Ensure we align zone memory at 8 byte boundaries on all platforms

BUG=v8:5668
R=verwaest@chromium.org

Review-Url: https://codereview.chromium.org/2672203002
Cr-Commit-Position: refs/heads/master@{#42959}

src/zone/zone.cc

@@ -63,15 +63,7 @@ Zone::~Zone() {
 
 void* Zone::New(size_t size) {
   // Round up the requested size to fit the alignment.
-  size = RoundUp(size, kAlignment);
-
-  // If the allocation size is divisible by 8 then we return an 8-byte aligned
-  // address.
-  if (kPointerSize == 4 && kAlignment == 4) {
-    position_ += ((~size) & 4) & (reinterpret_cast<intptr_t>(position_) & 4);
-  } else {
-    DCHECK(kAlignment >= kPointerSize);
-  }
+  size = RoundUp(size, kAlignmentInBytes);
 
   // Check if the requested size is available without expanding.
   Address result = position_;
@@ -91,7 +83,7 @@ void* Zone::New(size_t size) {
   ASAN_POISON_MEMORY_REGION(redzone_position, kASanRedzoneBytes);
 
   // Check that the result has the proper alignment and return it.
-  DCHECK(IsAddressAligned(result, kAlignment, 0));
+  DCHECK(IsAddressAligned(result, kAlignmentInBytes, 0));
   allocation_size_ += size;
   return reinterpret_cast<void*>(result);
 }
@@ -132,7 +124,7 @@ Segment* Zone::NewSegment(size_t requested_size) {
 Address Zone::NewExpand(size_t size) {
   // Make sure the requested size is already properly aligned and that
   // there isn't enough room in the Zone to satisfy the request.
-  DCHECK_EQ(size, RoundDown(size, kAlignment));
+  DCHECK_EQ(size, RoundDown(size, kAlignmentInBytes));
   DCHECK(limit_ < position_ ||
          reinterpret_cast<uintptr_t>(limit_) -
                  reinterpret_cast<uintptr_t>(position_) <
@@ -144,7 +136,7 @@ Address Zone::NewExpand(size_t size) {
   // is to avoid excessive malloc() and free() overhead.
   Segment* head = segment_head_;
   const size_t old_size = (head == nullptr) ? 0 : head->size();
-  static const size_t kSegmentOverhead = sizeof(Segment) + kAlignment;
+  static const size_t kSegmentOverhead = sizeof(Segment) + kAlignmentInBytes;
   const size_t new_size_no_overhead = size + (old_size << 1);
   size_t new_size = kSegmentOverhead + new_size_no_overhead;
   const size_t min_new_size = kSegmentOverhead + size;
@@ -173,7 +165,7 @@ Address Zone::NewExpand(size_t size) {
   }
 
   // Recompute 'top' and 'limit' based on the new segment.
-  Address result = RoundUp(segment->start(), kAlignment);
+  Address result = RoundUp(segment->start(), kAlignmentInBytes);
   position_ = result + size;
   // Check for address overflow.
   // (Should not happen since the segment is guaranteed to accomodate

src/zone/zone.h

@@ -63,16 +63,8 @@ class V8_EXPORT_PRIVATE Zone final {
   AccountingAllocator* allocator() const { return allocator_; }
 
  private:
-// All pointers returned from New() have this alignment.  In addition, if the
-// object being allocated has a size that is divisible by 8 then its alignment
-// will be 8. ASan requires 8-byte alignment. MIPS also requires 8-byte
-// alignment.
-#if defined(V8_USE_ADDRESS_SANITIZER) || defined(V8_TARGET_ARCH_MIPS)
-  static const size_t kAlignment = 8;
-  STATIC_ASSERT(kPointerSize <= 8);
-#else
-  static const size_t kAlignment = kPointerSize;
-#endif
+  // All pointers returned from New() are 8-byte aligned.
+  static const size_t kAlignmentInBytes = 8;
 
   // Never allocate segments smaller than this size in bytes.
   static const size_t kMinimumSegmentSize = 8 * KB;

test/unittests/BUILD.gn

@@ -143,6 +143,7 @@ v8_executable("unittests") {
     "wasm/wasm-opcodes-unittest.cc",
     "zone/segmentpool-unittest.cc",
     "zone/zone-chunk-list-unittest.cc",
+    "zone/zone-unittest.cc",
   ]
 
   if (v8_current_cpu == "arm") {

test/unittests/unittests.gyp

@@ -130,6 +130,7 @@
       'value-serializer-unittest.cc',
       'zone/segmentpool-unittest.cc',
       'zone/zone-chunk-list-unittest.cc',
+      'zone/zone-unittest.cc',
       'wasm/asm-types-unittest.cc',
       'wasm/control-transfer-unittest.cc',
       'wasm/decoder-unittest.cc',

test/unittests/zone/zone-unittest.cc

+// Copyright 2017 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.
+
+#include "src/zone/zone.h"
+
+#include "src/zone/accounting-allocator.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace v8 {
+namespace internal {
+
+TEST(Zone, 8ByteAlignment) {
+  AccountingAllocator allocator;
+  Zone zone(&allocator, ZONE_NAME);
+
+  for (size_t i = 0; i < 16; ++i) {
+    ASSERT_EQ(reinterpret_cast<intptr_t>(zone.New(i)) % 8, 0);
+  }
+}
+
+}  // namespace internal
+}  // namespace v8