[Wasm GC] Optimize away unnecessary non-null assertions (#3800)

ref.as_non_null is not needed if the value flows into a place that traps
on null anyhow. We replace a trap on one instruction with a trap on
another, but we allow such things (and even changing trap types, which
does not happen here).

1 files changed, 27 insertions, 0 deletions

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index e71506e28..1f55c0a4b 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -992,20 +992,45 @@ struct OptimizeInstructions
     }
   }
 
+  // If an instruction traps on a null input, there is no need for a
+  // ref.as_non_null on that input: we will trap either way (and the binaryen
+  // optimizer does not differentiate traps).
+  void skipNonNullCast(Expression*& input) {
+    while (1) {
+      if (auto* as = input->dynCast<RefAs>()) {
+        if (as->op == RefAsNonNull) {
+          input = as->value;
+          continue;
+        }
+      }
+      break;
+    }
+  }
+
+  void visitStructGet(StructGet* curr) { skipNonNullCast(curr->ref); }
+
   void visitStructSet(StructSet* curr) {
+    skipNonNullCast(curr->ref);
+
     if (curr->ref->type != Type::unreachable && curr->value->type.isInteger()) {
       const auto& fields = curr->ref->type.getHeapType().getStruct().fields;
       optimizeStoredValue(curr->value, fields[curr->index].getByteSize());
     }
   }
 
+  void visitArrayGet(ArrayGet* curr) { skipNonNullCast(curr->ref); }
+
   void visitArraySet(ArraySet* curr) {
+    skipNonNullCast(curr->ref);
+
     if (curr->ref->type != Type::unreachable && curr->value->type.isInteger()) {
       auto element = curr->ref->type.getHeapType().getArray().element;
       optimizeStoredValue(curr->value, element.getByteSize());
     }
   }
 
+  void visitArrayLen(ArrayLen* curr) { skipNonNullCast(curr->ref); }
+
   void visitRefCast(RefCast* curr) {
     if (curr->type == Type::unreachable) {
       return;
@@ -1110,6 +1135,8 @@ struct OptimizeInstructions
       return;
     }
 
+    skipNonNullCast(curr->value);
+
     // Check if the type is the kind we are checking for.
     auto result = GCTypeUtils::evaluateKindCheck(curr);