cppgc: young-gen: Return range size for AgeTable to 4KB

Allocation granularity (8 bytes on 64bit platforms) is already subsumed
by whatever range size we choose.

Bug: chromium:1029379
Change-Id: Iab95e6f36955c9ffbbe9ea0c98cb9d1f8dd0337b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3629869Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80415}

include/cppgc/internal/caged-heap-local-data.h

@@ -26,7 +26,7 @@ class HeapBase;
 #if defined(CPPGC_YOUNG_GENERATION)
 
 // AgeTable is the bytemap needed for the fast generation check in the write
-// barrier. AgeTable contains entries that correspond to 512 bytes memory
+// barrier. AgeTable contains entries that correspond to 4096 bytes memory
 // regions (cards). Each entry in the table represents generation of the objects
 // that reside on the corresponding card (young, old or mixed).
 class V8_EXPORT AgeTable final {
@@ -42,8 +42,7 @@ class V8_EXPORT AgeTable final {
   enum class AdjacentCardsPolicy : uint8_t { kConsider, kIgnore };
 
   static constexpr size_t kCardSizeInBytes =
-      (api_constants::kCagedHeapReservationSize / kAllocationGranularity) /
-      kRequiredSize;
+      api_constants::kCagedHeapReservationSize / kRequiredSize;
 
   void SetAge(uintptr_t cage_offset, Age age) {
     table_[card(cage_offset)] = age;
@@ -67,7 +66,7 @@ class V8_EXPORT AgeTable final {
         __builtin_ctz(static_cast<uint32_t>(kCardSizeInBytes));
 #else   //! V8_HAS_BUILTIN_CTZ
         // Hardcode and check with assert.
-        9;
+        12;
 #endif  // !V8_HAS_BUILTIN_CTZ
     static_assert((1 << kGranularityBits) == kCardSizeInBytes);
     const size_t entry = offset >> kGranularityBits;

test/unittests/heap/cppgc/age-table-unittest.cc

@@ -196,5 +196,15 @@ TEST_F(AgeTableTest, SetAgeForMultipleCardsConsiderAdjacentCards) {
   EXPECT_EQ(Age::kYoung, GetAge(object_end));
 }
 
+TEST_F(AgeTableTest, MarkAllCardsAsYoung) {
+  void* heap_start = Heap::From(GetHeap())->caged_heap().base();
+  void* heap_end =
+      static_cast<uint8_t*>(heap_start) + kCagedHeapReservationSize - 1;
+  AssertAgeForAddressRange(heap_start, heap_end, Age::kOld);
+  SetAgeForAddressRange(heap_start, heap_end, Age::kYoung,
+                        AdjacentCardsPolicy::kIgnore);
+  AssertAgeForAddressRange(heap_start, heap_end, Age::kYoung);
+}
+
 }  // namespace internal
 }  // namespace cppgc