cppgc: Mark object as constructed using std::atomic::fetch_or

Replace a manual fetch/or/store sequence with fetch_or.

Bug: chromium:1056170
Change-Id: Ib18d26686aa6e822f91b5b6662dd0bdcfafe5092
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2531788
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71129}

include/cppgc/allocation.h

@@ -39,9 +39,8 @@ class V8_EXPORT MakeGarbageCollectedTraitInternal {
             const_cast<uint16_t*>(reinterpret_cast<const uint16_t*>(
                 reinterpret_cast<const uint8_t*>(payload) -
                 api_constants::kFullyConstructedBitFieldOffsetFromPayload)));
-    uint16_t value = atomic_mutable_bitfield->load(std::memory_order_relaxed);
-    value = value | api_constants::kFullyConstructedBitMask;
-    atomic_mutable_bitfield->store(value, std::memory_order_release);
+    atomic_mutable_bitfield->fetch_or(api_constants::kFullyConstructedBitMask,
+                                      std::memory_order_release);
   }
 
   static void* Allocate(cppgc::AllocationHandle& handle, size_t size,

include/cppgc/internal/api-constants.h

@@ -28,7 +28,7 @@ constexpr size_t kGB = kMB * 1024;
 static constexpr size_t kFullyConstructedBitFieldOffsetFromPayload =
     2 * sizeof(uint16_t);
 // Mask for in-construction bit.
-static constexpr size_t kFullyConstructedBitMask = size_t{1};
+static constexpr uint16_t kFullyConstructedBitMask = uint16_t{1};
 
 static constexpr size_t kPageSize = size_t{1} << 17;