Re: [RFC] gimple ssa: SCCP - A new PHI optimization pass

[Richard Biener <rguenther@suse.de>]

On Thu, 31 Aug 2023, Andrew Pinski wrote:

> On Thu, Aug 31, 2023 at 5:15?AM Richard Biener via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > On Thu, 31 Aug 2023, Filip Kastl wrote:
> >
> > > > The most obvious places would be right after SSA construction and before RTL expansion.
> > > > Can you provide measurements for those positions?
> > >
> > > The algorithm should only remove PHIs that break SSA form minimality. Since
> > > GCC's SSA construction already produces minimal SSA form, the algorithm isn't
> > > expected to remove any PHIs if run right after the construction. I even
> > > measured it and indeed -- no PHIs got removed (except for 502.gcc_r, where the
> > > algorithm managed to remove exactly 1 PHI, which is weird).
> > >
> > > I tried putting the pass before pass_expand. There isn't a lot of PHIs to
> > > remove at that point, but there still are some.
> >
> > That's interesting.  Your placement at
> >
> >           NEXT_PASS (pass_cd_dce, false /* update_address_taken_p */);
> >           NEXT_PASS (pass_phiopt, true /* early_p */);
> > +         NEXT_PASS (pass_sccp);
> >
> > and
> >
> >        NEXT_PASS (pass_tsan);
> >        NEXT_PASS (pass_dse, true /* use DR analysis */);
> >        NEXT_PASS (pass_dce);
> > +      NEXT_PASS (pass_sccp);
> >
> > isn't immediately after the "best" existing pass we have to
> > remove dead PHIs which is pass_cd_dce.  phiopt might leave
> > dead PHIs around and the second instance runs long after the
> > last CD-DCE.
> 
> Actually the last phiopt is run before last pass_cd_dce:

I meant the second instance of pass_sccp, not phiopt.

Richard.