VM: Avoid smi-check when loading class-ids from known non-smi values

runtime/vm/intermediate_language_x64.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_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/compiler.h"
@@ skipping 908 matching lines @@
   return LocationSummary::Make(zone,
                                kNumInputs,
                                Location::RequiresRegister(),
                                LocationSummary::kNoCall);
 }
 
 
 void LoadClassIdInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   const Register object = locs()->in(0).reg();
   const Register result = locs()->out(0).reg();
-  Label load, done;
-
-  // We don't use Assembler::LoadTaggedClassIdMayBeSmi() here---which uses
-  // a conditional move instead---because it is slower, probably due to
-  // branch prediction usually working just fine in this case.
-  __ testq(object, Immediate(kSmiTagMask));
-  __ j(NOT_ZERO, &load, Assembler::kNearJump);
-  __ LoadImmediate(result, Immediate(Smi::RawValue(kSmiCid)));
-  __ jmp(&done);
-  __ Bind(&load);
-  __ LoadClassId(result, object);
-  __ SmiTag(result);
-  __ Bind(&done);
+  const AbstractType& value_type = *this->object()->Type()->ToAbstractType();
+  if (CompileType::Smi().IsAssignableTo(value_type) ||
+      value_type.IsTypeParameter()) {
+    // We don't use Assembler::LoadTaggedClassIdMayBeSmi() here---which uses
+    // a conditional move instead---because it is slower, probably due to
+    // branch prediction usually working just fine in this case.
+    Label load, done;
+    __ testq(object, Immediate(kSmiTagMask));
+    __ j(NOT_ZERO, &load, Assembler::kNearJump);
+    __ LoadImmediate(result, Immediate(Smi::RawValue(kSmiCid)));
+    __ jmp(&done);
+    __ Bind(&load);
+    __ LoadClassId(result, object);
+    __ SmiTag(result);
+    __ Bind(&done);
+  } else {
+    __ LoadClassId(result, object);
+    __ SmiTag(result);
+  }
 }
 
 
 CompileType LoadIndexedInstr::ComputeType() const {
   switch (class_id_) {
     case kArrayCid:
     case kImmutableArrayCid:
       return CompileType::Dynamic();
 
     case kTypedDataFloat32ArrayCid:
@@ skipping 5575 matching lines @@
   __ Drop(1);
   __ popq(result);
 }
 
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64