Uses a conditional move for usage counter increment instead of a branch

runtime/vm/intermediate_language_ia32.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_IA32.
 #if defined(TARGET_ARCH_IA32)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 917 matching lines @@
 void LoadUntaggedInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   Register object = locs()->in(0).reg();
   Register result = locs()->out(0).reg();
   __ movl(result, FieldAddress(object, offset()));
 }
 
 
 LocationSummary* LoadClassIdInstr::MakeLocationSummary(Isolate* isolate,
                                                        bool opt) const {
   const intptr_t kNumInputs = 1;
-  const intptr_t kNumTemps = 1;
-  LocationSummary* summary = new(isolate) LocationSummary(
-      isolate, kNumInputs, kNumTemps, LocationSummary::kNoCall);
-  summary->set_in(0, Location::RequiresRegister());
-  summary->set_out(0, Location::RequiresRegister());
-  summary->set_temp(0, Location::RequiresRegister());
-  return summary;
+  return LocationSummary::Make(isolate,
+                               kNumInputs,
+                               Location::RequiresRegister(),
+                               LocationSummary::kNoCall);
 }
 
 
 void LoadClassIdInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
-  Register object = locs()->in(0).reg();
-  Register result = locs()->out(0).reg();
-  Register tmp = locs()->temp(0).reg();
-  __ LoadTaggedClassIdMayBeSmi(result, object, tmp);
+  const Register object = locs()->in(0).reg();
+  const Register result = locs()->out(0).reg();
+  Label not_smi, done;
+
+  // We don't use Assembler::LoadTaggedClassIdMayBeSmi() here---which uses
+  // a conditional move instead, and requires an additional register---because
+  // it is slower, probably due to branch prediction usually working just fine
+  // in this case.

[Ivan Posva, 2014/07/15 08:47:09]

Do we know through type feedback whether a LoadClassIdInstr is used for values
that are guaranteed not to be Smi? Then we could save even more stuff here.

+  __ testl(object, Immediate(kSmiTagMask));
+  __ j(NOT_ZERO, &not_smi, Assembler::kNearJump);
+  __ movl(result, Immediate(Smi::RawValue(kSmiCid)));
+  __ jmp(&done, Assembler::kNearJump);
+  __ Bind(&not_smi);
+  __ LoadClassId(result, object);
+  __ SmiTag(result);
+  __ Bind(&done);
 }
 
 
 CompileType LoadIndexedInstr::ComputeType() const {
   switch (class_id_) {
     case kArrayCid:
     case kImmutableArrayCid:
       return CompileType::Dynamic();
 
     case kTypedDataFloat32ArrayCid:
@@ skipping 5400 matching lines @@
   __ movl(EDX, Immediate(kInvalidObjectPointer));
   __ movl(EDX, Immediate(kInvalidObjectPointer));
 #endif
 }
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_IA32