From a2a8d2a3a067a23c547b51b4544a933f77a1c03c Mon Sep 17 00:00:00 2001
From: Thomas Lively <tlively@google.com>
Date: Mon, 25 Nov 2024 12:05:15 -0800
Subject: Handle unoptimized branches in CodeFolding (#7111)

CodeFolding previously did not consider br_on_* instructions at all, so
it would happily merge tails even if there were br_on_* branches to the
same label with non-matching tails. Fix the bug by making any label
targeted by any instruction not explicitly handled by CodeFolding
unoptimizable. This will gracefully handle other branching instructions
like `resume` and `resume_throw` as well. Folding these branches
properly is left as future work.

Also rename the test file from code-folding_enable-threads.wast to just
code-folding.wast and enable all features instead of just threads. The
old name was left over from when the test was originally ported to lit,
and the new feature is necessary because the new test uses GC
instructions.
---
 src/passes/CodeFolding.cpp | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

(limited to 'src')

diff --git a/src/passes/CodeFolding.cpp b/src/passes/CodeFolding.cpp
index 21527da6b..0cddec4ca 100644
--- a/src/passes/CodeFolding.cpp
+++ b/src/passes/CodeFolding.cpp
@@ -84,7 +84,9 @@ struct ExpressionMarker
   void visitExpression(Expression* expr) { marked.insert(expr); }
 };
 
-struct CodeFolding : public WalkerPass<ControlFlowWalker<CodeFolding>> {
+struct CodeFolding
+  : public WalkerPass<
+      ControlFlowWalker<CodeFolding, UnifiedExpressionVisitor<CodeFolding>>> {
   bool isFunctionParallel() override { return true; }
 
   std::unique_ptr<Pass> create() override {
@@ -138,6 +140,17 @@ struct CodeFolding : public WalkerPass<ControlFlowWalker<CodeFolding>> {
 
   // walking
 
+  void visitExpression(Expression* curr) {
+    // For any branching instruction not explicitly handled by this pass, mark
+    // the labels it branches to unoptimizable.
+    // TODO: Handle folding br_on* instructions. br_on_null could be folded with
+    // other kinds of branches and br_on_non_null, br_on_cast, and
+    // br_on_cast_fail instructions could be folded with other copies of
+    // themselves.
+    BranchUtils::operateOnScopeNameUses(
+      curr, [&](Name label) { unoptimizables.insert(label); });
+  }
+
   void visitBreak(Break* curr) {
     if (curr->condition || curr->value) {
       unoptimizables.insert(curr->name);
@@ -155,13 +168,6 @@ struct CodeFolding : public WalkerPass<ControlFlowWalker<CodeFolding>> {
     }
   }
 
-  void visitSwitch(Switch* curr) {
-    for (auto target : curr->targets) {
-      unoptimizables.insert(target);
-    }
-    unoptimizables.insert(curr->default_);
-  }
-
   void visitUnreachable(Unreachable* curr) {
     // we can only optimize if we are at the end of the parent block
     if (!controlFlowStack.empty()) {
-- 
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