
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
-      [9] Call read useFreeze$5(freeze a$14_@3)
-      [10] Call read useFreeze$5(read a$14_@3)
-      [11] Call mutate call$6_@4(read a$14_@3)
+      [9] Call read useFreeze$5(freeze a$11_@3)
+      [10] Call read useFreeze$5(read a$11_@3)
+      [11] Call mutate call$6_@4(read a$11_@3)
     "]
-    bb1_instrs --> bb1_terminal(["Return read a$14_@3"])
+    bb1_instrs --> bb1_terminal(["Return read a$11_@3"])
   end
 
   %% Jumps
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 da7e9ce49f..3d5661bc78 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/inverted-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/inverted-if.expect.md
@@ -18,7 +18,7 @@ function foo(a, b, c) {
 
 ```
 bb0:
-  [1] Let mutate y$8_@0[1:6] = Array []
+  [1] Const mutate y$8_@0[1:6] = Array []
   [2] If (read a$5) then:bb3 else:bb1 fallthrough=bb1
 bb3:
   predecessor blocks: bb0
@@ -43,7 +43,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate y$8_@0[1:6] = Array []
+      [1] Const mutate y$8_@0[1:6] = Array []
     "]
     bb0_instrs --> bb0_terminal(["If (read a$5)"])
   end
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 c20f685f1f..75d1bfb61b 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/logical-expression.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/logical-expression.expect.md
@@ -24,19 +24,19 @@ function g() {}
 ```
 bb0:
   [1] Const mutate $5_@0[0:2] = Call mutate f$1_@0()
+  [2] Let mutate $6_@1[0:7] = undefined
   [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()
+  [3] Const mutate $6_@1[0:7] = Call mutate g$4_@1()
   [4] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [5] Const mutate $7_@1[0:6] = read $5_@0
+  [5] Const mutate $6_@1[0:7] = read $5_@0
   [6] Goto bb1
 bb1:
   predecessor blocks: bb2 bb3
-  $8_@1[0:6]: phi(bb2: $6_@1, bb3: $7_@1)
-  [7] Return freeze $8_@1
+  [7] Return freeze $6_@1
 
 ```
 
@@ -48,23 +48,24 @@ flowchart TB
   subgraph bb0
     bb0_instrs["
       [1] Const mutate $5_@0[0:2] = Call mutate f$1_@0()
+      [2] Let mutate $6_@1[0:7] = undefined
     "]
     bb0_instrs --> bb0_terminal(["If (read $5_@0)"])
   end
   subgraph bb2
     bb2_instrs["
-      [3] Const mutate $6_@1[0:6] = Call mutate g$4_@1()
+      [3] Const mutate $6_@1[0:7] = Call mutate g$4_@1()
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [5] Const mutate $7_@1[0:6] = read $5_@0
+      [5] Const mutate $6_@1[0:7] = read $5_@0
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb1
-    bb1_terminal(["Return freeze $8_@1"])
+    bb1_terminal(["Return freeze $6_@1"])
   end
 
   %% Jumps
@@ -92,19 +93,19 @@ function And$0() {
 ```
 bb0:
   [1] Const mutate $5_@0[0:2] = Call mutate f$1_@0()
+  [2] Let mutate $6_@1[0:7] = undefined
   [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
+  [3] Const mutate $6_@1[0:7] = read $5_@0
   [4] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [5] Const mutate $7_@1[0:6] = Call mutate g$4_@1()
+  [5] Const mutate $6_@1[0:7] = Call mutate g$4_@1()
   [6] Goto bb1
 bb1:
   predecessor blocks: bb2 bb3
-  $8_@1[0:6]: phi(bb2: $6_@1, bb3: $7_@1)
-  [7] Return freeze $8_@1
+  [7] Return freeze $6_@1
 
 ```
 
@@ -116,23 +117,24 @@ flowchart TB
   subgraph bb0
     bb0_instrs["
       [1] Const mutate $5_@0[0:2] = Call mutate f$1_@0()
+      [2] Let mutate $6_@1[0:7] = undefined
     "]
     bb0_instrs --> bb0_terminal(["If (read $5_@0)"])
   end
   subgraph bb2
     bb2_instrs["
-      [3] Const mutate $6_@1[0:6] = read $5_@0
+      [3] Const mutate $6_@1[0:7] = read $5_@0
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [5] Const mutate $7_@1[0:6] = Call mutate g$4_@1()
+      [5] Const mutate $6_@1[0:7] = Call mutate g$4_@1()
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb1
-    bb1_terminal(["Return freeze $8_@1"])
+    bb1_terminal(["Return freeze $6_@1"])
   end
 
   %% Jumps
@@ -162,19 +164,19 @@ 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] Let mutate $12_@3[0:9] = undefined
   [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
+  [5] Const mutate $12_@3[0:9] = read $9_@0
   [6] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [7] Const mutate $13_@3[0:8] = Call mutate g$7_@3()
+  [7] Const mutate $12_@3[0:9] = Call mutate g$7_@3()
   [8] Goto bb1
 bb1:
   predecessor blocks: bb2 bb3
-  $14_@3[0:8]: phi(bb2: $12_@3, bb3: $13_@3)
-  [9] Return freeze $14_@3
+  [9] Return freeze $12_@3
 scope2 [3:4]:
  - read $9_@0
  - read $10_@1
@@ -190,23 +192,24 @@ flowchart TB
       [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] Let mutate $12_@3[0:9] = undefined
     "]
     bb0_instrs --> bb0_terminal(["If (read $11_@2)"])
   end
   subgraph bb2
     bb2_instrs["
-      [5] Const mutate $12_@3[0:8] = read $9_@0
+      [5] Const mutate $12_@3[0:9] = read $9_@0
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [7] Const mutate $13_@3[0:8] = Call mutate g$7_@3()
+      [7] Const mutate $12_@3[0:9] = Call mutate g$7_@3()
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb1
-    bb1_terminal(["Return freeze $14_@3"])
+    bb1_terminal(["Return freeze $12_@3"])
   end
 
   %% Jumps
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 1c3331937f..038ffd893c 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
@@ -97,52 +97,44 @@ function cond$0(x$2) {}
 
 ```
 bb0:
-  [1] Let mutate a$13_@0[0:23] = Object {  }
-  [2] Let mutate b$14_@0[0:23] = Object {  }
-  [3] Let mutate c$15_@0[0:23] = Object {  }
-  [4] Let mutate d$16_@0[0:23] = Object {  }
+  [1] Let mutate a$18_@0[0:23] = Object {  }
+  [2] Let mutate b$20_@0[0:23] = Object {  }
+  [3] Let mutate c$22_@0[0:23] = Object {  }
+  [4] Let mutate d$24_@0[0:23] = Object {  }
   [5] While test=bb1 loop=bb3 fallthrough=bb2
 bb1:
   predecessor blocks: bb0 bb4
-  a$18_@0[0:23]: phi(bb0: a$13_@0, bb4: a$21_@0)
-  b$20_@0[0:23]: phi(bb0: b$14_@0, bb4: b$23_@0)
-  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 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
-  [8] Let mutate z$19_@0[0:23] = read a$18_@0
-  [9] Reassign mutate a$21_@0[0:23] = read b$20_@0
-  [10] Reassign mutate b$23_@0[0:23] = read c$22_@0
-  [11] Reassign mutate c$25_@0[0:23] = read d$24_@0
-  [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)
+  [8] Const mutate z$19_@0[0:23] = read a$18_@0
+  [9] Reassign mutate a$18_@0[0:23] = read b$20_@0
+  [10] Reassign mutate b$20_@0[0:23] = read c$22_@0
+  [11] Reassign mutate c$22_@0[0:23] = read d$24_@0
+  [12] Reassign mutate d$24_@0[0:23] = read z$19_@0
+  [13] Call mutate mutate$7_@0(mutate a$18_@0, mutate b$20_@0)
+  [14] Const mutate $29_@0[0:23] = Call mutate cond$8_@0(mutate a$18_@0)
   [15] If (read $29_@0) then:bb2 else:bb4 fallthrough=bb4
 bb4:
   predecessor blocks: bb3
   [16] Goto(Continue) bb1
 bb2:
   predecessor blocks: bb3 bb1
-  a$30_@0[0:23]: phi(bb3: a$21_@0, bb1: a$18_@0)
-  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 fallthrough=bb7
+  [17] If (read a$18_@0) then:bb7 else:bb7 fallthrough=bb7
 bb7:
   predecessor blocks: bb2
-  [18] If (read b$31_@0) then:bb9 else:bb9 fallthrough=bb9
+  [18] If (read b$20_@0) then:bb9 else:bb9 fallthrough=bb9
 bb9:
   predecessor blocks: bb7
-  [19] If (read c$32_@0) then:bb11 else:bb11 fallthrough=bb11
+  [19] If (read c$22_@0) then:bb11 else:bb11 fallthrough=bb11
 bb11:
   predecessor blocks: bb9
-  [20] If (read d$33_@0) then:bb13 else:bb13 fallthrough=bb13
+  [20] If (read d$24_@0) then:bb13 else:bb13 fallthrough=bb13
 bb13:
   predecessor blocks: bb11
   [21] Const mutate $34_@2 = null
-  [22] Call mutate mutate$7_@0(mutate d$33_@0, read $34_@2)
+  [22] Call mutate mutate$7_@0(mutate d$24_@0, read $34_@2)
   [23] Return
 
 ```
@@ -154,10 +146,10 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate a$13_@0[0:23] = Object {  }
-      [2] Let mutate b$14_@0[0:23] = Object {  }
-      [3] Let mutate c$15_@0[0:23] = Object {  }
-      [4] Let mutate d$16_@0[0:23] = Object {  }
+      [1] Let mutate a$18_@0[0:23] = Object {  }
+      [2] Let mutate b$20_@0[0:23] = Object {  }
+      [3] Let mutate c$22_@0[0:23] = Object {  }
+      [4] Let mutate d$24_@0[0:23] = Object {  }
     "]
     bb0_instrs --> bb0_terminal(["While"])
   end
@@ -169,13 +161,13 @@ flowchart TB
   end
   subgraph bb3
     bb3_instrs["
-      [8] Let mutate z$19_@0[0:23] = read a$18_@0
-      [9] Reassign mutate a$21_@0[0:23] = read b$20_@0
-      [10] Reassign mutate b$23_@0[0:23] = read c$22_@0
-      [11] Reassign mutate c$25_@0[0:23] = read d$24_@0
-      [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)
+      [8] Const mutate z$19_@0[0:23] = read a$18_@0
+      [9] Reassign mutate a$18_@0[0:23] = read b$20_@0
+      [10] Reassign mutate b$20_@0[0:23] = read c$22_@0
+      [11] Reassign mutate c$22_@0[0:23] = read d$24_@0
+      [12] Reassign mutate d$24_@0[0:23] = read z$19_@0
+      [13] Call mutate mutate$7_@0(mutate a$18_@0, mutate b$20_@0)
+      [14] Const mutate $29_@0[0:23] = Call mutate cond$8_@0(mutate a$18_@0)
     "]
     bb3_instrs --> bb3_terminal(["If (read $29_@0)"])
   end
@@ -183,21 +175,21 @@ flowchart TB
     bb4_terminal(["Goto"])
   end
   subgraph bb2
-    bb2_terminal(["If (read a$30_@0)"])
+    bb2_terminal(["If (read a$18_@0)"])
   end
   subgraph bb7
-    bb7_terminal(["If (read b$31_@0)"])
+    bb7_terminal(["If (read b$20_@0)"])
   end
   subgraph bb9
-    bb9_terminal(["If (read c$32_@0)"])
+    bb9_terminal(["If (read c$22_@0)"])
   end
   subgraph bb11
-    bb11_terminal(["If (read d$33_@0)"])
+    bb11_terminal(["If (read d$24_@0)"])
   end
   subgraph bb13
     bb13_instrs["
       [21] Const mutate $34_@2 = null
-      [22] Call mutate mutate$7_@0(mutate d$33_@0, read $34_@2)
+      [22] Call mutate mutate$7_@0(mutate d$24_@0, read $34_@2)
     "]
     bb13_instrs --> bb13_terminal(["Return"])
   end
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 58b79e8e30..d4f912a8c0 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
@@ -92,10 +92,10 @@ function cond$0() {}
 
 ```
 bb0:
-  [1] Let mutate a$12_@0[0:18] = Object {  }
-  [2] Let mutate b$13_@0[0:18] = Object {  }
-  [3] Let mutate c$14_@1 = Object {  }
-  [4] Let mutate d$15_@0[0:18] = Object {  }
+  [1] Const mutate a$12_@0[0:18] = Object {  }
+  [2] Const mutate b$13_@0[0:18] = Object {  }
+  [3] Const mutate c$14_@1 = Object {  }
+  [4] Const mutate d$15_@0[0:18] = Object {  }
   [5] While test=bb1 loop=bb3 fallthrough=bb2
 bb1:
   predecessor blocks: bb0 bb4
@@ -136,10 +136,10 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate a$12_@0[0:18] = Object {  }
-      [2] Let mutate b$13_@0[0:18] = Object {  }
-      [3] Let mutate c$14_@1 = Object {  }
-      [4] Let mutate d$15_@0[0:18] = Object {  }
+      [1] Const mutate a$12_@0[0:18] = Object {  }
+      [2] Const mutate b$13_@0[0:18] = Object {  }
+      [3] Const mutate c$14_@1 = Object {  }
+      [4] Const mutate d$15_@0[0:18] = Object {  }
     "]
     bb0_instrs --> bb0_terminal(["While"])
   end
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 97cc7a29ec..515a3143ff 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
@@ -15,8 +15,8 @@ function foo(a, b) {
 
 ```
 bb0:
-  [1] Let mutate x$7_@0[1:5] = Array []
-  [2] Let mutate y$8_@0[1:5] = Array []
+  [1] Const mutate x$7_@0[1:5] = Array []
+  [2] Const mutate y$8_@0[1:5] = Array []
   [3] Call mutate x$7_@0.push(read a$5)
   [4] Call mutate y$8_@0.push(read b$6)
   [5] Return
@@ -32,8 +32,8 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$7_@0[1:5] = Array []
-      [2] Let mutate y$8_@0[1:5] = Array []
+      [1] Const mutate x$7_@0[1:5] = Array []
+      [2] Const mutate y$8_@0[1:5] = Array []
       [3] Call mutate x$7_@0.push(read a$5)
       [4] Call mutate y$8_@0.push(read 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 e31eebbaa1..d62b0ae459 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
@@ -15,8 +15,8 @@ function foo(a, b) {
 
 ```
 bb0:
-  [1] Let mutate x$7_@0[1:5] = Array []
-  [2] Let mutate y$8_@1[2:4] = Array []
+  [1] Const mutate x$7_@0[1:5] = Array []
+  [2] Const mutate y$8_@1[2:4] = Array []
   [3] Call mutate y$8_@1.push(read b$6)
   [4] Call mutate x$7_@0.push(read a$5)
   [5] Return
@@ -32,8 +32,8 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$7_@0[1:5] = Array []
-      [2] Let mutate y$8_@1[2:4] = Array []
+      [1] Const mutate x$7_@0[1:5] = Array []
+      [2] Const mutate y$8_@1[2:4] = Array []
       [3] Call mutate y$8_@1.push(read b$6)
       [4] Call mutate x$7_@0.push(read 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 43f7316eb8..424d34233d 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
@@ -21,11 +21,11 @@ function foo(a, b, c) {
 
 ```
 bb0:
-  [1] Let mutate x$11_@0[1:11] = Array []
+  [1] Const mutate x$11_@0[1:11] = Array []
   [2] If (read a$8) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [3] Let mutate y$12_@1[3:7] = Array []
+  [3] Const mutate y$12_@1[3:7] = Array []
   [4] If (read b$9) then:bb4 else:bb3 fallthrough=bb3
 bb4:
   predecessor blocks: bb2
@@ -57,13 +57,13 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$11_@0[1:11] = Array []
+      [1] Const mutate x$11_@0[1:11] = Array []
     "]
     bb0_instrs --> bb0_terminal(["If (read a$8)"])
   end
   subgraph bb2
     bb2_instrs["
-      [3] Let mutate y$12_@1[3:7] = Array []
+      [3] Const mutate y$12_@1[3:7] = Array []
     "]
     bb2_instrs --> bb2_terminal(["If (read b$9)"])
   end
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 9bb61e3ccc..55f85d983e 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
@@ -17,8 +17,8 @@ function foo(a, b, c) {
 
 ```
 bb0:
-  [1] Let mutate x$9_@0[1:8] = Array []
-  [2] Let mutate y$10_@0[1:8] = Array []
+  [1] Const mutate x$9_@0[1:8] = Array []
+  [2] Const mutate y$10_@0[1:8] = Array []
   [3] While test=bb1 loop=bb3 fallthrough=bb2
 bb1:
   predecessor blocks: bb0 bb3
@@ -44,8 +44,8 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$9_@0[1:8] = Array []
-      [2] Let mutate y$10_@0[1:8] = Array []
+      [1] Const mutate x$9_@0[1:8] = Array []
+      [2] Const mutate y$10_@0[1:8] = Array []
     "]
     bb0_instrs --> bb0_terminal(["While"])
   end
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 fdf3a1010d..e471dd7995 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
@@ -21,11 +21,11 @@ function foo(a, b, c) {
 
 ```
 bb0:
-  [1] Let mutate x$9_@0[1:9] = Array []
+  [1] Const mutate x$9_@0[1:9] = Array []
   [2] If (read a$6) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [3] Let mutate y$10_@0[1:9] = Array []
+  [3] Const mutate y$10_@0[1:9] = Array []
   [4] If (read b$7) then:bb4 else:bb3 fallthrough=bb3
 bb4:
   predecessor blocks: bb2
@@ -51,13 +51,13 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$9_@0[1:9] = Array []
+      [1] Const mutate x$9_@0[1:9] = Array []
     "]
     bb0_instrs --> bb0_terminal(["If (read a$6)"])
   end
   subgraph bb2
     bb2_instrs["
-      [3] Let mutate y$10_@0[1:9] = Array []
+      [3] Const mutate y$10_@0[1:9] = Array []
     "]
     bb2_instrs --> bb2_terminal(["If (read b$7)"])
   end
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 008aca38f8..9173a7c6fa 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
@@ -18,9 +18,9 @@ function foo() {
 
 ```
 bb0:
-  [1] Let mutate x$4_@0[1:6] = Object {  }
-  [2] Let mutate y$5_@1[2:5] = Array []
-  [3] Let mutate z$6_@1[2:5] = Object {  }
+  [1] Const mutate x$4_@0[1:6] = Object {  }
+  [2] Const mutate y$5_@1[2:5] = Array []
+  [3] Const mutate z$6_@1[2:5] = Object {  }
   [4] Call mutate y$5_@1.push(mutate z$6_@1)
   [5] Reassign mutate x$4_@0.y[1:6] = read y$5_@1
   [6] Return freeze x$4_@0
@@ -35,9 +35,9 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$4_@0[1:6] = Object {  }
-      [2] Let mutate y$5_@1[2:5] = Array []
-      [3] Let mutate z$6_@1[2:5] = Object {  }
+      [1] Const mutate x$4_@0[1:6] = Object {  }
+      [2] Const mutate y$5_@1[2:5] = Array []
+      [3] Const mutate z$6_@1[2:5] = Object {  }
       [4] Call mutate y$5_@1.push(mutate z$6_@1)
       [5] Reassign mutate x$4_@0.y[1:6] = read y$5_@1
     "]
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 6aa83da0dd..aa6d913ec8 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/reactive-scopes.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/reactive-scopes.expect.md
@@ -19,7 +19,7 @@ function f(a, b) {
 
 ```
 bb0:
-  [1] Let mutate x$10_@0[1:8] = Array []
+  [1] Const 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 fallthrough=bb1
@@ -53,7 +53,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$10_@0[1:8] = Array []
+      [1] Const 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
     "]
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 fcefff2c63..8138b36630 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/reassignment-conditional.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/reassignment-conditional.expect.md
@@ -25,28 +25,29 @@ function Component(props) {
 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
+  [3] Const mutate y$8_@0[1:7] = read x$7_@0
   [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 []
+  [5] Reassign mutate x$7_@0[1:7] = Array []
   [6] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
-  x$11_@0[1:7]: phi(bb2: x$9_@0, bb0: x$7_@0)
-  [7] Let mutate _$12_@1 = JSX <read Component$0 x={freeze x$11_@0} ></read Component$0>
+  [7] Const mutate _$12_@1 = JSX <read Component$0 x={freeze x$7_@0} ></read Component$0>
   [8] Call read y$8_@0.push(read props$6.p2)
-  [9] Const mutate $15_@2 = JSX <read Component$0 x={read x$11_@0} y={read y$8_@0} ></read Component$0>
+  [9] Const mutate $15_@2 = JSX <read Component$0 x={read x$7_@0} y={read y$8_@0} ></read Component$0>
   [10] Return read $15_@2
 scope0 [1:7]:
  - read props$6.p0
  - read props$6.p1
 scope1 [7:8]:
  - read Component$0
+ - freeze x$7_@0
  - read y$8_@0.push
  - read props$6.p2
 scope2 [9:10]:
  - read Component$0
+ - read x$7_@0
  - read y$8_@0
 ```
 
@@ -59,21 +60,21 @@ flowchart TB
     bb0_instrs["
       [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
+      [3] Const mutate y$8_@0[1:7] = read x$7_@0
     "]
     bb0_instrs --> bb0_terminal(["If (read props$6.p1)"])
   end
   subgraph bb2
     bb2_instrs["
-      [5] Reassign mutate x$9_@0[1:7] = Array []
+      [5] Reassign mutate x$7_@0[1:7] = Array []
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
-      [7] Let mutate _$12_@1 = JSX <read Component$0 x={freeze x$11_@0} ></read Component$0>
+      [7] Const mutate _$12_@1 = JSX <read Component$0 x={freeze x$7_@0} ></read Component$0>
       [8] Call read y$8_@0.push(read props$6.p2)
-      [9] Const mutate $15_@2 = JSX <read Component$0 x={read x$11_@0} y={read y$8_@0} ></read Component$0>
+      [9] Const mutate $15_@2 = JSX <read Component$0 x={read x$7_@0} y={read y$8_@0} ></read Component$0>
     "]
     bb1_instrs --> bb1_terminal(["Return read $15_@2"])
   end
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 c15bd7a239..cf5079a158 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
@@ -34,7 +34,7 @@ function foo(a, b, c) {
 
 ```
 bb0:
-  [1] Let mutate x$16_@0[1:5] = Array []
+  [1] Const mutate x$16_@0[1:5] = Array []
   [2] If (read a$13) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
@@ -43,30 +43,30 @@ bb2:
 bb1:
   predecessor blocks: bb2 bb0
   [5] Const mutate $17_@1 = "div"
-  [6] Let mutate y$19_@2 = JSX <read $17_@1>{freeze x$16_@0}</read $17_@1>
+  [6] Const mutate y$19_@2 = JSX <read $17_@1>{freeze x$16_@0}</read $17_@1>
   [7] Const mutate $20_@3 = 0
+  [8] Let mutate x$22_@4[8:15] = undefined
   [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 []
+  [9] Reassign mutate x$22_@4[8:15] = Array []
   [10] Call mutate x$22_@4.push(read b$14)
   [11] Goto bb3
 bb4:
   predecessor blocks: bb1
-  [12] Reassign mutate x$23_@4[9:14] = Array []
-  [13] Call mutate x$23_@4.push(read c$15)
+  [12] Reassign mutate x$22_@4[8:15] = Array []
+  [13] Call mutate x$22_@4.push(read c$15)
   [14] Goto bb3
 bb3:
   predecessor blocks: bb5 bb4
-  x$30_@4[9:14]: phi(bb5: x$22_@4, bb4: x$23_@4)
   [15] Const mutate $25_@5 = "div"
   [16] Const mutate $26_@6 = "\n      "
   [17] Const mutate $27_@7 = "\n      "
   [18] Const mutate $28_@8 = "\n    "
-  [19] Const mutate $31_@9 = JSX <read $25_@5>{read $26_@6}{read y$19_@2}{read $27_@7}{freeze x$30_@4}{read $28_@8}</read $25_@5>
+  [19] Const mutate $31_@9 = JSX <read $25_@5>{read $26_@6}{read y$19_@2}{read $27_@7}{freeze x$22_@4}{read $28_@8}</read $25_@5>
   [20] Return read $31_@9
 scope0 [1:5]:
  - read a$13
@@ -74,7 +74,7 @@ scope0 [1:5]:
 scope2 [6:7]:
  - read $17_@1
  - freeze x$16_@0
-scope4 [9:14]:
+scope4 [8:15]:
  - read c$15
  - read b$14
  - read b$14
@@ -83,6 +83,7 @@ scope9 [19:20]:
  - read $26_@6
  - read y$19_@2
  - read $27_@7
+ - freeze x$22_@4
  - read $28_@8
 ```
 
@@ -93,7 +94,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$16_@0[1:5] = Array []
+      [1] Const mutate x$16_@0[1:5] = Array []
     "]
     bb0_instrs --> bb0_terminal(["If (read a$13)"])
   end
@@ -106,22 +107,23 @@ flowchart TB
   subgraph bb1
     bb1_instrs["
       [5] Const mutate $17_@1 = 'div'
-      [6] Let mutate y$19_@2 = JSX <read $17_@1>{freeze x$16_@0}</read $17_@1>
+      [6] Const mutate y$19_@2 = JSX <read $17_@1>{freeze x$16_@0}</read $17_@1>
       [7] Const mutate $20_@3 = 0
+      [8] Let mutate x$22_@4[8:15] = undefined
     "]
     bb1_instrs --> bb1_terminal(["Switch (read b$14)"])
   end
   subgraph bb5
     bb5_instrs["
-      [9] Reassign mutate x$22_@4[9:14] = Array []
+      [9] Reassign mutate x$22_@4[8:15] = Array []
       [10] Call mutate x$22_@4.push(read b$14)
     "]
     bb5_instrs --> bb5_terminal(["Goto"])
   end
   subgraph bb4
     bb4_instrs["
-      [12] Reassign mutate x$23_@4[9:14] = Array []
-      [13] Call mutate x$23_@4.push(read c$15)
+      [12] Reassign mutate x$22_@4[8:15] = Array []
+      [13] Call mutate x$22_@4.push(read c$15)
     "]
     bb4_instrs --> bb4_terminal(["Goto"])
   end
@@ -131,7 +133,7 @@ flowchart TB
       [16] Const mutate $26_@6 = '\n      '
       [17] Const mutate $27_@7 = '\n      '
       [18] Const mutate $28_@8 = '\n    '
-      [19] Const mutate $31_@9 = JSX <read $25_@5>{read $26_@6}{read y$19_@2}{read $27_@7}{freeze x$30_@4}{read $28_@8}</read $25_@5>
+      [19] Const mutate $31_@9 = JSX <read $25_@5>{read $26_@6}{read y$19_@2}{read $27_@7}{freeze x$22_@4}{read $28_@8}</read $25_@5>
     "]
     bb3_instrs --> bb3_terminal(["Return read $31_@9"])
   end
diff --git a/compiler/forget/src/__tests__/fixtures/hir/reassignment.expect.md b/compiler/forget/src/__tests__/fixtures/hir/reassignment.expect.md
index 2b5d7d45de..2f627e55ef 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/reassignment.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/reassignment.expect.md
@@ -21,11 +21,11 @@ function Component(props) {
 
 ```
 bb0:
-  [1] Let mutate x$7_@0[1:7] = Array []
+  [1] Const 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] Reassign mutate x$9_@1 = Array []
-  [5] Let mutate _$10_@2 = JSX <read Component$0 x={freeze x$9_@1} ></read Component$0>
+  [3] Const mutate y$8_@0[1:7] = read x$7_@0
+  [4] Const mutate x$9_@1 = Array []
+  [5] Const mutate _$10_@2 = JSX <read Component$0 x={freeze x$9_@1} ></read Component$0>
   [6] Call mutate y$8_@0.push(read props$6.p1)
   [7] Const mutate $11_@3 = JSX <read Component$0 x={read x$9_@1} y={freeze y$8_@0} ></read Component$0>
   [8] Return read $11_@3
@@ -48,11 +48,11 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$7_@0[1:7] = Array []
+      [1] Const 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] Reassign mutate x$9_@1 = Array []
-      [5] Let mutate _$10_@2 = JSX <read Component$0 x={freeze x$9_@1} ></read Component$0>
+      [3] Const mutate y$8_@0[1:7] = read x$7_@0
+      [4] Const mutate x$9_@1 = Array []
+      [5] Const mutate _$10_@2 = JSX <read Component$0 x={freeze x$9_@1} ></read Component$0>
       [6] Call mutate y$8_@0.push(read props$6.p1)
       [7] Const mutate $11_@3 = JSX <read Component$0 x={read x$9_@1} y={freeze y$8_@0} ></read 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 4133c31866..23006a17e7 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/reverse-postorder.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/reverse-postorder.expect.md
@@ -36,54 +36,56 @@ function Component(props) {
 
 ```
 bb0:
-  [1] Let mutate x$7_@0 = undefined
+  [1] Const mutate x$7_@0 = undefined
+  [2] Let mutate x$11_@1[2:18] = undefined
   [2] If (read props$6.cond) then:bb2 else:bb10 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [3] Const mutate $8_@1 = 2
-  [4] Const mutate $9_@2 = 1
-  [5] Const mutate $10_@3 = 0
+  [3] Const mutate $8_@2 = 2
+  [4] Const mutate $9_@3 = 1
+  [5] Const mutate $10_@4 = 0
   [6] Switch (read props$6.test)
-    Case read $10_@3: bb8
-    Case read $9_@2: bb6
-    Case read $8_@1: bb4
+    Case read $10_@4: bb8
+    Case read $9_@3: bb6
+    Case read $8_@2: bb4
     Default: bb4
     Fallthrough: bb1
 bb8:
   predecessor blocks: bb2
-  [7] Reassign mutate x$11_@4[7:17] = read props$6.v0
+  [7] Reassign mutate x$11_@1[2:18] = read props$6.v0
   [8] Goto bb1
 bb6:
   predecessor blocks: bb2
-  [9] Reassign mutate x$12_@4[7:17] = read props$6.v1
+  [9] Reassign mutate x$11_@1[2:18] = read props$6.v1
   [10] Goto bb1
 bb4:
   predecessor blocks: bb2
-  [11] Reassign mutate x$13_@4[7:17] = read props$6.v2
+  [11] Reassign mutate x$11_@1[2:18] = read props$6.v2
   [12] Goto bb1
 bb10:
   predecessor blocks: bb0
   [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
+  [14] Reassign mutate x$11_@1[2:18] = read props$6.b
   [15] Goto bb1
 bb13:
   predecessor blocks: bb10
-  [16] Reassign mutate x$15_@4[7:17] = read props$6.c
+  [16] Reassign mutate x$11_@1[2:18] = read props$6.c
   [17] Goto bb1
 bb1:
   predecessor blocks: bb8 bb6 bb4 bb12 bb13
-  x$16_@4[7:17]: phi(bb8: x$11_@4, bb6: x$12_@4, bb4: x$13_@4, bb12: x$14_@4, bb13: x$15_@4)
-  [18] read x$16_@4
+  [18] read x$11_@1
   [19] Return
-scope4 [7:17]:
+scope1 [2:18]:
  - read props$6.v2
  - read props$6.v1
  - read props$6.v0
  - read props$6.test
  - read props$6.b
  - read props$6.c
+ - read props$6.cond2
+ - read props$6.cond
 ```
 
 ### CFG
@@ -93,33 +95,34 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$7_@0 = undefined
+      [1] Const mutate x$7_@0 = undefined
+      [2] Let mutate x$11_@1[2:18] = undefined
     "]
     bb0_instrs --> bb0_terminal(["If (read props$6.cond)"])
   end
   subgraph bb2
     bb2_instrs["
-      [3] Const mutate $8_@1 = 2
-      [4] Const mutate $9_@2 = 1
-      [5] Const mutate $10_@3 = 0
+      [3] Const mutate $8_@2 = 2
+      [4] Const mutate $9_@3 = 1
+      [5] Const mutate $10_@4 = 0
     "]
     bb2_instrs --> bb2_terminal(["Switch (read props$6.test)"])
   end
   subgraph bb8
     bb8_instrs["
-      [7] Reassign mutate x$11_@4[7:17] = read props$6.v0
+      [7] Reassign mutate x$11_@1[2:18] = read props$6.v0
     "]
     bb8_instrs --> bb8_terminal(["Goto"])
   end
   subgraph bb6
     bb6_instrs["
-      [9] Reassign mutate x$12_@4[7:17] = read props$6.v1
+      [9] Reassign mutate x$11_@1[2:18] = read props$6.v1
     "]
     bb6_instrs --> bb6_terminal(["Goto"])
   end
   subgraph bb4
     bb4_instrs["
-      [11] Reassign mutate x$13_@4[7:17] = read props$6.v2
+      [11] Reassign mutate x$11_@1[2:18] = read props$6.v2
     "]
     bb4_instrs --> bb4_terminal(["Goto"])
   end
@@ -128,19 +131,19 @@ flowchart TB
   end
   subgraph bb12
     bb12_instrs["
-      [14] Reassign mutate x$14_@4[7:17] = read props$6.b
+      [14] Reassign mutate x$11_@1[2:18] = read props$6.b
     "]
     bb12_instrs --> bb12_terminal(["Goto"])
   end
   subgraph bb13
     bb13_instrs["
-      [16] Reassign mutate x$15_@4[7:17] = read props$6.c
+      [16] Reassign mutate x$11_@1[2:18] = read props$6.c
     "]
     bb13_instrs --> bb13_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
-      [18] read x$16_@4
+      [18] read x$11_@1
     "]
     bb1_instrs --> bb1_terminal(["Return"])
   end
@@ -149,9 +152,9 @@ flowchart TB
   bb0_terminal -- "then" --> bb2
   bb0_terminal -- "else" --> bb10
   bb0_terminal -- "fallthrough" --> bb1
-  bb2_terminal -- "read $10_@3" --> bb8
-  bb2_terminal -- "read $9_@2" --> bb6
-  bb2_terminal -- "read $8_@1" --> bb4
+  bb2_terminal -- "read $10_@4" --> bb8
+  bb2_terminal -- "read $9_@3" --> bb6
+  bb2_terminal -- "read $8_@2" --> bb4
   bb2_terminal -- "default" --> bb4
   bb2_terminal -- "fallthrough" --> bb1
   bb8_terminal --> bb1
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 f2ad814ebd..9292e01da1 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/simple-alias.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/simple-alias.expect.md
@@ -47,12 +47,12 @@ function mutate$0() {}
 
 ```
 bb0:
-  [1] Let mutate a$5_@0 = Object {  }
-  [2] Let mutate b$6_@1[0:8] = Object {  }
-  [3] Let mutate c$7_@1[0:8] = Object {  }
-  [4] Reassign mutate a$8_@1[0:8] = read b$6_@1
-  [5] Reassign mutate b$9_@1[0:8] = read c$7_@1
-  [6] Reassign mutate c$10_@1[0:8] = read a$8_@1
+  [1] Const mutate a$5_@0 = Object {  }
+  [2] Const mutate b$6_@1[0:8] = Object {  }
+  [3] Const mutate c$7_@1[0:8] = Object {  }
+  [4] Const mutate a$8_@1[0:8] = read b$6_@1
+  [5] Const mutate b$9_@1[0:8] = read c$7_@1
+  [6] Const mutate c$10_@1[0:8] = read a$8_@1
   [7] Call mutate mutate$4_@1(mutate a$8_@1, mutate b$9_@1)
   [8] Return freeze c$10_@1
 
@@ -65,12 +65,12 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate a$5_@0 = Object {  }
-      [2] Let mutate b$6_@1[0:8] = Object {  }
-      [3] Let mutate c$7_@1[0:8] = Object {  }
-      [4] Reassign mutate a$8_@1[0:8] = read b$6_@1
-      [5] Reassign mutate b$9_@1[0:8] = read c$7_@1
-      [6] Reassign mutate c$10_@1[0:8] = read a$8_@1
+      [1] Const mutate a$5_@0 = Object {  }
+      [2] Const mutate b$6_@1[0:8] = Object {  }
+      [3] Const mutate c$7_@1[0:8] = Object {  }
+      [4] Const mutate a$8_@1[0:8] = read b$6_@1
+      [5] Const mutate b$9_@1[0:8] = read c$7_@1
+      [6] Const mutate c$10_@1[0:8] = read a$8_@1
       [7] Call mutate mutate$4_@1(mutate a$8_@1, mutate b$9_@1)
     "]
     bb0_instrs --> bb0_terminal(["Return freeze c$10_@1"])
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 1112def4d9..3b4a9eda96 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
@@ -57,7 +57,7 @@ bb0:
 bb2:
   predecessor blocks: bb0
   [6] Const mutate $14_@1 = "div"
-  [7] Let mutate _$15_@2 = JSX <read $14_@1 a={freeze a$11_@0} ></read $14_@1>
+  [7] Const mutate _$15_@2 = JSX <read $14_@1 a={freeze a$11_@0} ></read $14_@1>
   [8] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
@@ -90,7 +90,7 @@ flowchart TB
   subgraph bb2
     bb2_instrs["
       [6] Const mutate $14_@1 = 'div'
-      [7] Let mutate _$15_@2 = JSX <read $14_@1 a={freeze a$11_@0} ></read $14_@1>
+      [7] Const mutate _$15_@2 = JSX <read $14_@1 a={freeze a$11_@0} ></read $14_@1>
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
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 002fba21e2..9052a9ed43 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
@@ -50,7 +50,7 @@ bb0:
   [2] Const mutate b$11_@0[0:7] = Object {  }
   [3] Call mutate foo$4_@0(mutate a$10_@0, mutate b$11_@0)
   [4] Const mutate $12_@1 = "div"
-  [5] Let mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
+  [5] Const mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
   [6] Call mutate foo$4_@0(read a$10_@0, mutate b$11_@0)
   [7] Const mutate $14_@3 = "div"
   [8] Const mutate $15_@4 = JSX <read $14_@3 a={read a$10_@0} b={freeze b$11_@0} ></read $14_@3>
@@ -74,7 +74,7 @@ flowchart TB
       [2] Const mutate b$11_@0[0:7] = Object {  }
       [3] Call mutate foo$4_@0(mutate a$10_@0, mutate b$11_@0)
       [4] Const mutate $12_@1 = 'div'
-      [5] Let mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
+      [5] Const mutate _$13_@2 = JSX <read $12_@1 a={freeze a$10_@0} ></read $12_@1>
       [6] Call mutate foo$4_@0(read a$10_@0, mutate b$11_@0)
       [7] Const mutate $14_@3 = 'div'
       [8] Const mutate $15_@4 = JSX <read $14_@3 a={read a$10_@0} b={freeze b$11_@0} ></read $14_@3>
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 9a5d73f716..31c40dda46 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
@@ -21,29 +21,28 @@ function foo() {
 
 ```
 bb0:
-  [1] Let mutate x$7_@0[1:14] = 1
-  [2] Let mutate y$8_@1 = 2
+  [1] Let mutate x$7_@0[1:8] = 1
+  [2] Const 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 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [6] Reassign mutate x$11_@0[1:14] = 3
+  [6] Reassign mutate x$7_@0[1:8] = 3
   [7] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
-  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] Let mutate x$15_@6[1:14] = undefined
   [10] If (read $14_@5) then:bb4 else:bb3 fallthrough=bb3
 bb4:
   predecessor blocks: bb1
-  [11] Reassign mutate x$15_@0[1:14] = 5
+  [11] Reassign mutate x$15_@6[1:14] = 5
   [12] Goto bb3
 bb3:
   predecessor blocks: bb4 bb1
-  x$16_@0[1:14]: phi(bb4: x$15_@0, bb1: x$17_@0)
-  [13] Reassign mutate y$18_@0[1:14] = read x$16_@0
+  [13] Const mutate y$18_@6[1:14] = read x$15_@6
   [14] Return
 scope3 [4:5]:
  - read y$8_@1
@@ -60,8 +59,8 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$7_@0[1:14] = 1
-      [2] Let mutate y$8_@1 = 2
+      [1] Let mutate x$7_@0[1:8] = 1
+      [2] Const mutate y$8_@1 = 2
       [3] Const mutate $9_@2 = 2
       [4] Const mutate $10_@3 = Binary read y$8_@1 === read $9_@2
     "]
@@ -69,7 +68,7 @@ flowchart TB
   end
   subgraph bb2
     bb2_instrs["
-      [6] Reassign mutate x$11_@0[1:14] = 3
+      [6] Reassign mutate x$7_@0[1:8] = 3
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
@@ -77,18 +76,19 @@ flowchart TB
     bb1_instrs["
       [8] Const mutate $12_@4 = 3
       [9] Const mutate $14_@5 = Binary read y$8_@1 === read $12_@4
+      [10] Let mutate x$15_@6[1:14] = undefined
     "]
     bb1_instrs --> bb1_terminal(["If (read $14_@5)"])
   end
   subgraph bb4
     bb4_instrs["
-      [11] Reassign mutate x$15_@0[1:14] = 5
+      [11] Reassign mutate x$15_@6[1:14] = 5
     "]
     bb4_instrs --> bb4_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [13] Reassign mutate y$18_@0[1:14] = read x$16_@0
+      [13] Const mutate y$18_@6[1:14] = read x$15_@6
     "]
     bb3_instrs --> bb3_terminal(["Return"])
   end
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 8f2d5e5db9..924f0f0f7e 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
@@ -19,18 +19,17 @@ function foo() {
 ```
 bb0:
   [1] Let mutate x$5_@0[1:9] = 1
-  [2] Let mutate y$6_@1 = 2
+  [2] Const 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 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [6] Reassign mutate x$9_@0[1:9] = 3
+  [6] Reassign mutate x$5_@0[1:9] = 3
   [7] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
-  x$10_@0[1:9]: phi(bb2: x$9_@0, bb0: x$5_@0)
-  [8] Reassign mutate y$11_@0[1:9] = read x$10_@0
+  [8] Const mutate y$11_@0[1:9] = read x$5_@0
   [9] Return
 scope3 [4:5]:
  - read y$6_@1
@@ -45,7 +44,7 @@ flowchart TB
   subgraph bb0
     bb0_instrs["
       [1] Let mutate x$5_@0[1:9] = 1
-      [2] Let mutate y$6_@1 = 2
+      [2] Const mutate y$6_@1 = 2
       [3] Const mutate $7_@2 = 2
       [4] Const mutate $8_@3 = Binary read y$6_@1 === read $7_@2
     "]
@@ -53,13 +52,13 @@ flowchart TB
   end
   subgraph bb2
     bb2_instrs["
-      [6] Reassign mutate x$9_@0[1:9] = 3
+      [6] Reassign mutate x$5_@0[1:9] = 3
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
-      [8] Reassign mutate y$11_@0[1:9] = read x$10_@0
+      [8] Const mutate y$11_@0[1:9] = read x$5_@0
     "]
     bb1_instrs --> bb1_terminal(["Return"])
   end
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 7aafc6d070..3b6f55073b 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
@@ -19,18 +19,18 @@ function foo(cond) {
 
 ```
 bb0:
-  [1] Let mutate items$5_@0 = Array []
+  [1] Const mutate items$5_@0 = Array []
   [2] Goto bb1
 bb1:
   predecessor blocks: bb0 bb4
   [3] If (read items$5_@0) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
-  [4] Let mutate y$7_@1 = 0
+  [4] Const mutate y$7_@1 = 0
   [5] If (read cond$4) then:bb5 else:bb4 fallthrough=bb4
 bb5:
   predecessor blocks: bb3
-  [6] Reassign mutate y$9_@2 = 1
+  [6] Const mutate y$9_@2 = 1
   [7] Goto bb4
 bb4:
   predecessor blocks: bb5 bb3
@@ -48,7 +48,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate items$5_@0 = Array []
+      [1] Const mutate items$5_@0 = Array []
     "]
     bb0_instrs --> bb0_terminal(["Goto"])
   end
@@ -57,13 +57,13 @@ flowchart TB
   end
   subgraph bb3
     bb3_instrs["
-      [4] Let mutate y$7_@1 = 0
+      [4] Const mutate y$7_@1 = 0
     "]
     bb3_instrs --> bb3_terminal(["If (read cond$4)"])
   end
   subgraph bb5
     bb5_instrs["
-      [6] Reassign mutate y$9_@2 = 1
+      [6] Const mutate y$9_@2 = 1
     "]
     bb5_instrs --> bb5_terminal(["Goto"])
   end
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 895cb9f422..bdc078f059 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-for.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-for.expect.md
@@ -20,18 +20,17 @@ bb0:
   [2] For init=bb3 test=bb1 loop=bb5 update=bb4 fallthrough=bb2
 bb3:
   predecessor blocks: bb0
-  [3] Let mutate i$8_@1[3:13] = 0
+  [3] Const mutate i$8_@1[3:13] = 0
   [4] Goto bb1
 bb1:
   predecessor blocks: bb3 bb4
-  x$13_@0[1:13]: 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 fallthrough=bb2
 bb5:
   predecessor blocks: bb1
   [8] Const mutate $12_@4 = 1
-  [9] Reassign mutate x$14_@0[1:13] = Binary read x$13_@0 + read $12_@4
+  [9] Reassign mutate x$7_@0[1:13] = Binary read x$7_@0 + read $12_@4
   [10] Goto(Continue) bb4
 bb4:
   predecessor blocks: bb5
@@ -39,7 +38,7 @@ bb4:
   [12] Goto bb1
 bb2:
   predecessor blocks: bb1
-  [13] Return read x$13_@0
+  [13] Return read x$7_@0
 scope3 [6:8]:
  - read $9_@2
 ```
@@ -57,7 +56,7 @@ flowchart TB
   end
   subgraph bb3
     bb3_instrs["
-      [3] Let mutate i$8_@1[3:13] = 0
+      [3] Const mutate i$8_@1[3:13] = 0
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
@@ -71,7 +70,7 @@ flowchart TB
   subgraph bb5
     bb5_instrs["
       [8] Const mutate $12_@4 = 1
-      [9] Reassign mutate x$14_@0[1:13] = Binary read x$13_@0 + read $12_@4
+      [9] Reassign mutate x$7_@0[1:13] = Binary read x$7_@0 + read $12_@4
     "]
     bb5_instrs --> bb5_terminal(["Goto"])
   end
@@ -82,7 +81,7 @@ flowchart TB
     bb4_instrs --> bb4_terminal(["Goto"])
   end
   subgraph bb2
-    bb2_terminal(["Return read x$13_@0"])
+    bb2_terminal(["Return read x$7_@0"])
   end
 
   %% Jumps
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 cc530e4c59..fa34e500b8 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
@@ -19,16 +19,16 @@ function foo() {
 
 ```
 bb0:
-  [1] Let mutate x$5_@0 = 1
-  [2] Let mutate y$6_@1 = 2
+  [1] Const mutate x$5_@0 = 1
+  [2] Const mutate y$6_@1 = 2
   [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
+  [4] Const mutate z$7_@2 = Binary read x$5_@0 + read y$6_@1
   [5] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [6] Let mutate z$8_@3 = read x$5_@0
+  [6] Const mutate z$8_@3 = read x$5_@0
   [7] Goto bb1
 bb1:
   predecessor blocks: bb2 bb3
@@ -47,20 +47,20 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$5_@0 = 1
-      [2] Let mutate y$6_@1 = 2
+      [1] Const mutate x$5_@0 = 1
+      [2] Const mutate y$6_@1 = 2
     "]
     bb0_instrs --> bb0_terminal(["If (read y$6_@1)"])
   end
   subgraph bb2
     bb2_instrs["
-      [4] Let mutate z$7_@2 = Binary read x$5_@0 + read y$6_@1
+      [4] Const mutate z$7_@2 = Binary read x$5_@0 + read y$6_@1
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [6] Let mutate z$8_@3 = read x$5_@0
+      [6] Const mutate z$8_@3 = read x$5_@0
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
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
index 9cc8ded1f7..f834e51a43 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-multiple-phis.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-multiple-phis.expect.md
@@ -28,25 +28,25 @@ function foo(a, b, c, d) {
 
 ```
 bb0:
-  [1] Let mutate x$13_@0 = 0
+  [1] Const mutate x$13_@0 = 0
   [2] Const mutate $14_@1 = true
+  [3] Let mutate x$18_@2[3:20] = undefined
   [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
+  [4] Const mutate $15_@3 = true
+  [5] If (read $15_@3) then:bb4 else:bb5 fallthrough=bb3
 bb4:
   predecessor blocks: bb2
-  [6] Reassign mutate x$16_@3[6:17] = read a$9
+  [6] Reassign mutate x$18_@2[3:20] = read a$9
   [7] Goto bb3
 bb5:
   predecessor blocks: bb2
-  [8] Reassign mutate x$17_@3[6:17] = read b$10
+  [8] Reassign mutate x$18_@2[3:20] = 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
+  [10] read x$18_@2
   [11] Goto bb1
 bb6:
   predecessor blocks: bb0
@@ -54,28 +54,26 @@ bb6:
   [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
+  [14] Reassign mutate x$18_@2[3:20] = read c$11
   [15] Goto bb7
 bb9:
   predecessor blocks: bb6
-  [16] Reassign mutate x$21_@3[6:17] = read d$12
+  [16] Reassign mutate x$18_@2[3:20] = 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
+  [18] read x$18_@2
   [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
+  [20] read x$18_@2
   [21] Return
-scope3 [6:17]:
+scope2 [3:20]:
  - read a$9
  - read b$10
- - read $15_@2
  - read c$11
  - read d$12
+ - read $14_@1
 ```
 
 ### CFG
@@ -85,32 +83,33 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$13_@0 = 0
+      [1] Const mutate x$13_@0 = 0
       [2] Const mutate $14_@1 = true
+      [3] Let mutate x$18_@2[3:20] = undefined
     "]
     bb0_instrs --> bb0_terminal(["If (read $14_@1)"])
   end
   subgraph bb2
     bb2_instrs["
-      [4] Const mutate $15_@2 = true
+      [4] Const mutate $15_@3 = true
     "]
-    bb2_instrs --> bb2_terminal(["If (read $15_@2)"])
+    bb2_instrs --> bb2_terminal(["If (read $15_@3)"])
   end
   subgraph bb4
     bb4_instrs["
-      [6] Reassign mutate x$16_@3[6:17] = read a$9
+      [6] Reassign mutate x$18_@2[3:20] = read a$9
     "]
     bb4_instrs --> bb4_terminal(["Goto"])
   end
   subgraph bb5
     bb5_instrs["
-      [8] Reassign mutate x$17_@3[6:17] = read b$10
+      [8] Reassign mutate x$18_@2[3:20] = read b$10
     "]
     bb5_instrs --> bb5_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [10] read x$18_@3
+      [10] read x$18_@2
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
@@ -122,25 +121,25 @@ flowchart TB
   end
   subgraph bb8
     bb8_instrs["
-      [14] Reassign mutate x$20_@3[6:17] = read c$11
+      [14] Reassign mutate x$18_@2[3:20] = read c$11
     "]
     bb8_instrs --> bb8_terminal(["Goto"])
   end
   subgraph bb9
     bb9_instrs["
-      [16] Reassign mutate x$21_@3[6:17] = read d$12
+      [16] Reassign mutate x$18_@2[3:20] = read d$12
     "]
     bb9_instrs --> bb9_terminal(["Goto"])
   end
   subgraph bb7
     bb7_instrs["
-      [18] read x$22_@3
+      [18] read x$18_@2
     "]
     bb7_instrs --> bb7_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
-      [20] read x$23_@3
+      [20] read x$18_@2
     "]
     bb1_instrs --> bb1_terminal(["Return"])
   end
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 e0f0d190f6..f7f1e79e03 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
@@ -21,7 +21,7 @@ function foo(a, b, c) {
 
 ```
 bb0:
-  [1] Let mutate x$9_@0 = 0
+  [1] Const mutate x$9_@0 = 0
   [2] While test=bb1 loop=bb3 fallthrough=bb2
 bb1:
   predecessor blocks: bb0 bb5
@@ -63,7 +63,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$9_@0 = 0
+      [1] Const mutate x$9_@0 = 0
     "]
     bb0_instrs --> bb0_terminal(["While"])
   end
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-phi.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-phi.expect.md
new file mode 100644
index 0000000000..3d00090cd2
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-phi.expect.md
@@ -0,0 +1,99 @@
+
+## Input
+
+```javascript
+function foo(a, b, c) {
+  let x = a;
+  if (b) {
+    if (c) {
+      x = c;
+    }
+    x;
+  }
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Const mutate x$8_@0[1:8] = read a$5
+  [2] If (read b$6) then:bb2 else:bb1 fallthrough=bb1
+bb2:
+  predecessor blocks: bb0
+  [3] If (read c$7) then:bb4 else:bb3 fallthrough=bb3
+bb4:
+  predecessor blocks: bb2
+  [4] Reassign mutate x$8_@0[1:8] = read c$7
+  [5] Goto bb3
+bb3:
+  predecessor blocks: bb4 bb2
+  [6] read x$8_@0
+  [7] Goto bb1
+bb1:
+  predecessor blocks: bb3 bb0
+  [8] Return
+scope0 [1:8]:
+ - read a$5
+ - read c$7
+ - read c$7
+ - read b$6
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Const mutate x$8_@0[1:8] = read a$5
+    "]
+    bb0_instrs --> bb0_terminal(["If (read b$6)"])
+  end
+  subgraph bb2
+    bb2_terminal(["If (read c$7)"])
+  end
+  subgraph bb4
+    bb4_instrs["
+      [4] Reassign mutate x$8_@0[1:8] = read c$7
+    "]
+    bb4_instrs --> bb4_terminal(["Goto"])
+  end
+  subgraph bb3
+    bb3_instrs["
+      [6] read x$8_@0
+    "]
+    bb3_instrs --> bb3_terminal(["Goto"])
+  end
+  subgraph bb1
+    bb1_terminal(["Return"])
+  end
+
+  %% Jumps
+  bb0_terminal -- "then" --> bb2
+  bb0_terminal -- "else" --> bb1
+  bb2_terminal -- "then" --> bb4
+  bb2_terminal -- "else" --> bb3
+  bb4_terminal --> bb3
+  bb3_terminal --> bb1
+
+```
+
+## Code
+
+```javascript
+function foo$0(a$5, b$6, c$7) {
+  const x$8 = a$5;
+  bb1: if (b$6) {
+    bb3: if (c$7) {
+      x$8 = c$7;
+    }
+
+    x$8;
+  }
+}
+
+```
+      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-phi.js b/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-phi.js
new file mode 100644
index 0000000000..9118aa4cc2
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-nested-partial-phi.js
@@ -0,0 +1,9 @@
+function foo(a, b, c) {
+  let x = a;
+  if (b) {
+    if (c) {
+      x = c;
+    }
+    x;
+  }
+}
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
index 69b0c972f1..1845d20cfc 100644
--- 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
@@ -24,19 +24,18 @@ bb0:
   [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
+  [3] Reassign mutate x$12_@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
+  [6] Reassign mutate x$12_@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
+  [8] Return read x$12_@0
 scope0 [1:8]:
  - read b$8
  - read d$10
@@ -57,7 +56,7 @@ flowchart TB
   end
   subgraph bb2
     bb2_instrs["
-      [3] Reassign mutate x$13_@0[1:8] = read b$8
+      [3] Reassign mutate x$12_@0[1:8] = read b$8
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
@@ -66,12 +65,12 @@ flowchart TB
   end
   subgraph bb5
     bb5_instrs["
-      [6] Reassign mutate x$14_@0[1:8] = read d$10
+      [6] Reassign mutate x$12_@0[1:8] = read d$10
     "]
     bb5_instrs --> bb5_terminal(["Goto"])
   end
   subgraph bb1
-    bb1_terminal(["Return read x$15_@0"])
+    bb1_terminal(["Return read x$12_@0"])
   end
 
   %% Jumps
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 c9bdd07aad..6a87423160 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-newexpression.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-newexpression.expect.md
@@ -46,7 +46,7 @@ function Foo$0() {}
 bb0:
   [1] Const mutate a$7_@0[0:4] = Array []
   [2] Const mutate b$8_@0[0:4] = Object {  }
-  [3] Let mutate c$9_@0[0:4] = New mutate Foo$5_@0(mutate a$7_@0, mutate b$8_@0)
+  [3] Const mutate c$9_@0[0:4] = New mutate Foo$5_@0(mutate a$7_@0, mutate b$8_@0)
   [4] Return freeze c$9_@0
 
 ```
@@ -60,7 +60,7 @@ flowchart TB
     bb0_instrs["
       [1] Const mutate a$7_@0[0:4] = Array []
       [2] Const mutate b$8_@0[0:4] = Object {  }
-      [3] Let mutate c$9_@0[0:4] = New mutate Foo$5_@0(mutate a$7_@0, mutate b$8_@0)
+      [3] Const mutate c$9_@0[0:4] = New mutate Foo$5_@0(mutate a$7_@0, mutate b$8_@0)
     "]
     bb0_instrs --> bb0_terminal(["Return freeze c$9_@0"])
   end
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 65082830ad..85b5e3dd7d 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
@@ -25,24 +25,23 @@ bb0:
   [1] Let mutate x$6_@0[1:10] = 1
   [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 fallthrough=bb1
+  [4] Const mutate $9_@0[1:10] = Binary read x$6_@0 > read $8_@2
+  [5] If (read $9_@0) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [6] Reassign mutate x$10_@0[1:10] = 2
+  [6] Reassign mutate x$6_@0[1:10] = 2
   [7] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [8] Reassign mutate y$11_@0[1:10] = 3
+  [8] Reassign mutate y$7_@0[1:10] = 3
   [9] Goto bb1
 bb1:
   predecessor blocks: bb2 bb3
-  x$12_@0[1:10]: phi(bb2: x$10_@0, bb3: x$6_@0)
-  y$13_@0[1:10]: phi(bb2: y$7_@0, bb3: y$11_@0)
-  [10] Let mutate t$14_@4 = Object { x: read x$12_@0, y: read y$13_@0 }
-  [11] Return freeze t$14_@4
-scope3 [4:5]:
- - read $8_@2
+  [10] Const mutate t$14_@3 = Object { x: read x$6_@0, y: read y$7_@0 }
+  [11] Return freeze t$14_@3
+scope3 [10:11]:
+ - read x$6_@0
+ - read y$7_@0
 ```
 
 ### CFG
@@ -55,27 +54,27 @@ flowchart TB
       [1] Let mutate x$6_@0[1:10] = 1
       [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
+      [4] Const mutate $9_@0[1:10] = Binary read x$6_@0 > read $8_@2
     "]
-    bb0_instrs --> bb0_terminal(["If (read $9_@3)"])
+    bb0_instrs --> bb0_terminal(["If (read $9_@0)"])
   end
   subgraph bb2
     bb2_instrs["
-      [6] Reassign mutate x$10_@0[1:10] = 2
+      [6] Reassign mutate x$6_@0[1:10] = 2
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [8] Reassign mutate y$11_@0[1:10] = 3
+      [8] Reassign mutate y$7_@0[1:10] = 3
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
-      [10] Let mutate t$14_@4 = Object { x: read x$12_@0, y: read y$13_@0 }
+      [10] Const mutate t$14_@3 = Object { x: read x$6_@0, y: read y$7_@0 }
     "]
-    bb1_instrs --> bb1_terminal(["Return freeze t$14_@4"])
+    bb1_instrs --> bb1_terminal(["Return freeze t$14_@3"])
   end
 
   %% Jumps
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-alias-mutate-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-alias-mutate-if.expect.md
index 3efa4ee7d0..97ab761b76 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-alias-mutate-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-alias-mutate-if.expect.md
@@ -27,12 +27,12 @@ bb0:
   [3] If (read a$7) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [4] Let mutate y$10_@0[0:11] = Object {  }
+  [4] Const mutate y$10_@0[0:11] = Object {  }
   [5] Reassign mutate x$9_@0.y[0:11] = read y$10_@0
   [6] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [7] Let mutate z$11_@0[0:11] = Object {  }
+  [7] Const mutate z$11_@0[0:11] = Object {  }
   [8] Reassign mutate x$9_@0.z[0:11] = read z$11_@0
   [9] Goto bb1
 bb1:
@@ -56,14 +56,14 @@ flowchart TB
   end
   subgraph bb2
     bb2_instrs["
-      [4] Let mutate y$10_@0[0:11] = Object {  }
+      [4] Const mutate y$10_@0[0:11] = Object {  }
       [5] Reassign mutate x$9_@0.y[0:11] = read y$10_@0
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [7] Let mutate z$11_@0[0:11] = Object {  }
+      [7] Const mutate z$11_@0[0:11] = Object {  }
       [8] Reassign mutate x$9_@0.z[0:11] = read z$11_@0
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-if.expect.md
index cf4d2aac0d..4e1602d91d 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-if.expect.md
@@ -24,12 +24,12 @@ bb0:
   [2] If (read a$5) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [3] Let mutate y$7_@1 = Object {  }
+  [3] Const mutate y$7_@1 = Object {  }
   [4] Reassign mutate x$6_@0.y[1:9] = read y$7_@1
   [5] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [6] Let mutate z$8_@2 = Object {  }
+  [6] Const mutate z$8_@2 = Object {  }
   [7] Reassign mutate x$6_@0.z[1:9] = read z$8_@2
   [8] Goto bb1
 bb1:
@@ -56,14 +56,14 @@ flowchart TB
   end
   subgraph bb2
     bb2_instrs["
-      [3] Let mutate y$7_@1 = Object {  }
+      [3] Const mutate y$7_@1 = Object {  }
       [4] Reassign mutate x$6_@0.y[1:9] = read y$7_@1
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [6] Let mutate z$8_@2 = Object {  }
+      [6] Const mutate z$8_@2 = Object {  }
       [7] Reassign mutate x$6_@0.z[1:9] = read z$8_@2
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-mutate-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-mutate-if.expect.md
index d61071c942..60db051fe8 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-mutate-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-mutate-if.expect.md
@@ -25,12 +25,12 @@ bb0:
   [2] If (read a$6) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [3] Let mutate y$8_@0[0:10] = Object {  }
+  [3] Const mutate y$8_@0[0:10] = Object {  }
   [4] Reassign mutate x$7_@0.y[0:10] = read y$8_@0
   [5] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [6] Let mutate z$9_@0[0:10] = Object {  }
+  [6] Const mutate z$9_@0[0:10] = Object {  }
   [7] Reassign mutate x$7_@0.z[0:10] = read z$9_@0
   [8] Goto bb1
 bb1:
@@ -53,14 +53,14 @@ flowchart TB
   end
   subgraph bb2
     bb2_instrs["
-      [3] Let mutate y$8_@0[0:10] = Object {  }
+      [3] Const mutate y$8_@0[0:10] = Object {  }
       [4] Reassign mutate x$7_@0.y[0:10] = read y$8_@0
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [6] Let mutate z$9_@0[0:10] = Object {  }
+      [6] Const mutate z$9_@0[0:10] = Object {  }
       [7] Reassign mutate x$7_@0.z[0:10] = read z$9_@0
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-mutate-inside-if.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-mutate-inside-if.expect.md
index 534b95ad2a..260f00877c 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-mutate-inside-if.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-property-alias-mutate-inside-if.expect.md
@@ -25,13 +25,13 @@ bb0:
   [2] If (read a$6) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [3] Let mutate y$8_@0[0:10] = Object {  }
+  [3] Const mutate y$8_@0[0:10] = Object {  }
   [4] Reassign mutate x$7_@0.y[0:10] = read y$8_@0
   [5] Call mutate mutate$4_@0(mutate y$8_@0)
   [6] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [7] Let mutate z$9_@1 = Object {  }
+  [7] Const mutate z$9_@1 = Object {  }
   [8] Reassign mutate x$7_@0.z[0:10] = read z$9_@1
   [9] Goto bb1
 bb1:
@@ -54,7 +54,7 @@ flowchart TB
   end
   subgraph bb2
     bb2_instrs["
-      [3] Let mutate y$8_@0[0:10] = Object {  }
+      [3] Const mutate y$8_@0[0:10] = Object {  }
       [4] Reassign mutate x$7_@0.y[0:10] = read y$8_@0
       [5] Call mutate mutate$4_@0(mutate y$8_@0)
     "]
@@ -62,7 +62,7 @@ flowchart TB
   end
   subgraph bb3
     bb3_instrs["
-      [7] Let mutate z$9_@1 = Object {  }
+      [7] Const mutate z$9_@1 = Object {  }
       [8] Reassign mutate x$7_@0.z[0:10] = read z$9_@1
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
diff --git a/compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.expect.md b/compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.expect.md
index e5c957e562..e579726fd4 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-reassign.expect.md
@@ -16,10 +16,10 @@ function foo(a, b, c) {
 
 ```
 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
+  [1] Const mutate x$8_@0 = 0
+  [2] Const mutate x$9_@1 = read a$5
+  [3] Const mutate x$10_@2 = read b$6
+  [4] Const mutate x$11_@3 = read c$7
   [5] Return read x$11_@3
 scope1 [2:3]:
  - read a$5
@@ -36,10 +36,10 @@ 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
+      [1] Const mutate x$8_@0 = 0
+      [2] Const mutate x$9_@1 = read a$5
+      [3] Const mutate x$10_@2 = read b$6
+      [4] Const mutate x$11_@3 = read c$7
     "]
     bb0_instrs --> bb0_terminal(["Return read x$11_@3"])
   end
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 33be65cfc8..0d055cbf5d 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-return.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-return.expect.md
@@ -19,18 +19,16 @@ function foo() {
 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 fallthrough=bb1
+  [3] Const mutate $6_@0[1:7] = Binary read x$4_@0 === read $5_@1
+  [4] If (read $6_@0) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [5] Reassign mutate x$7_@0[1:7] = 2
+  [5] Reassign mutate x$4_@0[1:7] = 2
   [6] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
-  x$8_@0[1:7]: phi(bb2: x$7_@0, bb0: x$4_@0)
-  [7] Return read x$8_@0
-scope2 [3:4]:
- - read $5_@1
+  [7] Return read x$4_@0
+
 ```
 
 ### CFG
@@ -42,18 +40,18 @@ flowchart TB
     bb0_instrs["
       [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
+      [3] Const mutate $6_@0[1:7] = Binary read x$4_@0 === read $5_@1
     "]
-    bb0_instrs --> bb0_terminal(["If (read $6_@2)"])
+    bb0_instrs --> bb0_terminal(["If (read $6_@0)"])
   end
   subgraph bb2
     bb2_instrs["
-      [5] Reassign mutate x$7_@0[1:7] = 2
+      [5] Reassign mutate x$4_@0[1:7] = 2
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb1
-    bb1_terminal(["Return read x$8_@0"])
+    bb1_terminal(["Return read x$4_@0"])
   end
 
   %% Jumps
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 3e6d8ad457..db3125e60a 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-shadowing.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-shadowing.expect.md
@@ -52,17 +52,16 @@ bb0:
   [2] If (read cond$5) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [3] Let mutate str$7_@1 = "other test"
+  [3] Const mutate str$7_@1 = "other test"
   [4] Call mutate log$4_@2(read str$7_@1)
   [5] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [6] Reassign mutate str$8_@0[1:8] = "fallthrough test"
+  [6] Reassign mutate str$6_@0[1:8] = "fallthrough test"
   [7] Goto bb1
 bb1:
   predecessor blocks: bb2 bb3
-  str$10_@0[1:8]: phi(bb2: str$6_@0, bb3: str$8_@0)
-  [8] Call mutate log$4_@2(read str$10_@0)
+  [8] Call mutate log$4_@2(read str$6_@0)
   [9] Return
 
 ```
@@ -80,20 +79,20 @@ flowchart TB
   end
   subgraph bb2
     bb2_instrs["
-      [3] Let mutate str$7_@1 = 'other test'
+      [3] Const mutate str$7_@1 = 'other test'
       [4] Call mutate log$4_@2(read str$7_@1)
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [6] Reassign mutate str$8_@0[1:8] = 'fallthrough test'
+      [6] Reassign mutate str$6_@0[1:8] = 'fallthrough test'
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
-      [8] Call mutate log$4_@2(read str$10_@0)
+      [8] Call mutate log$4_@2(read str$6_@0)
     "]
     bb1_instrs --> bb1_terminal(["Return"])
   end
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
index 4b02019526..c3e955e5a2 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-sibling-phis.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-sibling-phis.expect.md
@@ -28,52 +28,54 @@ function foo(a, b, c, d) {
 
 ```
 bb0:
-  [1] Let mutate x$13_@0 = 0
+  [1] Const 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] Let mutate x$16_@3[5:10] = undefined
   [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
+  [6] Reassign mutate x$16_@3[5:10] = read a$9
   [7] Goto bb3
 bb5:
   predecessor blocks: bb2
-  [8] Reassign mutate x$17_@3[6:9] = read b$10
+  [8] Reassign mutate x$16_@3[5:10] = 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
+  [10] read x$16_@3
   [11] Goto bb1
 bb6:
   predecessor blocks: bb0
   [12] Const mutate $19_@4 = true
+  [13] Let mutate x$20_@5[13:18] = undefined
   [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
+  [14] Reassign mutate x$20_@5[13:18] = read c$11
   [15] Goto bb7
 bb9:
   predecessor blocks: bb6
-  [16] Reassign mutate x$21_@5[14:17] = read d$12
+  [16] Reassign mutate x$20_@5[13:18] = 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
+  [18] read x$20_@5
   [19] Goto bb1
 bb1:
   predecessor blocks: bb3 bb7
   [20] Return
-scope3 [6:9]:
+scope3 [5:10]:
  - read a$9
  - read b$10
-scope5 [14:17]:
+ - read $15_@2
+scope5 [13:18]:
  - read c$11
  - read d$12
+ - read $19_@4
 ```
 
 ### CFG
@@ -83,7 +85,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$13_@0 = 0
+      [1] Const mutate x$13_@0 = 0
       [2] Const mutate $14_@1 = true
     "]
     bb0_instrs --> bb0_terminal(["If (read $14_@1)"])
@@ -91,48 +93,50 @@ flowchart TB
   subgraph bb2
     bb2_instrs["
       [4] Const mutate $15_@2 = true
+      [5] Let mutate x$16_@3[5:10] = undefined
     "]
     bb2_instrs --> bb2_terminal(["If (read $15_@2)"])
   end
   subgraph bb4
     bb4_instrs["
-      [6] Reassign mutate x$16_@3[6:9] = read a$9
+      [6] Reassign mutate x$16_@3[5:10] = read a$9
     "]
     bb4_instrs --> bb4_terminal(["Goto"])
   end
   subgraph bb5
     bb5_instrs["
-      [8] Reassign mutate x$17_@3[6:9] = read b$10
+      [8] Reassign mutate x$16_@3[5:10] = read b$10
     "]
     bb5_instrs --> bb5_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [10] read x$18_@3
+      [10] read x$16_@3
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb6
     bb6_instrs["
       [12] Const mutate $19_@4 = true
+      [13] Let mutate x$20_@5[13:18] = undefined
     "]
     bb6_instrs --> bb6_terminal(["If (read $19_@4)"])
   end
   subgraph bb8
     bb8_instrs["
-      [14] Reassign mutate x$20_@5[14:17] = read c$11
+      [14] Reassign mutate x$20_@5[13:18] = read c$11
     "]
     bb8_instrs --> bb8_terminal(["Goto"])
   end
   subgraph bb9
     bb9_instrs["
-      [16] Reassign mutate x$21_@5[14:17] = read d$12
+      [16] Reassign mutate x$20_@5[13:18] = read d$12
     "]
     bb9_instrs --> bb9_terminal(["Goto"])
   end
   subgraph bb7
     bb7_instrs["
-      [18] read x$22_@5
+      [18] read x$20_@5
     "]
     bb7_instrs --> bb7_terminal(["Goto"])
   end
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 90bdbee104..30d24d7585 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
@@ -20,27 +20,27 @@ function foo() {
 
 ```
 bb0:
-  [1] Let mutate y$5_@0 = 2
+  [1] Const 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] Let mutate y$8_@3[4:10] = undefined
   [4] If (read $7_@2) then:bb2 else:bb3 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [5] Reassign mutate y$8_@3[5:10] = 1
+  [5] Reassign mutate y$8_@3[4:10] = 1
   [6] Goto bb1
 bb3:
   predecessor blocks: bb0
-  [7] Reassign mutate y$9_@3[5:10] = 2
+  [7] Reassign mutate y$8_@3[4:10] = 2
   [8] Goto bb1
 bb1:
   predecessor blocks: bb2 bb3
-  y$10_@3[5:10]: phi(bb2: y$8_@3, bb3: y$9_@3)
-  [9] Let mutate x$11_@3[5:10] = read y$10_@3
+  [9] Const mutate x$11_@3[4:10] = read y$8_@3
   [10] Return
 scope2 [3:4]:
  - read y$5_@0
  - read $6_@1
-scope3 [5:10]:
+scope3 [4:10]:
  - read $7_@2
 ```
 
@@ -51,27 +51,28 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate y$5_@0 = 2
+      [1] Const 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] Let mutate y$8_@3[4:10] = undefined
     "]
     bb0_instrs --> bb0_terminal(["If (read $7_@2)"])
   end
   subgraph bb2
     bb2_instrs["
-      [5] Reassign mutate y$8_@3[5:10] = 1
+      [5] Reassign mutate y$8_@3[4:10] = 1
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [7] Reassign mutate y$9_@3[5:10] = 2
+      [7] Reassign mutate y$8_@3[4:10] = 2
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
-      [9] Let mutate x$11_@3[5:10] = read y$10_@3
+      [9] Const mutate x$11_@3[4:10] = read y$8_@3
     "]
     bb1_instrs --> bb1_terminal(["Return"])
   end
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 cc17b32e36..e935b42ec6 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-simple.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-simple.expect.md
@@ -13,8 +13,8 @@ function foo() {
 
 ```
 bb0:
-  [1] Let mutate x$3_@0 = 1
-  [2] Let mutate y$4_@1 = 2
+  [1] Const mutate x$3_@0 = 1
+  [2] Const mutate y$4_@1 = 2
   [3] Return
 
 ```
@@ -26,8 +26,8 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$3_@0 = 1
-      [2] Let mutate y$4_@1 = 2
+      [1] Const mutate x$3_@0 = 1
+      [2] Const mutate y$4_@1 = 2
     "]
     bb0_instrs --> bb0_terminal(["Return"])
   end
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 8d0a4b4338..9c90603e4c 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
@@ -17,12 +17,12 @@ function foo() {
 
 ```
 bb0:
-  [1] Let mutate x$4_@0 = 1
-  [2] Let mutate y$5_@1 = 2
+  [1] Const mutate x$4_@0 = 1
+  [2] Const mutate y$5_@1 = 2
   [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
+  [4] Const mutate z$6_@2 = Binary read x$4_@0 + read y$5_@1
   [5] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
@@ -39,14 +39,14 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$4_@0 = 1
-      [2] Let mutate y$5_@1 = 2
+      [1] Const mutate x$4_@0 = 1
+      [2] Const mutate y$5_@1 = 2
     "]
     bb0_instrs --> bb0_terminal(["If (read y$5_@1)"])
   end
   subgraph bb2
     bb2_instrs["
-      [4] Let mutate z$6_@2 = Binary read x$4_@0 + read y$5_@1
+      [4] Const mutate z$6_@2 = Binary read x$4_@0 + read y$5_@1
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
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 a358ffe9e7..83660156d9 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-switch.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-switch.expect.md
@@ -28,11 +28,12 @@ function foo() {
 
 ```
 bb0:
-  [1] Let mutate x$10_@0 = 1
+  [1] Const mutate x$10_@0 = 1
   [2] Const mutate $11_@1 = 2
   [3] Const mutate $12_@2 = Binary read x$10_@0 === read $11_@1
   [4] Const mutate $13_@3 = 1
   [5] Const mutate $14_@4 = Binary read x$10_@0 === read $13_@3
+  [6] Let mutate x$16_@5[6:17] = undefined
   [6] Switch (read x$10_@0)
     Case read $14_@4: bb5
     Case read $12_@2: bb3
@@ -40,23 +41,22 @@ bb0:
     Fallthrough: bb1
 bb5:
   predecessor blocks: bb0
-  [7] Const mutate $15_@5 = 1
-  [8] Reassign mutate x$16_@6[8:17] = Binary read x$10_@0 + read $15_@5
+  [7] Const mutate $15_@6 = 1
+  [8] Reassign mutate x$16_@5[6:17] = Binary read x$10_@0 + read $15_@6
   [9] Goto bb1
 bb3:
   predecessor blocks: bb0
   [10] Const mutate $17_@7 = 2
-  [11] Reassign mutate x$18_@6[8:17] = Binary read x$10_@0 + read $17_@7
+  [11] Reassign mutate x$16_@5[6:17] = Binary read x$10_@0 + read $17_@7
   [12] Goto bb1
 bb2:
   predecessor blocks: bb0
   [13] Const mutate $19_@8 = 3
-  [14] Reassign mutate x$20_@6[8:17] = Binary read x$10_@0 + read $19_@8
+  [14] Reassign mutate x$16_@5[6:17] = Binary read x$10_@0 + read $19_@8
   [15] Goto bb1
 bb1:
   predecessor blocks: bb5 bb3 bb2
-  x$21_@6[8:17]: phi(bb5: x$16_@6, bb3: x$18_@6, bb2: x$20_@6)
-  [16] Let mutate y$22_@6[8:17] = read x$21_@6
+  [16] Const mutate y$22_@5[6:17] = read x$16_@5
   [17] Return
 scope2 [3:4]:
  - read x$10_@0
@@ -64,12 +64,13 @@ scope2 [3:4]:
 scope4 [5:6]:
  - read x$10_@0
  - read $13_@3
-scope6 [8:17]:
+scope5 [6:17]:
  - read x$10_@0
+scope8 [13:14]:
  - read x$10_@0
 scope7 [10:11]:
  - read x$10_@0
-scope5 [7:8]:
+scope6 [7:8]:
  - read x$10_@0
 ```
 
@@ -80,38 +81,39 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$10_@0 = 1
+      [1] Const mutate x$10_@0 = 1
       [2] Const mutate $11_@1 = 2
       [3] Const mutate $12_@2 = Binary read x$10_@0 === read $11_@1
       [4] Const mutate $13_@3 = 1
       [5] Const mutate $14_@4 = Binary read x$10_@0 === read $13_@3
+      [6] Let mutate x$16_@5[6:17] = undefined
     "]
     bb0_instrs --> bb0_terminal(["Switch (read x$10_@0)"])
   end
   subgraph bb5
     bb5_instrs["
-      [7] Const mutate $15_@5 = 1
-      [8] Reassign mutate x$16_@6[8:17] = Binary read x$10_@0 + read $15_@5
+      [7] Const mutate $15_@6 = 1
+      [8] Reassign mutate x$16_@5[6:17] = Binary read x$10_@0 + read $15_@6
     "]
     bb5_instrs --> bb5_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
       [10] Const mutate $17_@7 = 2
-      [11] Reassign mutate x$18_@6[8:17] = Binary read x$10_@0 + read $17_@7
+      [11] Reassign mutate x$16_@5[6:17] = Binary read x$10_@0 + read $17_@7
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb2
     bb2_instrs["
       [13] Const mutate $19_@8 = 3
-      [14] Reassign mutate x$20_@6[8:17] = Binary read x$10_@0 + read $19_@8
+      [14] Reassign mutate x$16_@5[6:17] = Binary read x$10_@0 + read $19_@8
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
-      [16] Let mutate y$22_@6[8:17] = read x$21_@6
+      [16] Const mutate y$22_@5[6:17] = read x$16_@5
     "]
     bb1_instrs --> bb1_terminal(["Return"])
   end
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 669e36b3a7..cad5be1b3b 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-throw.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-throw.expect.md
@@ -18,18 +18,16 @@ function foo() {
 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 fallthrough=bb1
+  [3] Const mutate $6_@0[1:7] = Binary read x$4_@0 === read $5_@1
+  [4] If (read $6_@0) then:bb2 else:bb1 fallthrough=bb1
 bb2:
   predecessor blocks: bb0
-  [5] Reassign mutate x$7_@0[1:7] = 2
+  [5] Reassign mutate x$4_@0[1:7] = 2
   [6] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
-  x$8_@0[1:7]: phi(bb2: x$7_@0, bb0: x$4_@0)
-  [7] Throw read x$8_@0
-scope2 [3:4]:
- - read $5_@1
+  [7] Throw read x$4_@0
+
 ```
 
 ### CFG
@@ -41,18 +39,18 @@ flowchart TB
     bb0_instrs["
       [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
+      [3] Const mutate $6_@0[1:7] = Binary read x$4_@0 === read $5_@1
     "]
-    bb0_instrs --> bb0_terminal(["If (read $6_@2)"])
+    bb0_instrs --> bb0_terminal(["If (read $6_@0)"])
   end
   subgraph bb2
     bb2_instrs["
-      [5] Reassign mutate x$7_@0[1:7] = 2
+      [5] Reassign mutate x$4_@0[1:7] = 2
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb1
-    bb1_terminal(["Throw read x$8_@0"])
+    bb1_terminal(["Throw read x$4_@0"])
   end
 
   %% Jumps
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 e20b65de8f..e6ef3781b6 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
@@ -17,7 +17,7 @@ function foo() {
 
 ```
 bb0:
-  [1] Let mutate x$5_@0 = 1
+  [1] Const mutate x$5_@0 = 1
   [2] While test=bb1 loop=bb3 fallthrough=bb2
 bb1:
   predecessor blocks: bb0 bb3
@@ -46,7 +46,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$5_@0 = 1
+      [1] Const mutate x$5_@0 = 1
     "]
     bb0_instrs --> bb0_terminal(["While"])
   end
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 2789b37964..8fda351553 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/ssa-while.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/ssa-while.expect.md
@@ -21,18 +21,17 @@ bb0:
   [2] While test=bb1 loop=bb3 fallthrough=bb2
 bb1:
   predecessor blocks: bb0 bb3
-  x$7_@0[1:9]: phi(bb0: x$5_@0, bb3: x$10_@0)
   [3] Const mutate $6_@1 = 10
-  [4] Const mutate $8_@0[1:9] = Binary read x$7_@0 < read $6_@1
+  [4] Const mutate $8_@0[1:9] = Binary read x$5_@0 < read $6_@1
   [5] If (read $8_@0) then:bb3 else:bb2 fallthrough=bb2
 bb3:
   predecessor blocks: bb1
   [6] Const mutate $9_@2 = 1
-  [7] Reassign mutate x$10_@0[1:9] = Binary read x$7_@0 + read $9_@2
+  [7] Reassign mutate x$5_@0[1:9] = Binary read x$5_@0 + read $9_@2
   [8] Goto(Continue) bb1
 bb2:
   predecessor blocks: bb1
-  [9] Return read x$7_@0
+  [9] Return read x$5_@0
 
 ```
 
@@ -50,19 +49,19 @@ flowchart TB
   subgraph bb1
     bb1_instrs["
       [3] Const mutate $6_@1 = 10
-      [4] Const mutate $8_@0[1:9] = Binary read x$7_@0 < read $6_@1
+      [4] Const mutate $8_@0[1:9] = Binary read x$5_@0 < read $6_@1
     "]
     bb1_instrs --> bb1_terminal(["If (read $8_@0)"])
   end
   subgraph bb3
     bb3_instrs["
       [6] Const mutate $9_@2 = 1
-      [7] Reassign mutate x$10_@0[1:9] = Binary read x$7_@0 + read $9_@2
+      [7] Reassign mutate x$5_@0[1:9] = Binary read x$5_@0 + read $9_@2
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb2
-    bb2_terminal(["Return read x$7_@0"])
+    bb2_terminal(["Return read x$5_@0"])
   end
 
   %% Jumps
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 fdf868c6cc..bfb1e5bd77 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
@@ -32,7 +32,7 @@ function Component(props) {
 
 ```
 bb0:
-  [1] Let mutate x$10_@0[1:12] = Array []
+  [1] Const mutate x$10_@0[1:12] = Array []
   [2] Let mutate y$11_@0[1:12] = undefined
   [3] Const mutate $12_@1 = false
   [4] Const mutate $13_@2 = true
@@ -46,18 +46,17 @@ bb0:
 bb6:
   predecessor blocks: bb0
   [7] Call mutate x$10_@0.push(read props$9.p2)
-  [8] Reassign mutate y$15_@0[1:12] = Array []
+  [8] Reassign mutate y$11_@0[1:12] = Array []
   [9] Goto bb1
 bb2:
   predecessor blocks: bb0
-  [10] Reassign mutate y$16_@0[1:12] = read x$10_@0
+  [10] Reassign mutate y$11_@0[1:12] = read x$10_@0
   [11] Goto bb1
 bb1:
   predecessor blocks: bb0 bb6 bb2
-  y$20_@0[1:12]: phi(bb0: y$11_@0, bb6: y$15_@0, bb2: y$16_@0)
   [12] Const mutate child$19_@4 = JSX <read Component$0 data={freeze x$10_@0} ></read Component$0>
-  [13] Call read y$20_@0.push(read props$9.p4)
-  [14] Const mutate $22_@5 = JSX <read Component$0 data={freeze y$20_@0} >{read child$19_@4}</read Component$0>
+  [13] Call read y$11_@0.push(read props$9.p4)
+  [14] Const mutate $22_@5 = JSX <read Component$0 data={freeze y$11_@0} >{read child$19_@4}</read Component$0>
   [15] Return read $22_@5
 scope0 [1:12]:
  - read props$9.p2
@@ -65,9 +64,11 @@ scope0 [1:12]:
 scope4 [12:13]:
  - read Component$0
  - freeze x$10_@0
+ - read y$11_@0.push
  - read props$9.p4
 scope5 [14:15]:
  - read Component$0
+ - freeze y$11_@0
  - read child$19_@4
 ```
 
@@ -78,7 +79,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$10_@0[1:12] = Array []
+      [1] Const mutate x$10_@0[1:12] = Array []
       [2] Let mutate y$11_@0[1:12] = undefined
       [3] Const mutate $12_@1 = false
       [4] Const mutate $13_@2 = true
@@ -89,21 +90,21 @@ flowchart TB
   subgraph bb6
     bb6_instrs["
       [7] Call mutate x$10_@0.push(read props$9.p2)
-      [8] Reassign mutate y$15_@0[1:12] = Array []
+      [8] Reassign mutate y$11_@0[1:12] = Array []
     "]
     bb6_instrs --> bb6_terminal(["Goto"])
   end
   subgraph bb2
     bb2_instrs["
-      [10] Reassign mutate y$16_@0[1:12] = read x$10_@0
+      [10] Reassign mutate y$11_@0[1:12] = read x$10_@0
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
       [12] Const mutate child$19_@4 = JSX <read Component$0 data={freeze x$10_@0} ></read Component$0>
-      [13] Call read y$20_@0.push(read props$9.p4)
-      [14] Const mutate $22_@5 = JSX <read Component$0 data={freeze y$20_@0} >{read child$19_@4}</read Component$0>
+      [13] Call read y$11_@0.push(read props$9.p4)
+      [14] Const mutate $22_@5 = JSX <read Component$0 data={freeze y$11_@0} >{read child$19_@4}</read Component$0>
     "]
     bb1_instrs --> bb1_terminal(["Return read $22_@5"])
   end
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 267fbbf5dc..4c1fc55365 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
@@ -36,7 +36,7 @@ function foo(x) {
 
 ```
 bb0:
-  [1] Let mutate y$10_@0 = undefined
+  [1] Const mutate y$10_@0 = undefined
   [2] Const mutate $11_@1 = 5
   [3] Const mutate $12_@2 = 4
   [4] Const mutate $13_@3 = 3
@@ -54,27 +54,27 @@ bb0:
     Fallthrough: bb1
 bb10:
   predecessor blocks: bb0
-  [9] Reassign mutate y$17_@7 = 0
+  [9] Const mutate y$17_@7 = 0
   [10] Goto bb9
 bb9:
   predecessor blocks: bb10 bb0
-  [11] Reassign mutate y$18_@8 = 1
+  [11] Const mutate y$18_@8 = 1
   [12] Goto bb1
 bb5:
   predecessor blocks: bb0
-  [13] Reassign mutate y$19_@9 = 3
+  [13] Const mutate y$19_@9 = 3
   [14] Goto bb1
 bb4:
   predecessor blocks: bb0
-  [15] Reassign mutate y$20_@10 = 4
+  [15] Const mutate y$20_@10 = 4
   [16] Goto bb3
 bb3:
   predecessor blocks: bb4 bb0
-  [17] Reassign mutate y$21_@11 = 5
+  [17] Const mutate y$21_@11 = 5
   [18] Goto bb2
 bb2:
   predecessor blocks: bb3 bb0
-  [19] Reassign mutate y$22_@12 = 0
+  [19] Const mutate y$22_@12 = 0
   [20] Goto bb1
 bb1:
   predecessor blocks: bb9 bb0 bb5 bb2
@@ -89,7 +89,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate y$10_@0 = undefined
+      [1] Const mutate y$10_@0 = undefined
       [2] Const mutate $11_@1 = 5
       [3] Const mutate $12_@2 = 4
       [4] Const mutate $13_@3 = 3
@@ -101,37 +101,37 @@ flowchart TB
   end
   subgraph bb10
     bb10_instrs["
-      [9] Reassign mutate y$17_@7 = 0
+      [9] Const mutate y$17_@7 = 0
     "]
     bb10_instrs --> bb10_terminal(["Goto"])
   end
   subgraph bb9
     bb9_instrs["
-      [11] Reassign mutate y$18_@8 = 1
+      [11] Const mutate y$18_@8 = 1
     "]
     bb9_instrs --> bb9_terminal(["Goto"])
   end
   subgraph bb5
     bb5_instrs["
-      [13] Reassign mutate y$19_@9 = 3
+      [13] Const mutate y$19_@9 = 3
     "]
     bb5_instrs --> bb5_terminal(["Goto"])
   end
   subgraph bb4
     bb4_instrs["
-      [15] Reassign mutate y$20_@10 = 4
+      [15] Const mutate y$20_@10 = 4
     "]
     bb4_instrs --> bb4_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [17] Reassign mutate y$21_@11 = 5
+      [17] Const mutate y$21_@11 = 5
     "]
     bb3_instrs --> bb3_terminal(["Goto"])
   end
   subgraph bb2
     bb2_instrs["
-      [19] Reassign mutate y$22_@12 = 0
+      [19] Const mutate y$22_@12 = 0
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
diff --git a/compiler/forget/src/__tests__/fixtures/hir/switch.expect.md b/compiler/forget/src/__tests__/fixtures/hir/switch.expect.md
index 18676026ea..2cc0afbc80 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/switch.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/switch.expect.md
@@ -28,7 +28,7 @@ function Component(props) {
 
 ```
 bb0:
-  [1] Let mutate x$9_@0[1:12] = Array []
+  [1] Const mutate x$9_@0[1:12] = Array []
   [2] Let mutate y$10_@0[1:12] = undefined
   [3] Const mutate $11_@1 = false
   [4] Const mutate $12_@2 = true
@@ -41,18 +41,17 @@ bb4:
   predecessor blocks: bb0
   [6] Call mutate x$9_@0.push(read props$8.p2)
   [7] Call mutate x$9_@0.push(read props$8.p3)
-  [8] Reassign mutate y$13_@3 = Array []
+  [8] Const mutate y$13_@3 = Array []
   [9] Goto bb2
 bb2:
   predecessor blocks: bb4 bb0
-  [10] Reassign mutate y$15_@0[1:12] = read x$9_@0
+  [10] Reassign mutate y$10_@0[1:12] = read x$9_@0
   [11] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
-  y$20_@0[1:12]: phi(bb2: y$15_@0, bb0: y$10_@0)
   [12] Const mutate child$19_@4 = JSX <read Component$0 data={freeze x$9_@0} ></read Component$0>
-  [13] Call read y$20_@0.push(read props$8.p4)
-  [14] Const mutate $23_@5 = JSX <read Component$0 data={read y$20_@0} >{read child$19_@4}</read Component$0>
+  [13] Call read y$10_@0.push(read props$8.p4)
+  [14] Const mutate $23_@5 = JSX <read Component$0 data={read y$10_@0} >{read child$19_@4}</read Component$0>
   [15] Return read $23_@5
 scope0 [1:12]:
  - read props$8.p2
@@ -61,9 +60,11 @@ scope0 [1:12]:
 scope4 [12:13]:
  - read Component$0
  - freeze x$9_@0
+ - read y$10_@0.push
  - read props$8.p4
 scope5 [14:15]:
  - read Component$0
+ - read y$10_@0
  - read child$19_@4
 ```
 
@@ -74,7 +75,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$9_@0[1:12] = Array []
+      [1] Const mutate x$9_@0[1:12] = Array []
       [2] Let mutate y$10_@0[1:12] = undefined
       [3] Const mutate $11_@1 = false
       [4] Const mutate $12_@2 = true
@@ -85,21 +86,21 @@ flowchart TB
     bb4_instrs["
       [6] Call mutate x$9_@0.push(read props$8.p2)
       [7] Call mutate x$9_@0.push(read props$8.p3)
-      [8] Reassign mutate y$13_@3 = Array []
+      [8] Const mutate y$13_@3 = Array []
     "]
     bb4_instrs --> bb4_terminal(["Goto"])
   end
   subgraph bb2
     bb2_instrs["
-      [10] Reassign mutate y$15_@0[1:12] = read x$9_@0
+      [10] Reassign mutate y$10_@0[1:12] = read x$9_@0
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
   subgraph bb1
     bb1_instrs["
       [12] Const mutate child$19_@4 = JSX <read Component$0 data={freeze x$9_@0} ></read Component$0>
-      [13] Call read y$20_@0.push(read props$8.p4)
-      [14] Const mutate $23_@5 = JSX <read Component$0 data={read y$20_@0} >{read child$19_@4}</read Component$0>
+      [13] Call read y$10_@0.push(read props$8.p4)
+      [14] Const mutate $23_@5 = JSX <read Component$0 data={read y$10_@0} >{read child$19_@4}</read Component$0>
     "]
     bb1_instrs --> bb1_terminal(["Return read $23_@5"])
   end
diff --git a/compiler/forget/src/__tests__/hir-test.ts b/compiler/forget/src/__tests__/hir-test.ts
index 74f49383ed..cdb8288486 100644
--- a/compiler/forget/src/__tests__/hir-test.ts
+++ b/compiler/forget/src/__tests__/hir-test.ts
@@ -127,22 +127,7 @@ function transform(
   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,
-        });
-
-        // Print the HIR before leaving SSA.
-        const textHIR = printFunction(ir);
-        const visualization = visualizeHIRMermaid(ir);
-
-        const { ast } = run(nodePath, {
+        const { ir, ast } = run(nodePath, {
           eliminateRedundantPhi: true,
           inferReferenceEffects: true,
           inferMutableRanges: true,
@@ -153,6 +138,9 @@ function transform(
           codegen: true,
         });
 
+        const textHIR = printFunction(ir);
+        const visualization = visualizeHIRMermaid(ir);
+
         invariant(ast !== null, "ast is null when codegen option is enabled");
         const text = prettier.format(generate(ast).code.replace("\n\n", "\n"), {
           semi: true,