Add fake data dependencies (instead of disabling GVN) to fix code motion

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 8ee841937e8a145ed33ad1796e431a467003f0d3..ac9f41d4b6711e9c4e92185c41971050f349fc73 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -1668,29 +1668,6 @@ class HCallRuntime: public HCall<0> {
 };
 
 
-class HJSArrayLength: public HUnaryOperation {
- public:
-  explicit HJSArrayLength(HValue* value) : HUnaryOperation(value) {
-    // The length of an array is stored as a tagged value in the array
-    // object. It is guaranteed to be 32 bit integer, but it can be
-    // represented as either a smi or heap number.
-    set_representation(Representation::Tagged());
-    SetFlag(kUseGVN);
-    SetFlag(kDependsOnArrayLengths);
-    SetFlag(kDependsOnMaps);
-  }
-
-  virtual Representation RequiredInputRepresentation(int index) const {
-    return Representation::Tagged();
-  }
-
-  DECLARE_CONCRETE_INSTRUCTION(JSArrayLength)
-
- protected:
-  virtual bool DataEquals(HValue* other) { return true; }
-};
-
-
 class HFixedArrayLength: public HUnaryOperation {
  public:
   explicit HFixedArrayLength(HValue* value) : HUnaryOperation(value) {
@@ -1848,25 +1825,6 @@ class HUnaryMathOperation: public HUnaryOperation {
 };
 
 
-class HLoadElements: public HUnaryOperation {
- public:
-  explicit HLoadElements(HValue* value) : HUnaryOperation(value) {
-    set_representation(Representation::Tagged());
-    SetFlag(kUseGVN);
-    SetFlag(kDependsOnMaps);
-  }
-
-  virtual Representation RequiredInputRepresentation(int index) const {
-    return Representation::Tagged();
-  }
-
-  DECLARE_CONCRETE_INSTRUCTION(LoadElements)
-
- protected:
-  virtual bool DataEquals(HValue* other) { return true; }
-};
-
-
 class HLoadExternalArrayPointer: public HUnaryOperation {
  public:
   explicit HLoadExternalArrayPointer(HValue* value)
@@ -1905,10 +1863,6 @@ class HCheckMap: public HUnaryOperation {
   virtual void PrintDataTo(StringStream* stream);
   virtual HType CalculateInferredType();
 
-#ifdef DEBUG
-  virtual void Verify();
-#endif
-
   Handle<Map> map() const { return map_; }
 
   DECLARE_CONCRETE_INSTRUCTION(CheckMap)
@@ -1976,10 +1930,6 @@ class HCheckInstanceType: public HUnaryOperation {
     return Representation::Tagged();
   }
 
-#ifdef DEBUG
-  virtual void Verify();
-#endif
-
   virtual HValue* Canonicalize() {
     if (!value()->type().IsUninitialized() &&
         value()->type().IsString() &&
@@ -2446,6 +2396,32 @@ class HAccessArgumentsAt: public HTemplateInstruction<3> {
 };
 
 
+class HJSArrayLength: public HBinaryOperation {
+ public:
+  explicit HJSArrayLength(HValue* value, HValue* typecheck)

[Kevin Millikin (Chromium), 2011/07/11 15:03:54]

No need for explicit.

[Jakob Kummerow, 2011/07/19 14:55:44]

Done.

+      : HBinaryOperation(value, typecheck) {
+    // The length of an array is stored as a tagged value in the array
+    // object. It is guaranteed to be 32 bit integer, but it can be
+    // represented as either a smi or heap number.
+    set_representation(Representation::Tagged());
+    SetFlag(kUseGVN);
+    SetFlag(kDependsOnArrayLengths);
+    SetFlag(kDependsOnMaps);

[Kevin Millikin (Chromium), 2011/07/11 15:03:54]

I'm not positive we need this flag anymore, because it depends only on the map
that it's checking.

Everything should be made to work if we remove this flag.  That would enable
moving/eliminating this instruction in the case that some unrelated map has
changed.

The code as written is safe, so leave it as is and we can talk about changing it
when everyone gets back from vacation.

+  }
+
+  virtual Representation RequiredInputRepresentation(int index) const {
+    return Representation::Tagged();
+  }
+
+  HValue* value() { return OperandAt(0); }
+
+  DECLARE_CONCRETE_INSTRUCTION(JSArrayLength)
+
+ protected:
+  virtual bool DataEquals(HValue* other) { return true; }
+};
+
+
 class HBoundsCheck: public HBinaryOperation {
  public:
   HBoundsCheck(HValue* index, HValue* length)
@@ -2458,10 +2434,6 @@ class HBoundsCheck: public HBinaryOperation {
     return Representation::Integer32();
   }
 
-#ifdef DEBUG
-  virtual void Verify();
-#endif
-
   HValue* index() { return left(); }
   HValue* length() { return right(); }
 
@@ -3461,6 +3433,28 @@ class HLoadFunctionPrototype: public HUnaryOperation {
 };
 
 
+class HLoadElements: public HBinaryOperation {
+ public:
+  explicit HLoadElements(HValue* value, HValue* typecheck)

[Kevin Millikin (Chromium), 2011/07/11 15:03:54]

Same, no need for explicit.

[Jakob Kummerow, 2011/07/19 14:55:44]

Obsolete (I undid the changes to HLoadElements).

+      : HBinaryOperation(value, typecheck) {
+    set_representation(Representation::Tagged());
+    SetFlag(kUseGVN);
+    SetFlag(kDependsOnMaps);
+  }
+
+  virtual Representation RequiredInputRepresentation(int index) const {
+    return Representation::Tagged();
+  }
+
+  HValue* value() { return OperandAt(0); }
+
+  DECLARE_CONCRETE_INSTRUCTION(LoadElements)
+
+ protected:
+  virtual bool DataEquals(HValue* other) { return true; }
+};
+
+
 class HLoadKeyedFastElement: public HBinaryOperation {
  public:
   HLoadKeyedFastElement(HValue* obj, HValue* key) : HBinaryOperation(obj, key) {