Steel Bank Common Lisp

Guenther Thomsen <guenthert@...> writes:

> Am I wrong to expect an "ADD" in the disassembled code below?
> --8<-- 
> CL-USER> (defun h (x)
> 	   (declare (type fixnum x) (optimize (safety 0)))
> 	    (the fixnum (+ x 3)))
>
> STYLE-WARNING: redefining H in DEFUN
> H
> CL-USER> (disassemble 'h)
> ; 02C3D4D2:       488D5118         LEA RDX, [RCX+24]          ; no-arg-parsing entry point

Yes.  On x86oids, fixnum adds of constants can be implemented using
the Load Effective Address instruction.  (I haven't measured recently
to find whether or not this is faster than using ADD, but it has been
at least at some points in the past.

Best,

Christophe

On Tue, 2008-12-09 at 20:39 +0000, Christophe Rhodes wrote:

> Guenther Thomsen <guenthert@...> writes:
> 
> > Am I wrong to expect an "ADD" in the disassembled code below?
> > --8<-- 
> > CL-USER> (defun h (x)
> > 	   (declare (type fixnum x) (optimize (safety 0)))
> > 	    (the fixnum (+ x 3)))
> >
> > STYLE-WARNING: redefining H in DEFUN
> > H
> > CL-USER> (disassemble 'h)
> > ; 02C3D4D2:       488D5118         LEA RDX, [RCX+24]          ; no-arg-parsing entry point
> 
> Yes.  On x86oids, fixnum adds of constants can be implemented using
> the Load Effective Address instruction.  (I haven't measured recently
> to find whether or not this is faster than using ADD, but it has been
> at least at some points in the past.
> 
> Best,
> 
> Christophe


Ah, I see. With a variant of that function (adding 5 instead of 3, the
LEA instruction becomes "LEA RDX, [RCX+40]", the 3 bit left shift is due
to implied type information, I suppose).

Thanks
    Guenther