[GC] Add basic RTT support (#3432)

This adds rtt.canon and rtt.sub together with RTT type support
that is necessary for them. Together this lets us test roundtripping the
instructions and types.

Also fixes a missing traversal over globals in collectHeapTypes,
which the example from the GC docs requires, as the RTTs are in
globals there.

This does not yet add full interpreter support and other things. It
disables initial contents on GC in the fuzzer, to avoid the fuzzer
breaking.

Renames the binary ID for exnref, which is being removed from
the spec, and which overlaps with the binary ID for rtt.

1 files changed, 6 insertions, 6 deletions

diff --git a/src/wasm-interpreter.h b/src/wasm-interpreter.h
index a22bb6e54..65a4417c8 100644
--- a/src/wasm-interpreter.h
+++ b/src/wasm-interpreter.h
@@ -1388,13 +1388,13 @@ public:
     NOTE_ENTER("BrOnCast");
     WASM_UNREACHABLE("TODO (gc): br_on_cast");
   }
-  Flow visitRttCanon(RttCanon* curr) {
-    NOTE_ENTER("RttCanon");
-    WASM_UNREACHABLE("TODO (gc): rtt.canon");
-  }
+  Flow visitRttCanon(RttCanon* curr) { return Literal(curr->type); }
   Flow visitRttSub(RttSub* curr) {
-    NOTE_ENTER("RttSub");
-    WASM_UNREACHABLE("TODO (gc): rtt.sub");
+    Flow parent = this->visit(curr->parent);
+    if (parent.breaking()) {
+      return parent;
+    }
+    return Literal(curr->type);
   }
   Flow visitStructNew(StructNew* curr) {
     NOTE_ENTER("StructNew");