Allow optimistically hoisting elements transitions over accesses.

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4643 matching lines @@
 
 class HTransitionElementsKind: public HTemplateInstruction<1> {
  public:
   HTransitionElementsKind(HValue* object,
                           Handle<Map> original_map,
                           Handle<Map> transitioned_map)
       : original_map_(original_map),
         transitioned_map_(transitioned_map) {
     SetOperandAt(0, object);
     SetFlag(kUseGVN);
-    // Don't set GVN DependOn flags here. That would defeat GVN's detection of
-    // congruent HTransitionElementsKind instructions. Instruction hoisting
-    // handles HTransitionElementsKind instruction specially, explicitly adding
-    // DependsOn flags during its dependency calculations.
     SetGVNFlag(kChangesElementsKind);
-    if (original_map->has_fast_double_elements()) {
-      SetGVNFlag(kChangesElementsPointer);
-      SetGVNFlag(kChangesNewSpacePromotion);
-    }
-    if (transitioned_map->has_fast_double_elements()) {
-      SetGVNFlag(kChangesElementsPointer);
-      SetGVNFlag(kChangesNewSpacePromotion);
-    }
+    SetGVNFlag(kChangesElementsPointer);
+    SetGVNFlag(kChangesNewSpacePromotion);

[Michael Starzinger, 2012/10/01 13:13:48]

The kChangesNewSpacePromotion is mainly used for write-barrier removal. With
this change you defeat write-barrier removal for transitions that don't involve
doubles and hence won't allocate. Is this what you want?

[danno, 2012/10/01 14:08:47]

Please restore the original code with ifs, but still remove the comment, which
is no longer valid.

[Toon Verwaest, 2012/10/01 14:57:51]

Done.

     set_representation(Representation::Tagged());
   }
 
   virtual Representation RequiredInputRepresentation(int index) {
     return Representation::Tagged();
   }
 
   HValue* object() { return OperandAt(0); }
   Handle<Map> original_map() { return original_map_; }
   Handle<Map> transitioned_map() { return transitioned_map_; }
@@ skipping 595 matching lines @@
   DECLARE_CONCRETE_INSTRUCTION(LoadFieldByIndex);
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_