[turbofan] Fix bigint-to-word64 constant folding

Replacing a constant BigInt with a constant int64 is only valid
when the use site has truncating semantics. (For non-constant
values, the representation changer did correctly check for this.)

Bug: chromium:1028593
Change-Id: Ib58b16ece6f21ba30153fd6cfa0560cc2d78d6a2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1940263Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65262}

src/compiler/representation-change.cc

@@ -1042,7 +1042,8 @@ Node* RepresentationChanger::GetWord64RepresentationFor(
     }
     case IrOpcode::kHeapConstant: {
       HeapObjectMatcher m(node);
-      if (m.HasValue() && m.Ref(broker_).IsBigInt()) {
+      if (m.HasValue() && m.Ref(broker_).IsBigInt() &&
+          use_info.truncation().IsUsedAsWord64()) {
         auto bigint = m.Ref(broker_).AsBigInt();
         return jsgraph()->Int64Constant(
             static_cast<int64_t>(bigint.AsUint64()));

test/mjsunit/regress/regress-crbug-1028593.js

+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+function foo() {
+  try {
+    return "number".charCodeAt(0n);
+  } catch(e) {}
+}
+%PrepareFunctionForOptimization(foo);
+for (let i = 0; i < 3; i++) {
+  foo();
+}
+%OptimizeFunctionOnNextCall(foo);
+foo();