Bug 1830851 - Optimize post barrier code for JSAtom. r=jonco

Atoms are always tenured so treat them similar to `TenuredCell` types.
This should get rid of some post barrier code when atomizing strings.

Differential Revision: https://phabricator.services.mozilla.com/D176898

Author: jandem

js/src/gc/Cell.h

@@ -856,6 +856,14 @@ class alignas(gc::CellAlignBytes) CellWithTenuredGCPointer : public BaseCell {
 
 void CellHeaderPostWriteBarrier(JSObject** ptr, JSObject* prev, JSObject* next);
 
+template <typename T>
+constexpr inline bool GCTypeIsTenured() {
+  static_assert(std::is_base_of_v<Cell, T>);
+  static_assert(!std::is_same_v<Cell, T> && !std::is_same_v<TenuredCell, T>);
+
+  return std::is_base_of_v<TenuredCell, T> || std::is_base_of_v<JSAtom, T>;
+}
+
 template <class PtrT>
 class alignas(gc::CellAlignBytes) TenuredCellWithGCPointer
     : public TenuredCell {
@@ -869,7 +877,7 @@ class alignas(gc::CellAlignBytes) TenuredCellWithGCPointer
         std::is_base_of_v<Cell, PtrT>,
         "Only use TenuredCellWithGCPointer for pointers to GC things");
     static_assert(
-        !std::is_base_of_v<TenuredCell, PtrT>,
+        !GCTypeIsTenured<PtrT>,
         "Don't use TenuredCellWithGCPointer for always-tenured GC things");
   }
 

js/src/gc/StoreBuffer.h

@@ -15,6 +15,7 @@
 
 #include "ds/BitArray.h"
 #include "ds/LifoAlloc.h"
+#include "gc/Cell.h"
 #include "gc/Nursery.h"
 #include "gc/TraceKind.h"
 #include "js/AllocPolicy.h"
@@ -641,7 +642,7 @@ MOZ_ALWAYS_INLINE void PostWriteBarrier(T** vp, T* prev, T* next) {
   static_assert(std::is_base_of_v<Cell, T>);
   static_assert(!std::is_same_v<Cell, T> && !std::is_same_v<TenuredCell, T>);
 
-  if constexpr (!std::is_base_of_v<TenuredCell, T>) {
+  if constexpr (!GCTypeIsTenured<T>()) {
     using BaseT = typename BaseGCType<T>::type;
     PostWriteBarrierImpl<BaseT>(vp, prev, next);
     return;

js/src/gc/Tenuring.cpp

@@ -385,8 +385,7 @@ void js::gc::StoreBuffer::WholeCellBuffer::trace(TenuringTracer& mover) {
 template <typename T>
 void js::gc::StoreBuffer::CellPtrEdge<T>::trace(TenuringTracer& mover) const {
   static_assert(std::is_base_of_v<Cell, T>, "T must be a Cell type");
-  static_assert(!std::is_base_of_v<TenuredCell, T>,
-                "T must not be a tenured Cell type");
+  static_assert(!GCTypeIsTenured<T>(), "T must not be a tenured Cell type");
 
   T* thing = *edge;
   if (!thing) {