Bug 1863920 - Align mark bits to cache line size r=sfink

Differential Revision: https://phabricator.services.mozilla.com/D193187

js/public/HeapAPI.h

@@ -29,6 +29,10 @@ class NurseryDecommitTask;
 
 JS_PUBLIC_API bool CurrentThreadCanAccessZone(JS::Zone* zone);
 
+// To prevent false sharing, some data structures are aligned to a typical cache
+// line size.
+static constexpr size_t TypicalCacheLineSize = 64;
+
 namespace gc {
 
 class Arena;
@@ -183,7 +187,7 @@ enum class ColorBit : uint32_t { BlackBit = 0, GrayOrBlackBit = 1 };
 enum class MarkColor : uint8_t { Gray = 1, Black = 2 };
 
 // Mark bitmap for a tenured heap chunk.
-struct MarkBitmap {
+struct alignas(TypicalCacheLineSize) MarkBitmap {
   static constexpr size_t WordCount = ArenaBitmapWords * ArenasPerChunk;
   MarkBitmapWord bitmap[WordCount];
 

js/src/gc/Heap.cpp

@@ -610,7 +610,10 @@ void ChunkPool::verifyChunks() const {
   }
 }
 
-void TenuredChunk::verify() const {
+void TenuredChunk::verify() {
+  MOZ_ASSERT(
+      (uintptr_t(markBits.arenaBits(&arenas[0])) % TypicalCacheLineSize) == 0);
+
   MOZ_ASSERT(info.numArenasFree <= ArenasPerChunk);
   MOZ_ASSERT(info.numArenasFreeCommitted <= info.numArenasFree);
 

js/src/gc/Heap.h

@@ -21,10 +21,6 @@ class AutoLockGC;
 class AutoLockGCBgAlloc;
 class Nursery;
 
-// To prevent false sharing, some data structures are aligned to a typical cache
-// line size.
-static constexpr size_t TypicalCacheLineSize = 64;
-
 namespace gc {
 
 class Arena;
@@ -710,9 +706,9 @@ class TenuredChunk : public TenuredChunkBase {
   Arena* fetchNextFreeArena(GCRuntime* gc);
 
 #ifdef DEBUG
-  void verify() const;
+  void verify();
 #else
-  void verify() const {}
+  void verify() {}
 #endif
 
  private:

js/src/gc/Verifier.cpp

@@ -507,12 +507,15 @@ void js::gc::MarkingValidator::nonIncrementalMark(AutoGCSession& session) {
     AutoLockGC lock(gc);
     for (auto chunk = gc->allNonEmptyChunks(lock); !chunk.done();
          chunk.next()) {
-      MarkBitmap* bitmap = &chunk->markBits;
-      auto entry = MakeUnique<MarkBitmap>();
-      if (!entry) {
+      // Bug 1842582: Allocate mark bit buffer in two stages to avoid alignment
+      // restriction which we currently can't support.
+      void* buffer = js_malloc(sizeof(MarkBitmap));
+      if (!buffer) {
         return;
       }
+      UniquePtr<MarkBitmap> entry(new (buffer) MarkBitmap);
 
+      MarkBitmap* bitmap = &chunk->markBits;
       memcpy((void*)entry->bitmap, (void*)bitmap->bitmap,
              sizeof(bitmap->bitmap));