Re: rawhide report: 20050510 changes

[Nils Philippsen <nphilipp redhat com>]

On Tue, 2005-05-10 at 08:26 -0400, Jakub Jelinek wrote:
> On Tue, May 10, 2005 at 02:19:32PM +0200, Ralf Ertzinger wrote:
> > On Tue, May 10, 2005 at 08:12:35AM -0400, Build System wrote:
> > 
> > > - on x86 and x86_64, use -msse and -msse2 to accomodate newer compilers
> > 
> > Not that I want to revive the whole .i386.rpm vs. i686.rpm discussion,
> > but... how does gcc handle these flags? Does it build alternative
> > code for processors that do not have these features? Or does the binary
> > not run on anything below PII or compatible?
> 
> The way it is done in current rawhide's GIMP is definitely wrong.
> %ifarch %ix86 x86_64
> CFLAGS="%optflags -msse -msse2" CXXFLAGS="%optflags -msse -msse2" \
> %endif
> %configure \
> 
> a) on x86-64 this is unnecessary, all x86-64's have -msse2 by default
> b) -msse2 implies -msse

ACK

> c) if you use -msse2 in CFLAGS for all files, you can't run the latest
>    GIMP on e.g. Pentium2, or pre-x86_64 AMD chips.
>    -msse2 should be ONLY used on sources that have SSE/SSE2 stuff in it,
>    and GIMP should make sure that no routine from those sources will be
>    ever called on pre-SSE2 chips

According to the gcc man page, you need to use "-mfpmath=sse" if you
want to get SSE/SSE2 instructions generated from FP code. As I
understand the manpage, "-msse" and the like only _enable_ its use in
inline assembler code. GIMP does check whether MMX/SSE etc. are
available when starting and only uses the optimized code paths if
possible.

Nils
-- 
     Nils Philippsen    /    Red Hat    /    nphilipp redhat com
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