LeaveSSA: handle phi as operand to subsequent phi

Fixes https://github.com/facebook/react-forget/pull/858#discussion_r1044626137. 
The case is 

```javascript 

function foo() { 

let x$1 = 1; 

let y = 2; 

if (y === 2) { 

x$2 = 3; 

} 

x$3 = phi(x$1, x$2) 

if (y === 3) { 

x4 = 5; 

} 

x$5 = phi(x$3, x$4); 

y = x$5; 

} 

``` 

What happens here is that there are two _sequential_ phis for `x`. Previously 
when we encountered the second phi we would find that there is no `let` 
declaration for the phi or any of its operands, and create a new one before the 
second `if`. That's incorrect, these should all merge into a single `x` 
declaration. We now look up the phi operands to see if they are part of a 
previous phi, and merge them correctly.

compiler/forget/src/HIR/LeaveSSA.ts

@@ -78,8 +78,9 @@ export function leaveSSA(fn: HIRFunction) {
         // options we'll choose it and can reuse the declaration.
         canonicalId = phi.id;
         for (const [, operand] of phi.operands) {
-          if (operand.id < canonicalId.id) {
-            canonicalId = operand;
+          let canonicalOperand = variableMapping.get(operand) ?? operand;
+          if (canonicalOperand.id < canonicalId.id) {
+            canonicalId = canonicalOperand;
           }
         }
         canonicalId.mutableRange.start = Math.min(

compiler/forget/src/HIR/Pipeline.ts

@@ -18,6 +18,8 @@ import { inferMutableRanges } from "./InferMutableRanges";
 import { inferReactiveScopeDependencies } from "./InferReactiveScopeDependencies";
 import { inferReactiveScopes } from "./InferReactiveScopes";
 import { inferReactiveScopeVariables } from "./InferReactiveScopeVariables";
+import { log } from "./logger";
+import { printFunction } from "./PrintHIR";
 
 export type CompilerFlags = {
   eliminateRedundantPhi: boolean;
@@ -40,29 +42,48 @@ export default function (
   flags: CompilerFlags
 ): CompilerResult {
   const env = new Environment();
+
   const ir = lower(func, env);
+  logStep("HIR", ir);
+
   enterSSA(ir, env);
+  logStep("SSA", ir);
+
   if (flags.eliminateRedundantPhi) {
     eliminateRedundantPhi(ir);
+    logStep("eliminateRedundantPhi", ir);
   }
+
   if (flags.inferReferenceEffects) {
     inferReferenceEffects(ir);
+    logStep("inferReferenceEffects", ir);
   }
+
   if (flags.inferMutableRanges) {
     inferMutableRanges(ir);
+    logStep("inferMutableRanges", ir);
   }
+
   if (flags.leaveSSA) {
     leaveSSA(ir);
+    logStep("leaveSSA", ir);
   }
+
   if (flags.inferReactiveScopeVariables) {
     inferReactiveScopeVariables(ir);
+    logStep("inferReactiveScopeVariables", ir);
   }
+
   if (flags.inferReactiveScopes) {
     inferReactiveScopes(ir);
+    logStep("inferReactiveScopes", ir);
   }
+
   if (flags.inferReactiveScopeDependencies) {
     inferReactiveScopeDependencies(ir);
+    logStep("inferReactiveScopeDependencies", ir);
   }
+
   if (flags.codegen) {
     return {
       ast: codegen(ir),
@@ -72,3 +93,7 @@ export default function (
 
   return { ast: null, ir: ir };
 }
+
+function logStep(step: string, ir: HIRFunction) {
+  log(() => `${step}:\n${printFunction(ir)}`);
+}

compiler/forget/src/HIR/logger.ts

@@ -0,0 +1,19 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+let ENABLED: boolean = false;
+
+export function toggleLogging(enabled: boolean) {
+  ENABLED = enabled;
+}
+
+export function log(fn: () => string) {
+  if (ENABLED) {
+    const message = fn();
+    process.stdout.write(message.trim() + "\n\n");
+  }
+}

compiler/forget/src/__tests__/fixtures/hir/ssa-complex-multiple-if.expect.md

@@ -21,28 +21,27 @@ function foo() {
 
 ```
 bb0:
-  [1] Let mutate x$7_@0[1:8] = 1
+  [1] Let mutate x$7_@0[1:14] = 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$7_@0[1:8] = 3
+  [6] Reassign mutate x$7_@0[1:14] = 3
   [7] Goto bb1
 bb1:
   predecessor blocks: bb2 bb0
   [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_@6[1:14] = 5
+  [11] Reassign mutate x$7_@0[1:14] = 5
   [12] Goto bb3
 bb3:
   predecessor blocks: bb4 bb1
-  [13] Const mutate y$18_@6[1:14] = read x$15_@6
+  [13] Const mutate y$18_@0[1:14] = read x$7_@0
   [14] Return
 scope3 [4:5]:
  - read y$8_@1
@@ -59,7 +58,7 @@ flowchart TB
   %% Basic Blocks
   subgraph bb0
     bb0_instrs["
-      [1] Let mutate x$7_@0[1:8] = 1
+      [1] Let mutate x$7_@0[1:14] = 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
@@ -68,7 +67,7 @@ flowchart TB
   end
   subgraph bb2
     bb2_instrs["
-      [6] Reassign mutate x$7_@0[1:8] = 3
+      [6] Reassign mutate x$7_@0[1:14] = 3
     "]
     bb2_instrs --> bb2_terminal(["Goto"])
   end
@@ -76,19 +75,18 @@ 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_@6[1:14] = 5
+      [11] Reassign mutate x$7_@0[1:14] = 5
     "]
     bb4_instrs --> bb4_terminal(["Goto"])
   end
   subgraph bb3
     bb3_instrs["
-      [13] Const mutate y$18_@6[1:14] = read x$15_@6
+      [13] Const mutate y$18_@0[1:14] = read x$7_@0
     "]
     bb3_instrs --> bb3_terminal(["Return"])
   end
@@ -113,13 +111,11 @@ function foo$0() {
     x$7 = 3;
   }
 
-  let x$15 = undefined;
-
   bb3: if (y$8 === 3) {
-    x$15 = 5;
+    x$7 = 5;
   }
 
-  const y$18 = x$15;
+  const y$18 = x$7;
 }
 
 ```

compiler/forget/src/__tests__/hir-test.ts

@@ -14,6 +14,7 @@ import { wasmFolder } from "@hpcc-js/wasm";
 import invariant from "invariant";
 import path from "path";
 import prettier from "prettier";
+import { toggleLogging } from "../HIR/logger";
 import run from "../HIR/Pipeline";
 import { printFunction } from "../HIR/PrintHIR";
 import visualizeHIRMermaid from "../HIR/VisualizeHIRMermaid";
@@ -34,7 +35,7 @@ const Pragma_RE = /\/\/\s*@enable\((\w+)\)$/gm;
 describe("React Forget (HIR version)", () => {
   generateTestsFromFixtures(
     path.join(__dirname, "fixtures", "hir"),
-    (input, file) => {
+    (input, file, options) => {
       const matches = input.matchAll(Pragma_RE);
 
       for (const match of matches) {
@@ -50,6 +51,9 @@ describe("React Forget (HIR version)", () => {
 
       let items: Array<[string, string, string]> | null = null;
       let error: Error | null = null;
+      if (options.debug) {
+        toggleLogging(options.debug);
+      }
       try {
         items = transform(input, file);
       } catch (e) {

compiler/forget/src/__tests__/test-utils/generateTestsFromFixtures.ts

@@ -42,7 +42,7 @@ expect.extend({
 
 export default function generateTestsFromFixtures(
   fixturesPath: string,
-  transform: (input: string, file: any) => string
+  transform: (input: string, file: any, options: { debug: boolean }) => string
 ) {
   const files = fs.readdirSync(fixturesPath);
   const fixtures = matchInputOutputFixtures(files, fixturesPath);
@@ -87,18 +87,20 @@ export default function generateTestsFromFixtures(
         }
 
         let input: string | null = null;
+        let debug = false;
         if (inputFile != null) {
           input = fs.readFileSync(inputFile, "utf8");
           const lines = input.split("\n");
           if (lines[0]!.indexOf("@only") !== -1) {
             testCommand = test.only;
+            debug = true;
           }
         }
 
         testCommand(basename, () => {
           let receivedOutput;
           if (input !== null) {
-            receivedOutput = transform(input, basename);
+            receivedOutput = transform(input, basename, { debug });
           } else {
             receivedOutput = "<<input deleted>>";
           }