Optimizations to IC stub for unoptimized code performance on x64.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
diff --git a/runtime/vm/assembler_x64.cc b/runtime/vm/assembler_x64.cc
index 20541ef5e69d2c9eb0194b38fce817784577eb87..38ecf1c0e0432a1402c7c5ee3e6e06eca6a85537 100644
--- a/runtime/vm/assembler_x64.cc
+++ b/runtime/vm/assembler_x64.cc
@@ -3676,29 +3676,33 @@ void Assembler::SmiUntagOrCheckClass(Register object,
 
 
 void Assembler::LoadClassIdMayBeSmi(Register result, Register object) {
-  ASSERT(result != object);
+  Label smi, join;
 
-  // Load up a null object. We only need it so we can use LoadClassId on it in
-  // the case that object is a Smi.
-  LoadObject(result, Object::null_object());
-  // Check if the object is a Smi.
   testq(object, Immediate(kSmiTagMask));
-  // If the object *is* a Smi, use the null object instead.
-  cmoveq(object, result);
-  // Loads either the cid of the object if it isn't a Smi, or the cid of null
-  // if it is a Smi, which will be ignored.
+  j(EQUAL, &smi, Assembler::kNearJump);
   LoadClassId(result, object);
+  jmp(&join, Assembler::kNearJump);
 
-  movq(TMP, Immediate(kSmiCid));
-  // If object is a Smi, move the Smi cid into result. o/w leave alone.
-  cmoveq(result, TMP);
+  Bind(&smi);
+  movq(result, Immediate(kSmiCid));
+
+  Bind(&join);
 }
 
 
 void Assembler::LoadTaggedClassIdMayBeSmi(Register result, Register object) {
-  LoadClassIdMayBeSmi(result, object);
-  // Finally, tag the result.
+  Label smi, join;
+
+  testq(object, Immediate(kSmiTagMask));
+  j(EQUAL, &smi, Assembler::kNearJump);
+  LoadClassId(result, object);
   SmiTag(result);
+  jmp(&join, Assembler::kNearJump);
+
+  Bind(&smi);
+  movq(result, Immediate(Smi::RawValue(kSmiCid)));
+
+  Bind(&join);
 }