cppgc: Fix GetHeapFromPayload

GetHeapFromPayload returned the page header address instead of the
value of the first field of the header.

Bug: chromium:1056170
Change-Id: I4de5be975accced32460d6fab91543e6a5b07ba0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2143825
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67079}

include/cppgc/internal/accessors.h

@@ -5,14 +5,16 @@
 #ifndef INCLUDE_CPPGC_INTERNAL_ACCESSORS_H_
 #define INCLUDE_CPPGC_INTERNAL_ACCESSORS_H_
 
-#include "include/cppgc/heap.h"
 #include "include/cppgc/internal/api-constants.h"
 
 namespace cppgc {
+
+class Heap;
+
 namespace internal {
 
 inline cppgc::Heap* GetHeapFromPayload(const void* payload) {
-  return reinterpret_cast<cppgc::Heap*>(
+  return *reinterpret_cast<cppgc::Heap**>(
       (reinterpret_cast<uintptr_t>(payload) & api_constants::kPageBaseMask) +
       api_constants::kHeapOffset);
 }

test/unittests/heap/cppgc/heap-page_unittest.cc

@@ -3,6 +3,8 @@
 // found in the LICENSE file.
 
 #include "src/heap/cppgc/heap-page.h"
+#include "include/cppgc/allocation.h"
+#include "include/cppgc/internal/accessors.h"
 #include "test/unittests/heap/cppgc/tests.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -13,6 +15,8 @@ namespace {
 
 class PageTest : public testing::TestWithHeap {};
 
+class GCed : public GarbageCollected<GCed> {};
+
 }  // namespace
 
 TEST_F(PageTest, PageLayout) {
@@ -20,5 +24,10 @@ TEST_F(PageTest, PageLayout) {
   NormalPage::Destroy(np);
 }
 
+TEST_F(PageTest, GetHeapForAllocatedObject) {
+  GCed* gced = MakeGarbageCollected<GCed>(GetHeap());
+  EXPECT_EQ(GetHeap(), GetHeapFromPayload(gced));
+}
+
 }  // namespace internal
 }  // namespace cppgc