cpgpc: pointer-compression: Use global variable for base pointer

Now that the cage is shared, we can remove the thread-local base and
instead use a simple global.

Bug: chromium:1325007
Change-Id: I05019de83868f823c66003740e277578f2a1d439
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3688051
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81037}

include/cppgc/member.h

@@ -40,7 +40,7 @@ class CageBaseGlobal final {
   static constexpr uintptr_t kLowerHalfWordMask =
       (api_constants::kCagedHeapReservationAlignment - 1);
 
-  static thread_local V8_EXPORT uintptr_t g_base_
+  static V8_EXPORT uintptr_t g_base_
 #if !V8_CC_MSVC
       __attribute__((require_constant_initialization))
 #endif  // !V8_CC_MSVC

src/heap/cppgc/caged-heap.cc

@@ -84,8 +84,7 @@ CagedHeap::CagedHeap(PageAllocator& platform_allocator)
 #if defined(CPPGC_POINTER_COMPRESSION)
   // With pointer compression only single heap per thread is allowed.
   CHECK(!CageBaseGlobal::IsSet());
-  CageBaseGlobalUpdater::UpdateCageBase(
-      reinterpret_cast<uintptr_t>(reserved_area_.address()));
+  CageBaseGlobalUpdater::UpdateCageBase(CagedHeapBase::g_heap_base_);
 #endif  // defined(CPPGC_POINTER_COMPRESSION)
 
   const bool is_not_oom = platform_allocator.SetPermissions(

src/heap/cppgc/concurrent-marker.cc

@@ -54,32 +54,6 @@ bool HasWorkForConcurrentMarking(MarkingWorklists& marking_worklists) {
               ->IsEmpty();
 }
 
-#if defined(CPPGC_POINTER_COMPRESSION)
-namespace {
-
-// The concurrent marking task can run from a thread where no cage-base is set.
-// Moreover, it can run from a thread which has another heap attached. Make sure
-// to set/reset the base. This also works for the main thread joining the
-// marking.
-class PointerCompressionCageScope final {
- public:
-  explicit PointerCompressionCageScope(HeapBase& heap)
-      : prev_cage_base_(CageBaseGlobalUpdater::GetCageBase()) {
-    CageBaseGlobalUpdater::UpdateCageBase(
-        reinterpret_cast<uintptr_t>(heap.caged_heap().base()));
-  }
-
-  ~PointerCompressionCageScope() {
-    CageBaseGlobalUpdater::UpdateCageBase(prev_cage_base_);
-  }
-
- private:
-  const uintptr_t prev_cage_base_;
-};
-
-}  // namespace
-#endif  // defined(CPPGC_POINTER_COMPRESSION)
-
 class ConcurrentMarkingTask final : public v8::JobTask {
  public:
   explicit ConcurrentMarkingTask(ConcurrentMarkerBase&);
@@ -102,10 +76,6 @@ void ConcurrentMarkingTask::Run(JobDelegate* job_delegate) {
   StatsCollector::EnabledConcurrentScope stats_scope(
       concurrent_marker_.heap().stats_collector(),
       StatsCollector::kConcurrentMark);
-#if defined(CPPGC_POINTER_COMPRESSION)
-  PointerCompressionCageScope cage_base_resetter(concurrent_marker_.heap());
-#endif  // defined(CPPGC_POINTER_COMPRESSION)
-
   if (!HasWorkForConcurrentMarking(concurrent_marker_.marking_worklists()))
     return;
   ConcurrentMarkingState concurrent_marking_state(

src/heap/cppgc/member.cc

@@ -8,8 +8,7 @@ namespace cppgc {
 namespace internal {
 
 #if defined(CPPGC_POINTER_COMPRESSION)
-thread_local uintptr_t CageBaseGlobal::g_base_ =
-    CageBaseGlobal::kLowerHalfWordMask;
+uintptr_t CageBaseGlobal::g_base_ = CageBaseGlobal::kLowerHalfWordMask;
 #endif  // defined(CPPGC_POINTER_COMPRESSION)
 
 }  // namespace internal