What is a difference between `-march=pentiumpro' and `-march=i686'

[Joe Buck]

> What is a difference between:
>  `-march=pentiumpro' and `-march=i686' ?
>  `-mcpu=pentiumpro' and `-mcpu=i686' ?

"pentiumpro" and "i686" are idential.

The difference between -march and -mcpu is this: -mcpu says "tune the
code for this processor", -march says "you are allowed to use special
instructions that exist only on this processor".  By default, you get
-march=i386 and -mcpu=<the processor detected by configure>, meaning
the code you get will run on any Intel-compatible hardware.

> And what are best optimization options (for code speed, of course) 
> flags for PIII and Athlon processors in gcc 2.95.2 ?

There is specific code for the Athlon (-mcpu=athlon) and K6 (-mcpu=k6).
There does not seem to be any specific PIII code; I think people
are using i686 but don't know.

I don't have any numbers or data saying how good any of this tuning is.