HSubAllocatedObject instruction allows hydrogen code to carve up

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
diff --git a/src/ia32/lithium-ia32.cc b/src/ia32/lithium-ia32.cc
index 69a0e7f5fd0650cc6f7bb6d50c470f04372f3dcf..52b23c4cfc6cbe4e5f0cc8ad9887b45f40229944 100644
--- a/src/ia32/lithium-ia32.cc
+++ b/src/ia32/lithium-ia32.cc
@@ -292,6 +292,13 @@ void LTypeofIsAndBranch::PrintDataTo(StringStream* stream) {
 }
 
 
+void LSubAllocatedObject::PrintDataTo(StringStream* stream) {
+  stream->Add(" = ");
+  base_object()->PrintTo(stream);
+  stream->Add("+%d", offset());

[Hannes Payer (out of office), 2013/03/13 08:48:39]

do you need a blank before +?

[mvstanton, 2013/03/13 10:14:40]

I'll do a blank before and after the plus, that way it'll be like
"sub-allocated-object @ecx = @eax + 16"

+}
+
+
 void LCallConstantFunction::PrintDataTo(StringStream* stream) {
   stream->Add("#%d / ", arity());
 }
@@ -1081,6 +1088,16 @@ LInstruction* LChunkBuilder::DoPushArgument(HPushArgument* instr) {
 }
 
 
+LInstruction* LChunkBuilder::DoSubAllocatedObject(
+    HSubAllocatedObject* sub_object) {
+  bool zero_offset = sub_object->offset() == 0;

[Hannes Payer (out of office), 2013/03/13 08:48:39]

remove this and just just do return sub_object->offset() == 0 ?
DefineSameAsFirst(result) : DefineAsRegister(result);

[Michael Starzinger, 2013/03/13 09:12:35]

Does this actually happen in practice? If not I would just remove this
optimization for now. The instruction should be removed completely from the
graph during the canonicalization phase for zero offsets if we ever need this.

[mvstanton, 2013/03/13 10:14:40]

good point, thanks. done.

[mvstanton, 2013/03/13 10:14:40]

I got rid of zero_offset, per Michael's advice.

+  LOperand* base_object = UseRegister(sub_object->base_object());

[Michael Starzinger, 2013/03/13 09:12:35]

The base register and the result register can be the same, better use
"UseRegisterAtStart" for the base register.

[mvstanton, 2013/03/13 10:14:40]

Done.

+  LSubAllocatedObject* result =
+      new(zone()) LSubAllocatedObject(base_object);
+  return zero_offset ? DefineSameAsFirst(result) : DefineAsRegister(result);
+}
+
+
 LInstruction* LChunkBuilder::DoThisFunction(HThisFunction* instr) {
   return instr->HasNoUses()
       ? NULL