heap: Untangle HeapAllocator object size computation from Oilpan

- Move kMaxHeapObjectSize to HeapAllocator as Oilpan has no internal
  restrictions on object sizing other than ultimately running OOM.
- Size quantization for Oilpan can just return whatever is required to
  match capacity needs. Internally, sizes may be padded and aligned
  accordingly.

Bug: 1056170
Change-Id: I5c833a2b6cb5354bd24b117406e68fe18e622f71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2551121Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829657}

third_party/blink/renderer/platform/heap/impl/heap_allocator.h

@@ -68,6 +68,10 @@ class PLATFORM_EXPORT HeapAllocator {
   using Visitor = blink::Visitor;
   static constexpr bool kIsGarbageCollected = true;
 
+  // See wtf/size_t.h for details.
+  static constexpr size_t kMaxHeapObjectSizeLog2 = 27;
+  static constexpr size_t kMaxHeapObjectSize = 1 << kMaxHeapObjectSizeLog2;
+
   template <typename T>
   static size_t MaxElementCountInBackingStore() {
     return kMaxHeapObjectSize / sizeof(T);
@@ -75,10 +79,12 @@ class PLATFORM_EXPORT HeapAllocator {
 
   template <typename T>
   static size_t QuantizedSize(size_t count) {
-    CHECK(count <= MaxElementCountInBackingStore<T>());
-    return ThreadHeap::AllocationSizeFromSize(count * sizeof(T)) -
-           sizeof(HeapObjectHeader);
+    CHECK_LE(count, MaxElementCountInBackingStore<T>());
+    // Oilpan's internal size is independent of MaxElementCountInBackingStore()
+    // and the required size to match capacity needs.
+    return count * sizeof(T);
   }
+
   template <typename T>
   static T* AllocateVectorBacking(size_t size) {
     return reinterpret_cast<T*>(

third_party/blink/renderer/platform/heap/impl/heap_page.h

@@ -92,8 +92,6 @@ BlinkGuardPageSize() {
 static_assert(8 == sizeof(double), "We expect sizeof(double) to be 8");
 constexpr size_t kAllocationGranularity = sizeof(double);
 constexpr size_t kAllocationMask = kAllocationGranularity - 1;
-constexpr size_t kMaxHeapObjectSizeLog2 = 27;
-constexpr size_t kMaxHeapObjectSize = 1 << kMaxHeapObjectSizeLog2;
 constexpr size_t kLargeObjectSizeThreshold = kBlinkPageSize / 2;
 
 // A zap value used for freed memory that is allowed to be added to the free

third_party/blink/renderer/platform/heap/test/heap_test.cc

@@ -2191,7 +2191,7 @@ TEST_F(HeapTest, MAYBE_LargeHashMap) {
 
   // Try to allocate a HashTable larger than kMaxHeapObjectSize
   // (crbug.com/597953).
-  wtf_size_t size = kMaxHeapObjectSize /
+  wtf_size_t size = HeapAllocator::kMaxHeapObjectSize /
                     sizeof(HeapHashMap<int, Member<IntWrapper>>::ValueType);
   Persistent<HeapHashMap<int, Member<IntWrapper>>> map =
       MakeGarbageCollected<HeapHashMap<int, Member<IntWrapper>>>();
@@ -2204,7 +2204,8 @@ TEST_F(HeapTest, LargeVector) {
 
   // Try to allocate a HeapVectors larger than kMaxHeapObjectSize
   // (crbug.com/597953).
-  const wtf_size_t size = kMaxHeapObjectSize / sizeof(Member<IntWrapper>);
+  const wtf_size_t size =
+      HeapAllocator::kMaxHeapObjectSize / sizeof(Member<IntWrapper>);
   Persistent<HeapVector<Member<IntWrapper>>> vector =
       MakeGarbageCollected<HeapVector<Member<IntWrapper>>>(size);
   EXPECT_LE(size, vector->capacity());