cppgc: Drop high-address stack assumption

The assumption doesn't necessarily hold on linux and Android either.

Bug: chromium:1056170, chromium:1239287
Change-Id: Ibb0d8f5f814580bff4e8a7dce9a3397df1385896
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3097273Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76299}

include/cppgc/internal/write-barrier.h

@@ -168,17 +168,9 @@ class V8_EXPORT WriteBarrierTypeForCagedHeapPolicy final {
 
   static V8_INLINE bool TryGetCagedHeap(const void* slot, const void* value,
                                         WriteBarrier::Params& params) {
-    if (!Platform::StackAddressesSmallerThanHeapAddresses()) {
-      // This method assumes that the stack is allocated in high
-      // addresses. That is not guaranteed on Windows and Fuchsia. Having a
-      // low-address (below api_constants::kCagedHeapReservationSize) on-stack
-      // slot with a nullptr value would cause this method to erroneously return
-      // that the slot resides in a caged heap that starts at a null address.
-      // This check is applied only on Windows because it is not an issue on
-      // other OSes where the stack resides in higher adderesses, and to keep
-      // the write barrier as cheap as possible.
-      if (!value) return false;
-    }
+    // TODO(chromium:1056170): Check if the null check can be folded in with
+    // the rest of the write barrier.
+    if (!value) return false;
     params.start = reinterpret_cast<uintptr_t>(value) &
                    ~(api_constants::kCagedHeapReservationAlignment - 1);
     const uintptr_t slot_offset =

include/cppgc/platform.h

@@ -29,8 +29,6 @@ using TracingController = v8::TracingController;
  */
 class V8_EXPORT Platform {
  public:
-  static constexpr bool StackAddressesSmallerThanHeapAddresses();
-
   virtual ~Platform() = default;
 
   /**
@@ -151,15 +149,6 @@ V8_EXPORT void Abort();
 
 }  // namespace internal
 
-// static
-constexpr bool Platform::StackAddressesSmallerThanHeapAddresses() {
-#if V8_OS_WIN || V8_OS_FUCHSIA || V8_OS_MACOSX
-  return false;
-#else
-  return true;
-#endif  // V8_OS_WIN || V8_OS_FUCHSIA || V8_OS_MACOSX
-}
-
 }  // namespace cppgc
 
 #endif  // INCLUDE_CPPGC_PLATFORM_H_

src/heap/cppgc/caged-heap.cc

@@ -74,11 +74,6 @@ CagedHeap::CagedHeap(HeapBase* heap_base, PageAllocator* platform_allocator)
     : reserved_area_(ReserveCagedHeap(platform_allocator)) {
   using CagedAddress = CagedHeap::AllocatorType::Address;
 
-  if (Platform::StackAddressesSmallerThanHeapAddresses()) {
-    // Write barrier assumes that caged heap is allocated below the stack.
-    CHECK_LT(reserved_area_.address(), v8::base::Stack::GetStackStart());
-  }
-
   DCHECK_NOT_NULL(heap_base);
 
   CHECK(platform_allocator->SetPermissions(