Flat IR: Allow ref.as_non_null in nested positions (#3732)

We can't disallow it, as its result is non-null which we can't spill to a local.

This may cause issues eventually in the combination of GC + flatten, but
I don't expect it to. If it does we may need to revisit.

1 files changed, 10 insertions, 3 deletions

diff --git a/src/ir/flat.h b/src/ir/flat.h
index 706ca5a86..273acc42f 100644
--- a/src/ir/flat.h
+++ b/src/ir/flat.h
@@ -43,8 +43,8 @@
 // making the AST have these properties:
 //
 //  1. Aside from a local.set, the operands of an instruction must be a
-//     local.get, a const, or an unreachable. Anything else is written
-//     to a local earlier.
+//     local.get, a const, an unreachable, or a ref.as_non_null. Anything else
+//     is written to a local earlier.
 //  2. Disallow control flow (block, loop, if, and try) return values, and do
 //     not allow the function body to have a concrete type, i.e., do not use
 //     control flow to pass around values.
@@ -54,6 +54,10 @@
 //     values is prohibited already, but e.g. a block ending in unreachable,
 //     which can normally be nested, is also disallowed).
 //
+// Note: ref.as_non_null must be allowed in a nested position because we cannot
+// spill it to a local - the result is non-null, which is not allowable in a
+// local.
+//
 
 #ifndef wasm_ir_flat_h
 #define wasm_ir_flat_h
@@ -82,8 +86,11 @@ inline void verifyFlatness(Function* func) {
                "set values cannot be control flow");
       } else {
         for (auto* child : ChildIterator(curr)) {
+          bool isRefAsNonNull =
+            child->is<RefAs>() && child->cast<RefAs>()->op == RefAsNonNull;
           verify(Properties::isConstantExpression(child) ||
-                   child->is<LocalGet>() || child->is<Unreachable>(),
+                   child->is<LocalGet>() || child->is<Unreachable>() ||
+                   isRefAsNonNull,
                  "instructions must only have constant expressions, local.get, "
                  "or unreachable as children");
         }