Improve GVN handling of ElementTransitions.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index b2b3a6183047e2cc0290e2a28fec101b0289e9fb..b55fe25b38bbbe0f6b49bda5dd274575e19ec32d 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -186,6 +186,7 @@ class LChunkBuilder;
   V(InobjectFields)                            \
   V(BackingStoreFields)                        \
   V(ElementsKind)                              \
+  V(ElementsPointer)                           \
   V(ArrayElements)                             \
   V(DoubleArrayElements)                       \
   V(SpecializedArrayElements)                  \
@@ -646,6 +647,18 @@ class HValue: public ZoneObject {
     return gvn_flags_.ContainsAnyOf(AllObservableSideEffectsFlagSet());
   }
 
+  GVNFlagSet DependsOnFlags() const {
+    GVNFlagSet result = gvn_flags_;
+    result.Intersect(AllDependsOnFlagSet());
+    return result;
+  }
+
+  GVNFlagSet SideEffectFlags() const {
+    GVNFlagSet result = gvn_flags_;
+    result.Intersect(AllSideEffectsFlagSet());
+    return result;
+  }
+
   GVNFlagSet ChangesFlags() const {
     GVNFlagSet result = gvn_flags_;
     result.Intersect(AllChangesFlagSet());
@@ -659,6 +672,10 @@ class HValue: public ZoneObject {
     return result;
   }
 
+  virtual bool HasOneTimeSideEffects() const {
+    return false;
+  }
+
   Range* range() const { return range_; }
   bool HasRange() const { return range_ != NULL; }
   void AddNewRange(Range* r);
@@ -722,6 +739,15 @@ class HValue: public ZoneObject {
     representation_ = r;
   }
 
+  static GVNFlagSet AllDependsOnFlagSet() {
+    GVNFlagSet result;
+    // Create changes mask.
+#define ADD_FLAG(type) result.Add(kDependsOn##type);
+  GVN_FLAG_LIST(ADD_FLAG)
+#undef ADD_FLAG
+    return result;
+  }
+
   static GVNFlagSet AllChangesFlagSet() {
     GVNFlagSet result;
     // Create changes mask.
@@ -743,6 +769,8 @@ class HValue: public ZoneObject {
   static GVNFlagSet AllObservableSideEffectsFlagSet() {
     GVNFlagSet result = AllChangesFlagSet();
     result.Remove(kChangesElementsKind);
+    result.Remove(kChangesElementsPointer);
+    result.Remove(kChangesMaps);
     return result;
   }
 
@@ -1920,8 +1948,7 @@ class HLoadElements: public HUnaryOperation {
   explicit HLoadElements(HValue* value) : HUnaryOperation(value) {
     set_representation(Representation::Tagged());
     SetFlag(kUseGVN);
-    SetGVNFlag(kDependsOnMaps);
-    SetGVNFlag(kDependsOnElementsKind);
+    SetGVNFlag(kDependsOnElementsPointer);
   }
 
   virtual Representation RequiredInputRepresentation(int index) {
@@ -1972,6 +1999,11 @@ class HCheckMap: public HTemplateInstruction<2> {
     set_representation(Representation::Tagged());
     SetFlag(kUseGVN);
     SetGVNFlag(kDependsOnMaps);
+    // If the map to check doesn't have the untransitioned elements, it must not
+    // be hoisted above TransitionElements instructions.
+    if (mode == REQUIRE_EXACT_MAP || !map->has_fast_smi_only_elements()) {
+      SetGVNFlag(kDependsOnElementsKind);
+    }
     has_element_transitions_ =
         map->LookupElementsTransitionMap(FAST_DOUBLE_ELEMENTS, NULL) != NULL ||
         map->LookupElementsTransitionMap(FAST_ELEMENTS, NULL) != NULL;
@@ -4135,7 +4167,17 @@ class HTransitionElementsKind: public HTemplateInstruction<1> {
         transitioned_map_(transitioned_map) {
     SetOperandAt(0, object);
     SetFlag(kUseGVN);
+    SetGVNFlag(kDependsOnMaps);

[fschneider, 2012/02/02 14:59:50]

There is currently no instruction that has kChangesMaps set (except calls that
have all side effects).

[danno, 2012/02/06 16:54:57]

But if we ever add one, then it will be incorrect to hoist the transitions about
it, so I'd prefer to keep this in.
On 2012/02/02 14:59:50, fschneider wrote:

     SetGVNFlag(kChangesElementsKind);
+    if (original_map->has_fast_double_elements()) {
+      SetGVNFlag(kChangesElementsPointer);
+      SetGVNFlag(kDependsOnElementsPointer);
+      SetGVNFlag(kDependsOnDoubleArrayElements);
+    } else if (transitioned_map->has_fast_double_elements()) {
+      SetGVNFlag(kChangesElementsPointer);
+      SetGVNFlag(kDependsOnElementsPointer);
+      SetGVNFlag(kDependsOnArrayElements);
+    }
     set_representation(Representation::Tagged());
   }
 
@@ -4143,6 +4185,10 @@ class HTransitionElementsKind: public HTemplateInstruction<1> {
     return Representation::Tagged();
   }
 
+  virtual bool HasOneTimeSideEffects() const {

[fschneider, 2012/02/02 14:59:50]

It may be not worth making a predicate that applies to exactly one instruction a
separate virtual function (unless we plan to get more instructions like this).

 I'd just use IsTransitionElementsKind() for now.

[danno, 2012/02/06 16:54:57]

Done.

+    return true;
+  }
+
   HValue* object() { return OperandAt(0); }
   Handle<Map> original_map() { return original_map_; }
   Handle<Map> transitioned_map() { return transitioned_map_; }