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

src/x64/assembler-x64.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // 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.
 //
@@ skipping 959 matching lines @@
   void fprem();
   void fprem1();
 
   void fxch(int i = 1);
   void fincstp();
   void ffree(int i = 0);
 
   void ftst();
   void fucomp(int i);
   void fucompp();
+  void fucomip();
+
   void fcompp();
   void fnstsw_ax();
   void fwait();
   void fnclex();
 
   void fsin();
   void fcos();
 
   void frndint();
 
@@ skipping 305 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_