Mark map-transition stores as map-changing instructions.

This prevents code motion from hoisting map-checks across such stores
which may result in unnecessary deoptimizations.

In the following example program we would move a map-check from the inner loop out before the outer loop which is not desirable:

function f() {
  var o = {};
  var j = 0;
  o.a = 1;
  do {
    o.b = 6;  // Map transition
    for (var i=0; i<10; i++) {
       o.a = o.b + i;
    }
  } while(++j < 1) {}
}

for (var i = 0; i < 1000000; i++) f();


Review URL: http://codereview.chromium.org/5991001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6071 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen-instructions.cc

@@ -579,6 +579,13 @@ void HBranch::PrintDataTo(StringStream* stream) const {
 }
 
 
+void HCompareMapAndBranch::PrintDataTo(StringStream* stream) const {
+  stream->Add("on ");
+  value()->PrintNameTo(stream);
+  stream->Add(" (%p)", *map());
+}
+
+
 void HGoto::PrintDataTo(StringStream* stream) const {
   stream->Add("B%d", FirstSuccessor()->block_id());
 }

src/hydrogen-instructions.h

@@ -905,6 +905,8 @@ class HCompareMapAndBranch: public HUnaryControlInstruction {
   virtual HBasicBlock* FirstSuccessor() const { return true_destination_; }
   virtual HBasicBlock* SecondSuccessor() const { return false_destination_; }
 
+  virtual void PrintDataTo(StringStream* stream) const;
+
   Handle<Map> map() const { return map_; }
 
   DECLARE_CONCRETE_INSTRUCTION(CompareMapAndBranch, "compare_map_and_branch")

src/hydrogen.cc

@@ -3165,6 +3165,9 @@ HInstruction* HGraphBuilder::BuildStoreNamedField(HValue* object,
   if (lookup->type() == MAP_TRANSITION) {
     Handle<Map> transition(lookup->GetTransitionMapFromMap(*type));
     instr->set_transition(transition);
+    // TODO(fschneider): Record the new map type of the object in the IR to
+    // enable elimination of redundant checks after the transition store.
+    instr->SetFlag(HValue::kChangesMaps);
   }
   return instr;
 }