On Thu, May 13, 2021 at 7:52 PM Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> Jakub Jelinek <ja...@redhat.com> writes:
> > On Thu, May 13, 2021 at 12:32:26PM +0100, Richard Sandiford wrote:
> >> Jakub Jelinek <ja...@redhat.com> writes:
> >> > On Thu, May 13, 2021 at 11:43:19AM +0200, Uros Bizjak wrote:
> >> >> > > Bootstrapped and regtested on X86_64-linux-gnu{-m32,}
> >> >> > > Ok for trunk?
> >> >> >
> >> >> > Some time ago a support for CLOBBER_HIGH RTX was added (and later
> >> >> > removed for some reason). Perhaps we could resurrect the patch for the
> >> >> > purpose of ferrying 128bit modes via vzeroupper RTX?
> >> >>
> >> >> https://gcc.gnu.org/legacy-ml/gcc-patches/2017-11/msg01325.html
> >> >
> >> > https://gcc.gnu.org/legacy-ml/gcc-patches/2019-09/msg01468.html
> >> > is where it got removed, CCing Richard.
> >>
> >> Yeah. Initially clobber_high seemed like the best appraoch for
> >> handling the tlsdesc thing, but in practice it was too difficult
> >> to shoe-horn the concept in after the fact, when so much rtl
> >> infrastructure wasn't prepared to deal with it. The old support
> >> didn't handle all cases and passes correctly, and handled others
> >> suboptimally.
> >>
> >> I think it would be worth using the same approach as
> >> https://gcc.gnu.org/legacy-ml/gcc-patches/2019-09/msg01466.html for
> >> vzeroupper: represent the instructions as call_insns in which the
> >> call has a special vzeroupper ABI. I think that's likely to lead
> >> to better code than clobber_high would (or at least, it did for tlsdesc).
>From an implementation perspective， I guess you're meaning we should
implement TARGET_INSN_CALLEE_ABI and TARGET_FNTYPE_ABI in the i386
backend.
> >
> > Perhaps a magic call_insn that is split post-reload into a normal insn
> > with the sets then?
>
> I'd be tempted to treat it is a call_insn throughout. The unspec_volatile
> means that we can't move the instruction, so converting a call_insn to an
> insn isn't likely to help from that point of view. The sets are also
> likely to be handled suboptimally compared to the more accurate register
> information attached to the call: all code that handles calls has to be
> prepared to deal with partial clobbers, whereas most code dealing with
> sets will assume that the set does useful work, and that the rhs of the
> set is live.
>
> Thanks,
> Richard
>
--
BR,
Hongtao
