[GC] br_on_null (#3528)

This is only partial support, as br_on_null also has an extra optional
value in the spec. Implementing that is cumbersome in binaryen, and
there is ongoing spec discussions about it (see
https://github.com/WebAssembly/function-references/issues/45 ), so
for now we only support the simple case without the default value.

Also fix prefixed opcodes to be LEBs in RefAs, which was noticed here
as the change here made it noticeable whether the values were int8 or
LEBs.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/wasm-interpreter.h b/src/wasm-interpreter.h
index 26c86efe6..ac64a30c0 100644
--- a/src/wasm-interpreter.h
+++ b/src/wasm-interpreter.h
@@ -1490,6 +1490,15 @@ public:
     }
     const auto& value = flow.getSingleValue();
     NOTE_EVAL1(value);
+    if (curr->op == BrOnNull) {
+      // Unlike the others, BrOnNull does not propagate the value if it takes
+      // the branch.
+      if (value.isNull()) {
+        return Flow(curr->name);
+      }
+      // If the branch is not taken, we return the non-null value.
+      return {value};
+    }
     if (value.isNull()) {
       return {value};
     }