Allocation folding integrated into the GVN phase.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 741b8c53ada0e4d19399e4e72830b1b9f18446b0..ae5d7d40b915aa0dcab3e746f94c308cdca3f919 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -663,6 +663,23 @@ DEFINE_GET_CONSTANT(False, false, HType::Boolean(), false)
 DEFINE_GET_CONSTANT(Hole, the_hole, HType::Tagged(), false)
 DEFINE_GET_CONSTANT(Null, null, HType::Tagged(), false)
 
+HConstant* HGraph::GetConstantFreeSpaceMap() {

[danno, 2013/07/08 14:35:39]

Please don't introduce another constant at the top of the graph. They
artificially extend live ranges, and given some changes I have planned for
better live range handling of constants, they are counter-productive.

[Hannes Payer (out of office), 2013/07/09 08:26:15]

Done.

+  if (!constant_free_space_map_.is_set()) {
+    HConstant* constant = new(zone()) HConstant(
+        isolate()->factory()->free_space_map(),
+        UniqueValueId(isolate()->heap()->free_space_map()),
+        Representation::None(),
+        HType::Tagged(),
+        false,
+        true,
+        false,
+        false);
+    constant->InsertAfter(GetConstantUndefined());
+    constant_free_space_map_.set(constant);
+  }
+  return constant_free_space_map_.get();
+}
+
 
 #undef DEFINE_GET_CONSTANT
 
@@ -3440,6 +3457,7 @@ bool HGraph::Optimize(SmartArrayPointer<char>* bailout_reason) {
   // The constant is needed to make idef-based bounds check work: the pass
   // evaluates relations with "zero" and that zero cannot be created after GVN.
   GetConstant0();
+  GetConstantFreeSpaceMap();
 
 #ifdef DEBUG
   // Do a full verify after building the graph and computing dominators.