Code optimization <LEA Instruction>

[H. Peter Anvin]

Followup to:  <Pine.LNX.3.95.1000127182026.1435C-100000 en chaos.analogic.com>
By author:    "Richard B. Johnson" <root en chaos.analogic.com>
In newsgroup: linux.dev.kernel
>
> On Thu, 27 Jan 2000, Alan Cox wrote:
> 
> > > One of the things it states is that the LEA instruction can
> > > be used to change the value of an index register faster than
> > > using the ADD instruction (Page G-10, Intel '486 Rag).
> > 
> > It is
> 
> It is not. Never was and can't possibly be. Further, tests show
> as expected, that address generation takes more time than register
> addition.
> 

Huh?  LEA is just accessing what is, in effect, a third ALU.  Either
should be the same speed, *but* you have to worry about scheduling,
which is CPU-dependent.  P6 cores have two "normal" ALUs and one
address-generation one.  One thing LEA lets you do is a three-register
operation, which the normal ALUs can't do:

	lea eax,[ebx+esi]

.. instead of ...

	mov eax,ebx
	add eax,esi

(On Crusoe, of course, either of these become a single VLIW "add" atom
and thus are completely equivalent.)

	-hpa
-- 
<hpa en transmeta.com> at work, <hpa en zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."

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