gcc-2.7 creates faster code than pgcc-1.1.1

[Richard Henderson]

On Fri, Mar 05, 1999 at 12:23:46PM -0500, Alfred Perlstein wrote:
> > I any way "movzb? %al,%?ax" and "and? $255,%?ax" takes 1 tick both.
> > So this is a kind of mistery with this instructions.
> 
> I think the magic lies in that with register renaming, instruction
> caches and all the 'behind the scenes' optimizations PPro and later
> versions of x86 chips can do.  It really should be investigated more.

It has nothing to do with register renaming. 

It is most likely to be related to instruction alignment -- some
important insn in the loop is straddling a 16-byte boundary, which
requires an extra cycle to decode.

I've seen such create up to a 20% difference in runtime on a small loop.


r~