Optimizations

Jeremy Herbison herbie at autobotcity.net
Thu Nov 28 19:08:59 PST 2002


I have a further question...

It appears from reading between the lines on the gcc page that to fully
utilize the pentium3 i would need to
pass --march=pentium3 --mmmx --msse -mfpmath=sse,mmx is this correct? or is
it sufficient to pass --march=pentium3 --mfpmath=sse,mmx? i guess i mean
will mmx and sse be implied by --march=pentium3 or not?

<nholland at tisys.org> wrote in message
news:20021127192115.GA3474 at tisys.org...
> Hallo everybody,
>
> there are a few things that I'm not entirely sure about, so I thought I'd
> post a short question here.
>
> Well, imagine I want to build a LFS system that will run on a wide range
of
> machines, let's say from genuine Pentium machines to everything up to AMD
> Athlons. Let's furthermore asume that the machine I use for building the
> stuff is an AMD Athlon box. Now, I basically wonder what that would mean
> for the build process.
>
> Look at GCC for example. When I run the ./configure script, it will detect
> three things automatically: --build, --host and --target. These will be
set
> to the value corresponding to the machine I am building on (in case of an
> Athlon, this would be i686-pc-linux-gnu). While --build and --target are
of
> lesser interest here, I wonder if --host=i686-pc-linux-gnu actually means
> that GCC would be optimized for running on i686 class machines, which
would
> mean that I could probably not simply put it on a i586 machine later on.
> *Or* does the --build variable not really have any influence on that, so
> that I could simply let GCC's configure auto-detect everything as
> i686-pc-linux-gnu and then still let it run on an i586 machine later?
>
> While we're at it, there's another thing: I can, if I wanted to, set my
> CFLAGS to something like "-O2 -march=k6", which would, to my
understanding,
> produce code optimized for AMD K6 machines. The question is: Does GCC
> always produce i386 code by default unless I set something else, or is it
> probably so that a GCC that has been build with configure
> --host=i686-pc-linux-gnu would produce i686 optimized code unless
> instructed otherwise?
>
> These two rather long questions can probably be summed up like this: If I
> build a LFS system on an Athlon box and never manually specify anything
for
> --build, nor manually set -march, what kind of code do I get? i386
> optimized code, which would work fine on other machines, or i686 code,
which
> would only run on my build machine (or other i686-class machines)?
>
> I've been wondering about this quite a bit lately and have not yet been
> able to find anything that clarifies this properly for me, so I hope that
> someone can give me some short information ;-)
>
> Greetings
> Nils
>
> --
> Nils Holland <nholland at tisys.org>
> *Ti Systems - Linux in Tiddische, Germany - Since 1996*
> --
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