Allow removal of obsolete map checks after transitions.

This allows side effect dominator tracking to remove map checks that are
dominated by a single HStoreNamedField that performs a transition on the
same object. A similar trick could be applied to HAllocateObject.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/12035026

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13488 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen-instructions.cc

@@ -1167,6 +1167,26 @@ void HCheckInstanceType::GetCheckMaskAndTag(uint8_t* mask, uint8_t* tag) {
 }
 
 
+void HCheckMaps::SetSideEffectDominator(GVNFlag side_effect,
+                                        HValue* dominator) {
+  ASSERT(side_effect == kChangesMaps);
+  // TODO(mstarzinger): For now we specialize on HStoreNamedField, but once
+  // type information is rich enough we should generalize this to any HType
+  // for which the map is known.
+  if (dominator->IsStoreNamedField()) {
+    HStoreNamedField* store = HStoreNamedField::cast(dominator);
+    Handle<Map> map = store->transition();
+    if (map.is_null() || store->object() != value()) return;
+    for (int i = 0; i < map_set()->length(); i++) {
+      if (map.is_identical_to(map_set()->at(i))) {
+        DeleteAndReplaceWith(NULL);
+        return;
+      }
+    }
+  }
+}
+
+
 void HLoadElements::PrintDataTo(StringStream* stream) {
   value()->PrintNameTo(stream);
   if (HasTypeCheck()) {

src/hydrogen-instructions.h

@@ -193,6 +193,7 @@ class LChunkBuilder;
   V(WrapReceiver)
 
 #define GVN_TRACKED_FLAG_LIST(V)               \
+  V(Maps)                                      \
   V(NewSpacePromotion)
 
 #define GVN_UNTRACKED_FLAG_LIST(V)             \
@@ -205,7 +206,6 @@ class LChunkBuilder;
   V(DoubleArrayElements)                       \
   V(SpecializedArrayElements)                  \
   V(GlobalVars)                                \
-  V(Maps)                                      \
   V(ArrayLengths)                              \
   V(ContextSlots)                              \
   V(OsrEntries)
@@ -2248,6 +2248,7 @@ class HCheckMaps: public HTemplateInstruction<2> {
     SetOperandAt(1, typecheck != NULL ? typecheck : value);
     set_representation(Representation::Tagged());
     SetFlag(kUseGVN);
+    SetFlag(kTrackSideEffectDominators);
     SetGVNFlag(kDependsOnMaps);
     SetGVNFlag(kDependsOnElementsKind);
     map_set()->Add(map, zone);
@@ -2257,6 +2258,7 @@ class HCheckMaps: public HTemplateInstruction<2> {
     SetOperandAt(1, value);
     set_representation(Representation::Tagged());
     SetFlag(kUseGVN);
+    SetFlag(kTrackSideEffectDominators);
     SetGVNFlag(kDependsOnMaps);
     SetGVNFlag(kDependsOnElementsKind);
     for (int i = 0; i < maps->length(); i++) {
@@ -2291,7 +2293,7 @@ class HCheckMaps: public HTemplateInstruction<2> {
   virtual Representation RequiredInputRepresentation(int index) {
     return Representation::Tagged();
   }
-
+  virtual void SetSideEffectDominator(GVNFlag side_effect, HValue* dominator);
   virtual void PrintDataTo(StringStream* stream);
   virtual HType CalculateInferredType();