[strings] Fix hash for exactly 512MB long strings

Bug: chromium:1016237
Change-Id: Idda1e44b5d578d1213aa54927ca68289bcdce8ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1878487
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64552}

src/strings/string-hasher-inl.h

@@ -33,9 +33,18 @@ uint32_t StringHasher::GetHashCore(uint32_t running_hash) {
 
 uint32_t StringHasher::GetTrivialHash(int length) {
   DCHECK_GT(length, String::kMaxHashCalcLength);
-  // String hash of a large string is simply the length.
-  return (static_cast<uint32_t>(length) << String::kHashShift) |
-         String::kIsNotArrayIndexMask | String::kIsNotIntegerIndexMask;
+  // The hash of a large string is simply computed from the length. We don't
+  // have quite enough bits, so we drop the least significant bit.
+  // TODO(9904): Free up one bit, so we don't have to drop anything here.
+  constexpr int kDroppedBits = 1;
+  // Ensure that the max length after dropping bits is small enough to be
+  // shifted without losing information.
+  STATIC_ASSERT(base::bits::CountLeadingZeros32(String::kMaxLength) +
+                    kDroppedBits >=
+                String::kHashShift);
+  uint32_t hash = static_cast<uint32_t>(length) >> kDroppedBits;
+  return (hash << String::kHashShift) | String::kIsNotArrayIndexMask |
+         String::kIsNotIntegerIndexMask;
 }
 
 template <typename schar>

test/mjsunit/regress/regress-crbug-1016237.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
+
+const kLength = Math.min(536870912, %StringMaxLength());
+const v2 = "foo".padEnd(kLength);
+delete v2[v2];