Bug 1686219 - Store chunk information at the start rather than the en…

…d of the chunk r=sfink

This moves the chunk metadata to the start of the chunk and defines the data structures in the public header.  This simplifies accessing this data and removes the need for hardcoded offsets.

Requesting review from jandem for JIT updates.

Differential Revision: https://phabricator.services.mozilla.com/D101778

js/src/gc/Allocator.cpp

@@ -788,7 +788,7 @@ Chunk* GCRuntime::getOrAllocChunk(AutoLockGCBgAlloc& lock) {
 }
 
 void GCRuntime::recycleChunk(Chunk* chunk, const AutoLockGC& lock) {
-  AlwaysPoison(&chunk->trailer, JS_FREED_CHUNK_PATTERN, sizeof(ChunkTrailer),
+  AlwaysPoison(&chunk->header, JS_FREED_CHUNK_PATTERN, sizeof(ChunkHeader),
                MemCheckKind::MakeNoAccess);
   emptyChunks(lock).push(chunk);
 }
@@ -861,6 +861,9 @@ void Chunk::init(GCRuntime* gc) {
   Poison(this, JS_FRESH_TENURED_PATTERN, ChunkSize,
          MemCheckKind::MakeUndefined);
 
+  new (&header) ChunkHeader(gc->rt);
+  info.init();
+
   /*
    * We clear the bitmap to guard against JS::GCThingIsMarkedGray being called
    * on uninitialized data, which would happen before the first GC cycle.
@@ -873,10 +876,6 @@ void Chunk::init(GCRuntime* gc) {
    */
   decommitAllArenas();
 
-  /* Initialize the chunk info. */
-  info.init();
-  new (&trailer) ChunkTrailer(gc->rt);
-
   /* The rest of info fields are initialized in pickChunk. */
 }
 

js/src/gc/Cell.h

@@ -330,13 +330,13 @@ MOZ_ALWAYS_INLINE bool Cell::isMarkedAtLeast(gc::MarkColor color) const {
 }
 
 inline JSRuntime* Cell::runtimeFromMainThread() const {
-  JSRuntime* rt = chunk()->trailer.runtime;
+  JSRuntime* rt = chunk()->header.runtime;
   MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
   return rt;
 }
 
 inline JSRuntime* Cell::runtimeFromAnyThread() const {
-  return chunk()->trailer.runtime;
+  return chunk()->header.runtime;
 }
 
 inline uintptr_t Cell::address() const {
@@ -354,7 +354,7 @@ Chunk* Cell::chunk() const {
 }
 
 inline StoreBuffer* Cell::storeBuffer() const {
-  return chunk()->trailer.storeBuffer;
+  return chunk()->header.storeBuffer;
 }
 
 JS::Zone* Cell::zone() const {

js/src/gc/GCEnum.h

@@ -18,10 +18,6 @@
 namespace js {
 namespace gc {
 
-// Mark colors. Order is important here: the greater value the 'more marked' a
-// cell is.
-enum class MarkColor : uint8_t { Gray = 1, Black = 2 };
-
 // The phases of an incremental GC.
 #define GCSTATES(D) \
   D(NotActive)      \