[Wasm GC] OptimizeInstructions: Don't turn ref.test into unreachable immediately (#5673)

Emit an unreachable, but guarded by a block as we do in other cases in this pass, to avoid
having unreachable code that is not fully propagated during the pass (as we only do a full
refinalize at the end). See existing comments starting with
"Make sure to emit a block with the same type as us" in the pass.

This is mostly not a problem with other casts, but ref.test returns an i32 which we have
lots of code that tries to optimize.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 70df652b8..6251348af 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -2169,8 +2169,12 @@ struct OptimizeInstructions
           {builder.makeDrop(curr->ref), builder.makeConst(int32_t(1))}));
         break;
       case GCTypeUtils::Unreachable:
-        replaceCurrent(builder.makeSequence(builder.makeDrop(curr->ref),
-                                            builder.makeUnreachable()));
+        // Make sure to emit a block with the same type as us, to avoid other
+        // code in this pass needing to handle unexpected unreachable code
+        // (which is only properly propagated at the end of this pass when we
+        // refinalize).
+        replaceCurrent(builder.makeBlock(
+          {builder.makeDrop(curr->ref), builder.makeUnreachable()}, Type::i32));
         break;
       case GCTypeUtils::Failure:
         replaceCurrent(builder.makeSequence(builder.makeDrop(curr->ref),