Use fucomi instruction to compare floats and put results directly into EFLAGS...

src/x64/macro-assembler-x64.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 579 matching lines @@
   movq(Operand(kScratchRegister, 0), rsp);
 }
 
 
 void MacroAssembler::Ret() {
   ret(0);
 }
 
 
 void MacroAssembler::FCmp() {
-  fcompp();
-  push(rax);
-  fnstsw_ax();
-  if (CpuFeatures::IsSupported(CpuFeatures::SAHF)) {
-    sahf();
-  } else {
-    shrl(rax, Immediate(8));
-    and_(rax, Immediate(0xFF));
-    push(rax);
-    popfq();
-  }
-  pop(rax);
+  fucomip();
+  ffree(0);
+  fincstp();
 }
 
 
 void MacroAssembler::CmpObjectType(Register heap_object,
                                    InstanceType type,
                                    Register map) {
   movq(map, FieldOperand(heap_object, HeapObject::kMapOffset));
   CmpInstanceType(map, type);
 }
 
@@ skipping 755 matching lines @@
   movq(kScratchRegister, new_space_allocation_top);
 #ifdef DEBUG
   cmpq(object, Operand(kScratchRegister, 0));
   Check(below, "Undo allocation of non allocated memory");
 #endif
   movq(Operand(kScratchRegister, 0), object);
 }
 
 
 } }  // namespace v8::internal