RemoveUnusedBrs: Do not create a select with a multivalue result (#4005)

The spec disallows that.

Fixes #3990

1 files changed, 13 insertions, 3 deletions

diff --git a/src/passes/RemoveUnusedBrs.cpp b/src/passes/RemoveUnusedBrs.cpp
index 5631c9d5f..157cfc1f3 100644
--- a/src/passes/RemoveUnusedBrs.cpp
+++ b/src/passes/RemoveUnusedBrs.cpp
@@ -97,6 +97,12 @@ static bool tooCostlyToRunUnconditionally(const PassOptions& passOptions,
   return total >= TOO_MUCH;
 }
 
+static bool canEmitSelectWithArms(Expression* ifTrue, Expression* ifFalse) {
+  // A select only allows a single value in its arms in the spec:
+  // https://webassembly.github.io/spec/core/valid/instructions.html#xref-syntax-instructions-syntax-instr-parametric-mathsf-select-t-ast
+  return ifTrue->type.isSingle() && ifFalse->type.isSingle();
+}
+
 struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
   bool isFunctionParallel() override { return true; }
 
@@ -1023,7 +1029,8 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
                     EffectAnalyzer(passOptions, features, curr)
                       .hasSideEffects();
                   list[0] = old;
-                  if (canReorder && !hasSideEffects) {
+                  if (canReorder && !hasSideEffects &&
+                      canEmitSelectWithArms(br->value, curr)) {
                     ExpressionManipulator::nop(list[0]);
                     replaceCurrent(
                       builder.makeSelect(br->condition, br->value, curr));
@@ -1044,8 +1051,11 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
 
       // Convert an if into a select, if possible and beneficial to do so.
       Select* selectify(If* iff) {
-        if (!iff->ifFalse || !iff->ifTrue->type.isSingle() ||
-            !iff->ifFalse->type.isSingle()) {
+        // Only an if-else can be turned into a select.
+        if (!iff->ifFalse) {
+          return nullptr;
+        }
+        if (!canEmitSelectWithArms(iff->ifTrue, iff->ifFalse)) {
           return nullptr;
         }
         if (iff->condition->type == Type::unreachable) {