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SubjectRe: Code optimization <LEA Instruction>
Followup to:  <>
By author: "Richard B. Johnson" <>
In newsgroup:
> 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.)

<> at work, <> in private!
"Unix gives you enough rope to shoot yourself in the foot."

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