Fix RefNull issues (#3123)

* ExpressionAnalyzer: Fix `ref.null ht` equality check to include `ht`.
* Precompute: Fix `ref.null ht` expression reuse to also update `ht`.
* Fuzzing: Fix `ref.null func` becoming canonicalized to `ref.func $funcref`
  when evaluating execution results, by adding a check for `isNull`.
* Fuzzing: Print actual and expected execution results when aborting.
* Tests: Update `if-arms-subtype` test in `optimize-instructions` to check
  that identical `if` arms become folded while not identical arms are kept.

3 files changed, 10 insertions, 6 deletions

diff --git a/src/ir/ExpressionAnalyzer.cpp b/src/ir/ExpressionAnalyzer.cpp
index cdbcd9ae0..d2f9611f9 100644
--- a/src/ir/ExpressionAnalyzer.cpp
+++ b/src/ir/ExpressionAnalyzer.cpp
@@ -219,7 +219,7 @@ template<typename T> void visitImmediates(Expression* curr, T& visitor) {
       visitor.visitInt(curr->op);
       visitor.visitNonScopeName(curr->nameOperand);
     }
-    void visitRefNull(RefNull* curr) {}
+    void visitRefNull(RefNull* curr) { visitor.visitType(curr->type); }
     void visitRefIsNull(RefIsNull* curr) {}
     void visitRefFunc(RefFunc* curr) { visitor.visitNonScopeName(curr->func); }
     void visitTry(Try* curr) {}
diff --git a/src/passes/Precompute.cpp b/src/passes/Precompute.cpp
index 0b67a25aa..d8a1fa9fc 100644
--- a/src/passes/Precompute.cpp
+++ b/src/passes/Precompute.cpp
@@ -134,9 +134,12 @@ struct Precompute
             curr->finalize();
             return;
           }
-        } else if (singleValue.isNull() &&
-                   curr->value->template is<RefNull>()) {
-          return;
+        } else if (singleValue.isNull()) {
+          if (auto* n = curr->value->template dynCast<RefNull>()) {
+            n->finalize(singleValue.type);
+            curr->finalize();
+            return;
+          }
         } else if (singleValue.type == Type::funcref) {
           if (auto* r = curr->value->template dynCast<RefFunc>()) {
             r->func = singleValue.getFunc();
diff --git a/src/tools/execution-results.h b/src/tools/execution-results.h
index c41b0c208..0881163f1 100644
--- a/src/tools/execution-results.h
+++ b/src/tools/execution-results.h
@@ -73,7 +73,7 @@ struct ExecutionResults {
           // change (after duplicate function elimination or roundtripping)
           // while the function contents are still the same
           for (Literal& val : ret) {
-            if (val.type == Type::funcref) {
+            if (val.type == Type::funcref && !val.isNull()) {
               val = Literal::makeFunc(Name("funcref"));
             }
           }
@@ -112,7 +112,8 @@ struct ExecutionResults {
       }
       std::cout << "[fuzz-exec] comparing " << name << '\n';
       if (results[name] != other.results[name]) {
-        std::cout << "not identical!\n";
+        std::cout << "not identical! " << results[name]
+                  << " != " << other.results[name] << "\n";
         return false;
       }
     }