[Wasm GC] Optimize static casts in br_on_cast* (#4520)

We were missing this particular case, which we can in fact handle
when the cast is static.

1 files changed, 12 insertions, 2 deletions

diff --git a/src/ir/gc-type-utils.h b/src/ir/gc-type-utils.h
index 80a935462..584cde83c 100644
--- a/src/ir/gc-type-utils.h
+++ b/src/ir/gc-type-utils.h
@@ -52,9 +52,19 @@ inline EvaluationResult evaluateKindCheck(Expression* curr) {
       // We don't check nullability here.
       case BrOnNull:
       case BrOnNonNull:
-      // Casts can only be known at runtime using RTTs.
-      case BrOnCast:
       case BrOnCastFail:
+        flip = true;
+        [[fallthrough]];
+      case BrOnCast:
+        if (!br->rtt) {
+          // This is a static cast check, which we may be able to resolve at
+          // compile time. Note that the type must be non-nullable for us to
+          // succeed at that inference, as otherwise a null can make us fail.
+          if (Type::isSubType(br->ref->type,
+                              Type(br->intendedType, NonNullable))) {
+            return flip ? Failure : Success;
+          }
+        }
         return Unknown;
       case BrOnNonFunc:
         flip = true;