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

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
===================================================================
--- runtime/vm/intermediate_language_x64.cc	(revision 38122)
+++ runtime/vm/intermediate_language_x64.cc	(working copy)
@@ -872,9 +872,21 @@
 
 
 void LoadClassIdInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
-  Register object = locs()->in(0).reg();
-  Register result = locs()->out(0).reg();
-  __ LoadTaggedClassIdMayBeSmi(result, object);
+  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)), PP);
+  __ jmp(&done);
+  __ Bind(&load);
+  __ LoadClassId(result, object);
+  __ SmiTag(result);
+  __ Bind(&done);
 }