Short check for zero is tst not cmp...

runtime/vm/stub_code_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"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/compiler.h"
 #include "vm/dart_entry.h"
@@ skipping 2145 matching lines @@
   // R13: first IC entry
   __ LoadTaggedClassIdMayBeSmi(RAX, RDI);
   // RAX: receiver cid as Smi
 
   __ Bind(&loop);
   __ movq(R9, Address(R13, 0));
   __ cmpq(RAX, R9);
   __ j(EQUAL, &found, Assembler::kNearJump);
 
   ASSERT(Smi::RawValue(kIllegalCid) == 0);
-  __ cmpq(R9, R9);
-  __ j(EQUAL, &miss, Assembler::kNearJump);
+  __ testq(R9, R9);
+  __ j(ZERO, &miss, Assembler::kNearJump);
 
   const intptr_t entry_length = ICData::TestEntryLengthFor(1) * kWordSize;
   __ addq(R13, Immediate(entry_length));  // Next entry.
   __ jmp(&loop);
 
   __ Bind(&found);
   const intptr_t target_offset = ICData::TargetIndexFor(1) * kWordSize;
   __ movq(RAX, Address(R13, target_offset));
   __ movq(RCX, FieldAddress(RAX, Function::entry_point_offset()));
   __ movq(CODE_REG, FieldAddress(RAX, Function::code_offset()));
   __ ret();
 
   __ Bind(&miss);
   __ LoadIsolate(RAX);
   __ movq(CODE_REG, Address(RAX, Isolate::ic_miss_code_offset()));
   __ movq(RCX, FieldAddress(CODE_REG, Code::entry_point_offset()));
   __ ret();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64