[runtime] Allow Map::HasFastPointerLayout during GC

Use relaxed reads to allow changing the pointer concurrently in the 
GC. The layout descriptor will either be fast and there's nothing
to be done, or it will be slow, and we will use the forwarding
pointer to determine the proper version to use.

Bug: chromium:726040
Change-Id: I0a376752c3a99abf0874070387fcaeb3cee0dcb2
Reviewed-on: https://chromium-review.googlesource.com/519346
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45652}

src/objects-inl.h

@@ -5177,15 +5177,14 @@ void Map::set_prototype(Object* value, WriteBarrierMode mode) {
   CONDITIONAL_WRITE_BARRIER(GetHeap(), this, kPrototypeOffset, value, mode);
 }
 
-
-LayoutDescriptor* Map::layout_descriptor_gc_safe() {
-  Object* layout_desc = READ_FIELD(this, kLayoutDescriptorOffset);
+LayoutDescriptor* Map::layout_descriptor_gc_safe() const {
+  Object* layout_desc = RELAXED_READ_FIELD(this, kLayoutDescriptorOffset);
   return LayoutDescriptor::cast_gc_safe(layout_desc);
 }
 
 
 bool Map::HasFastPointerLayout() const {
-  Object* layout_desc = READ_FIELD(this, kLayoutDescriptorOffset);
+  Object* layout_desc = RELAXED_READ_FIELD(this, kLayoutDescriptorOffset);
   return LayoutDescriptor::IsFastPointerLayout(layout_desc);
 }
 

src/objects/map.h

@@ -354,7 +354,7 @@ class Map : public HeapObject {
   // [layout descriptor]: describes the object layout.
   DECL_ACCESSORS(layout_descriptor, LayoutDescriptor)
   // |layout descriptor| accessor which can be used from GC.
-  inline LayoutDescriptor* layout_descriptor_gc_safe();
+  inline LayoutDescriptor* layout_descriptor_gc_safe() const;
   inline bool HasFastPointerLayout() const;
 
   // |layout descriptor| accessor that is safe to call even when