Reland r20974: Unify and simplify the FastCloneShallowArrayStub

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HYDROGEN_INSTRUCTIONS_H_
 #define V8_HYDROGEN_INSTRUCTIONS_H_
 
 #include "v8.h"
 
 #include "allocation.h"
@@ skipping 601 matching lines @@
     // Set after an instruction is killed.
     kIsDead,
     // Instructions that are allowed to produce full range unsigned integer
     // values are marked with kUint32 flag. If arithmetic shift or a load from
     // EXTERNAL_UINT32_ELEMENTS array is not marked with this flag
     // it will deoptimize if result does not fit into signed integer range.
     // HGraph::ComputeSafeUint32Operations is responsible for setting this
     // flag.
     kUint32,
     kHasNoObservableSideEffects,
+    // Indicates an instruction shouldn't be replaced by optimization, this flag
+    // is useful to set in cases where recomputing a value is cheaper than
+    // extending the value's live range and spilling it.
+    kCantBeReplaced,
     // Indicates the instruction is live during dead code elimination.
     kIsLive,
 
     // HEnvironmentMarkers are deleted before dead code
     // elimination takes place, so they can repurpose the kIsLive flag:
     kEndsLiveRange = kIsLive,
 
     // TODO(everyone): Don't forget to update this!
     kLastFlag = kIsLive
   };
@@ skipping 5618 matching lines @@
     if (index == 0 && access().IsExternalMemory()) {
       // object must be external in case of external memory access
       return Representation::External();
     }
     return Representation::Tagged();
   }
   virtual Range* InferRange(Zone* zone) V8_OVERRIDE;
   virtual void PrintDataTo(StringStream* stream) V8_OVERRIDE;
 
   bool CanBeReplacedWith(HValue* other) const {
+    if (!CheckFlag(HValue::kCantBeReplaced)) return false;
     if (!type().Equals(other->type())) return false;
     if (!representation().Equals(other->representation())) return false;
     if (!other->IsLoadNamedField()) return true;
     HLoadNamedField* that = HLoadNamedField::cast(other);
     if (this->maps_ == that->maps_) return true;
     if (this->maps_ == NULL || that->maps_ == NULL) return false;
     return this->maps_->IsSubset(that->maps_);
   }
 
   DECLARE_CONCRETE_INSTRUCTION(LoadNamedField)
@@ skipping 1438 matching lines @@
   virtual bool IsDeletable() const V8_OVERRIDE { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_