3 files changed, 34 insertions, 12 deletions

diff --git a/src/ir/effects.h b/src/ir/effects.h
index 3ecb54641..ef9aceb37 100644
--- a/src/ir/effects.h
+++ b/src/ir/effects.h
@@ -989,7 +989,14 @@ private:
       // traps when an input is null.
       parent.implicitTrap = true;
     }
-    void visitStringEq(StringEq* curr) {}
+    void visitStringEq(StringEq* curr) {
+      if (curr->op == StringEqCompare) {
+        // traps when either input is null.
+        if (curr->left->type.isNullable() || curr->right->type.isNullable()) {
+          parent.implicitTrap = true;
+        }
+      }
+    }
     void visitStringAs(StringAs* curr) {
       // traps when ref is null.
       parent.implicitTrap = true;
diff --git a/src/tools/fuzzing.h b/src/tools/fuzzing.h
index dc17f7c91..c60744dea 100644
--- a/src/tools/fuzzing.h
+++ b/src/tools/fuzzing.h
@@ -327,6 +327,7 @@ private:
   Expression* makeStringNewArray();
   Expression* makeStringNewCodePoint();
   Expression* makeStringConcat();
+  Expression* makeStringEq(Type type);
   Expression* makeStringEncode(Type type);
 
   // Similar to makeBasic/CompoundRef, but indicates that this value will be
@@ -398,7 +399,7 @@ private:
   Nullability getSuperType(Nullability nullability);
   HeapType getSuperType(HeapType type);
   Type getSuperType(Type type);
-  Type getArrayTypeForString();
+  HeapType getArrayTypeForString();
 
   // Utilities
   Name getTargetName(Expression* target);
diff --git a/src/tools/fuzzing/fuzzing.cpp b/src/tools/fuzzing/fuzzing.cpp
index 541b58b0f..add0494ea 100644
--- a/src/tools/fuzzing/fuzzing.cpp
+++ b/src/tools/fuzzing/fuzzing.cpp
@@ -1368,7 +1368,8 @@ Expression* TranslateToFuzzReader::_makeConcrete(Type type) {
                 &Self::makeI31Get);
     options.add(FeatureSet::ReferenceTypes | FeatureSet::GC |
                   FeatureSet::Strings,
-                &Self::makeStringEncode);
+                &Self::makeStringEncode,
+                &Self::makeStringEq);
   }
   if (type.isTuple()) {
     options.add(FeatureSet::Multivalue, &Self::makeTupleMake);
@@ -2755,7 +2756,7 @@ Expression* TranslateToFuzzReader::makeCompoundRef(Type type) {
 }
 
 Expression* TranslateToFuzzReader::makeStringNewArray() {
-  auto* array = make(getArrayTypeForString());
+  auto* array = makeTrappingRefUse(getArrayTypeForString());
   auto* start = make(Type::i32);
   auto* end = make(Type::i32);
   return builder.makeStringNew(StringNewWTF16Array, array, start, end, false);
@@ -2812,11 +2813,26 @@ Expression* TranslateToFuzzReader::makeStringConst() {
 }
 
 Expression* TranslateToFuzzReader::makeStringConcat() {
-  auto* left = make(Type(HeapType::string, getNullability()));
-  auto* right = make(Type(HeapType::string, getNullability()));
+  auto* left = makeTrappingRefUse(HeapType::string);
+  auto* right = makeTrappingRefUse(HeapType::string);
   return builder.makeStringConcat(left, right);
 }
 
+Expression* TranslateToFuzzReader::makeStringEq(Type type) {
+  assert(type == Type::i32);
+
+  if (oneIn(2)) {
+    auto* left = make(Type(HeapType::string, getNullability()));
+    auto* right = make(Type(HeapType::string, getNullability()));
+    return builder.makeStringEq(StringEqEqual, left, right);
+  }
+
+  // string.compare may trap if the either input is null.
+  auto* left = makeTrappingRefUse(HeapType::string);
+  auto* right = makeTrappingRefUse(HeapType::string);
+  return builder.makeStringEq(StringEqCompare, left, right);
+}
+
 Expression* TranslateToFuzzReader::makeTrappingRefUse(HeapType type) {
   auto percent = upTo(100);
   // Only give a low probability to emit a nullable reference.
@@ -3920,8 +3936,8 @@ Expression* TranslateToFuzzReader::makeArrayBulkMemoryOp(Type type) {
 Expression* TranslateToFuzzReader::makeStringEncode(Type type) {
   assert(type == Type::i32);
 
-  auto* ref = make(Type(HeapType::string, getNullability()));
-  auto* array = make(getArrayTypeForString());
+  auto* ref = makeTrappingRefUse(HeapType::string);
+  auto* array = makeTrappingRefUse(getArrayTypeForString());
   auto* start = make(Type::i32);
 
   // Only rarely emit without a bounds check, which might trap. See related
@@ -4307,12 +4323,10 @@ Type TranslateToFuzzReader::getSuperType(Type type) {
   return superType;
 }
 
-Type TranslateToFuzzReader::getArrayTypeForString() {
+HeapType TranslateToFuzzReader::getArrayTypeForString() {
   // Emit an array that can be used with JS-style strings, containing 16-bit
   // elements. For now, this must be a mutable type as that is all V8 accepts.
-  auto arrayHeapType = HeapType(Array(Field(Field::PackedType::i16, Mutable)));
-  auto nullability = getNullability();
-  return Type(arrayHeapType, nullability);
+  return HeapType(Array(Field(Field::PackedType::i16, Mutable)));
 }
 
 Name TranslateToFuzzReader::getTargetName(Expression* target) {