Re: Will GCC eventually support SSE2 or SSE4.1?

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Fri, 26 May 2023 at 14:55, Stefan Kanthak <stefan.kanthak@nexgo.de> wrote:
>
> "Jakub Jelinek" <jakub@redhat.com> wrote:
>
> > On Fri, May 26, 2023 at 02:19:54PM +0200, Stefan Kanthak wrote:
> >> > I find it very SURPRISING that you're only just learning the basics of
> >> > how to use gcc NOW, after YELLING about all the OUCH.
> >>
> >> I'm NOT surprised that you don't grok it!
> >>
> >> gcc -msse4.1 -m32 -march=core2 ...
> >>
> >> Which -m* options win here?
> >> Do -m32 or -march=core2 override -msse4.1?
> >
> > Jonathan told you what to use to find it out (-Q --help=target).
> > -m32/-m64/-mx32/-m16 options don't affect the ISA, they switch the
> > main ABI (ilp32 32-bit code, lp64 64-bit code, ilp32 code running
> > in 64-bit mode, 16-bit code).
>
> PLEASE read your own documentation and specify ANY:
>
> | -m32
> ...
> |    generates code that runs on any i386 system.
>
> The first example of my initial posts falsifies "runs on any i386 system"!

Which is why that word "any" has been acknowledged as a bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109954
The word "any" is wrong. We know it's wrong. We've acknowledged it's
wrong. I've even proposed wording to fix it in that bug report.
What part of this is still hard to understand?


>
> > -march= options selects the ISA base (which CPU family to compile form
> > as minimum), if you don't supply it, the default from how gcc has been
> > configured is selected (e.g. if you configure --with-arch-32=core2, then
> > that will be the -m32 default, if you configure --with-arch=x86-64, that
> > will be the -march default if --with-arch-32= isn't specified, etc.).
> > If more than one -march= is specified, the last one wins.
>
> But "the last one wins" does NOT apply to -m32 or -m<feature>: the latter
> are additive, INDEPENDENT from their position before or after -march=,
> what is NOT documented, and -m32 fails to disable all -m<feature> not
> common to ANY i386 system set before, thus contradicting the documentation.

See above.


> > And, the -mISA options then tweak the ISA set.  Most ISAs have dependencies,
> > say -msse4.1 enables -mssse3 which enables -msse3 which enables -msse2 etc.,
> > and similarly the -mno-ISA options disable what ISAs depend on it, so
> > -mno-avx disables -mno-avx2 which disables -mno-avx512f which disables ...
>
> These points are obvious.
> NOT obvious is but that -m<feature> -march=<lowerISA> does not clear any
> <feature> not supported in <lowerISA>, i.e the last one does NOT win here.

The last -march option selects the base set of instructions. The -mISA
options modify that base.