Fix assertion in OptimizeInstructions for exact refs

scripts/test/fuzzing.py

@@ -103,6 +103,7 @@
     'stack_switching_switch.wast',
     # TODO: fuzzer support for exact references
     'exact-references.wast',
+    'optimize-instructions-exact.wast',
 ]
 
 

src/passes/OptimizeInstructions.cpp

@@ -2346,18 +2346,20 @@ struct OptimizeInstructions
       }
         [[fallthrough]];
       case GCTypeUtils::SuccessOnlyIfNull: {
-        auto nullType = Type(curr->type.getHeapType().getBottom(), Nullable);
         // The cast either returns null or traps. In trapsNeverHappen mode
         // we know the result, since by assumption it will not trap.
         if (getPassOptions().trapsNeverHappen) {
-          replaceCurrent(builder.makeBlock(
-            {builder.makeDrop(curr->ref), builder.makeRefNull(nullType)},
-            curr->type));
+          replaceCurrent(
+            builder.makeBlock({builder.makeDrop(curr->ref),
+                               builder.makeRefNull(curr->type.getHeapType())},
+                              curr->type));
           return;
         }
         // Otherwise, we should have already refined the cast type to cast
-        // directly to null.
-        assert(curr->type == nullType);
+        // directly to null. We do not further refine the cast type to exact
+        // null because the extra precision is not useful and doing so would
+        // increase the size of the instruction encoding.
+        assert(curr->type.isNull());
         break;
       }
       case GCTypeUtils::Unreachable:

test/lit/passes/optimize-instructions-exact.wast

@@ -0,0 +1,24 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+
+;; Check that optimizations on casts involving exact reference types work
+;; correctly.
+
+;; RUN: wasm-opt %s -all --optimize-instructions -S -o - | filecheck %s
+
+(module
+ ;; CHECK:      (func $cast-to-exact-none (type $0) (param $0 anyref)
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (ref.cast (exact nullref)
+ ;; CHECK-NEXT:    (local.get $0)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $cast-to-exact-none (param anyref)
+  (drop
+   ;; This will not be changed, but should not trigger an assertion.
+   (ref.cast (exact nullref)
+    (local.get 0)
+   )
+  )
+ )
+)