On Wed, Sep 30, 2020 at 04:05:41PM +0200, Florian Weimer wrote:
> > I think the documentation should state that these are not valid in -mtune=,
> > just in -march=, and that using -march=x86-64-v* will not change tuning.
> > I guess there should be some testsuite coverage for the for some unexpected
> > behavior of
> > -march=skylake -march=x86-64-v3
> > actually acting as
> > -march=x86-64-v3 -mtune=skylake
> > though perhaps it needs to be skipped if user used explicit -mtune= and
> > not sure how to actually test that (-fverbose-asm doesn't print -mtune=
> > when it is not explicit).
>
> I think the compiler driver collapses -march=skylake -march=x86-64-v3
> to -march=x86-64-v3, dropping the tuning. The cc1 option parser also
> drops the first -march=. That's a bit surprising to me. It means
> that we can't use multiple tuning/non-tuning -march= switches, and
> that tuning with (say) -march=x86-64-v3 needs to use -mtune.
>
> PTA_NO_TUNE is still needed because we'd define __tune_k8__ otherwise
> (and switch to K8 tuning internally).
>
> Is it okay to simply document this? Perhaps like this?
Thinking about it more, wouldn't it better to just imply generic tuning
for these -march= options?
Jakub
