Bug 1841040 - Remove over-alignment from GCMarker and Nursery,...

Bug 1841040 - Remove over-alignment from GCMarker and Nursery, r=spidermonkey-reviewers,jonco a=RyanVM

js_new<T> cannot guarantee the alignment if T is over-aligned, and this
issue is not trivial to fix (blocked by Bug 1842582).

Add a static assert to detect the attempt using js_new<T> for
over-aligned T, and remove the problematic alignas() attributes as a
short-term fix.

Differential Revision: https://phabricator.services.mozilla.com/D182546

js/public/Utility.h

@@ -478,6 +478,9 @@ static inline void js_free(void* p) {
 #define JS_DECLARE_NEW_METHODS(NEWNAME, ALLOCATOR, QUALIFIERS)              \
   template <class T, typename... Args>                                      \
   QUALIFIERS T* MOZ_HEAP_ALLOCATOR NEWNAME(Args&&... args) {                \
+    static_assert(                                                          \
+        alignof(T) <= alignof(max_align_t),                                 \
+        "over-aligned type is not supported by JS_DECLARE_NEW_METHODS");    \
     void* memory = ALLOCATOR(sizeof(T));                                    \
     return MOZ_LIKELY(memory) ? new (memory) T(std::forward<Args>(args)...) \
                               : nullptr;                                    \
@@ -494,6 +497,9 @@ static inline void js_free(void* p) {
 #define JS_DECLARE_NEW_ARENA_METHODS(NEWNAME, ALLOCATOR, QUALIFIERS)           \
   template <class T, typename... Args>                                         \
   QUALIFIERS T* MOZ_HEAP_ALLOCATOR NEWNAME(arena_id_t arena, Args&&... args) { \
+    static_assert(                                                             \
+        alignof(T) <= alignof(max_align_t),                                    \
+        "over-aligned type is not supported by JS_DECLARE_NEW_ARENA_METHODS"); \
     void* memory = ALLOCATOR(arena, sizeof(T));                                \
     return MOZ_LIKELY(memory) ? new (memory) T(std::forward<Args>(args)...)    \
                               : nullptr;                                       \

js/src/gc/GCMarker.h

@@ -274,7 +274,7 @@ enum ShouldReportMarkTime : bool {
 
 } /* namespace gc */
 
-class alignas(TypicalCacheLineSize) GCMarker {
+class GCMarker {
   enum MarkingState : uint8_t {
     // Have not yet started marking.
     NotActive,

js/src/gc/Nursery.h

@@ -67,7 +67,7 @@ class GCSchedulingTunables;
 class TenuringTracer;
 }  // namespace gc
 
-class alignas(TypicalCacheLineSize) Nursery {
+class Nursery {
  public:
   explicit Nursery(gc::GCRuntime* gc);
   ~Nursery();