Clear next map word when folding allocations into js arrays.

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
diff --git a/src/hydrogen-instructions.cc b/src/hydrogen-instructions.cc
index b0045b8751b06258f0a241416e54615924ea910f..61eb1a098c58bfe6364a6fbf03c5bbf035e16969 100644
--- a/src/hydrogen-instructions.cc
+++ b/src/hydrogen-instructions.cc
@@ -3209,6 +3209,7 @@ void HAllocate::HandleSideEffectDominator(GVNFlag side_effect,
   // First update the size of the dominator allocate instruction.
   int32_t dominator_size_constant =
       HConstant::cast(dominator_size)->GetInteger32Constant();
+  int32_t object_offset = dominator_size_constant;
   int32_t current_size_constant =
       HConstant::cast(current_size)->GetInteger32Constant();
   int32_t new_dominator_size = dominator_size_constant + current_size_constant;
@@ -3238,6 +3239,20 @@ void HAllocate::HandleSideEffectDominator(GVNFlag side_effect,
   new_dominator_size_constant->InsertBefore(dominator_allocate_instr);
   dominator_allocate_instr->UpdateSize(new_dominator_size_constant);
 
+  // TODO(hpayer): This is a short-term hack to make allocation mementos
+  // work again in new space.
+  if (dominator_allocate_instr->type().Equals(HType::JSArray()) &&

[mvstanton, 2013/08/26 10:01:27]

Actually here could you make use of Map::CanTrackAllocationSite(), or at least
extend/generalize that function to also work with an HType? That is a
centralized place for deciding which types have "the right" to use allocation
mementos.

[Hannes Payer (out of office), 2013/08/26 11:51:07]

Thanks for the comment, makes sense. I made a static check method in the
AlloationSite class.

+      !dominator_allocate_instr->clear_next_map_word_) {
+    HObjectAccess access =
+        HObjectAccess::ForJSObjectOffset(object_offset);
+    HStoreNamedField* clear_next_map =
+        HStoreNamedField::New(zone, context(), dominator_allocate_instr, access,
+            block->graph()->GetConstantNull());
+    clear_next_map->ClearAllSideEffects();
+    clear_next_map->InsertAfter(dominator_allocate_instr);
+    dominator_allocate_instr->clear_next_map_word_ = true;
+  }
+
 #ifdef VERIFY_HEAP
   if (FLAG_verify_heap) {
     dominator_allocate_instr->MakePrefillWithFiller();