Re: cselib: add function to check if SET is redundant [PR106187]

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Jul 29, 2022 at 11:52 AM Richard Earnshaw
<Richard.Earnshaw@foss.arm.com> wrote:
>
>
>
> On 29/07/2022 08:06, Richard Biener via Gcc-patches wrote:
> > On Thu, Jul 28, 2022 at 6:46 PM Richard Earnshaw
> > <Richard.Earnshaw@foss.arm.com> wrote:
> >>
> >> [resend with correct subject line]
> >>
> >> A SET operation that writes memory may have the same value as an earlier
> >> store but if the alias sets of the new and earlier store do not conflict
> >> then the set is not truly redundant.  This can happen, for example, if
> >> objects of different types share a stack slot.
> >>
> >> To fix this we define a new function in cselib that first checks for
> >> equality and if that is successful then finds the earlier store in the
> >> value history and checks the alias sets.
> >>
> >> The routine is used in two places elsewhere in the compiler.  Firstly
> >> in cfgcleanup and secondly in postreload.
> >
> > I can't comment on the stripping on SUBREGs and friends but it seems
> > to be conservative apart from
> >
> > +  if (!flag_strict_aliasing || !MEM_P (dest))
> > +    return true;
> >
> > where if dest is not a MEM but were to contain one we'd miss it.
> > Double-checking
> > from more RTL literate people appreciated.
>
> There are very few things that can wrap a MEM in a SET_DEST.  I'm pretty
> sure that's all of them.  It certainly matches the code in
> cselib_invalidate_rtx which has to deal with this sort of case.
>
> >
> > +  /* Lookup the equivalents to the dest.  This is more likely to succeed
> > +     than looking up the equivalents to the source (for example, when the
> > +     src is some form of constant).  */
> >
> > I think the comment is misleading - we _do_ have to lookup the MEM,
> > looking up equivalences of a reg or an expression on the RHS isn't
> > what we are interested in.
>
> OK, I'll try to reword it.
>
> >
> > +               return alias_sets_conflict_p (MEM_ALIAS_SET (dest),
> > +                                             MEM_ALIAS_SET (src_equiv));
> >
> > that's not conservative enough - dse.cc has correct boilerplate, we have
> > to check both MEM_ALIAS_SET and MEM_EXPR here (the latter only
> > if the former load/store has a MEM_EXPR).  Note in particular
> > using alias_set_subset_of instead of alias_sets_conflict_p.
> >
> >                /* We can only remove the later store if the earlier aliases
> >                   at least all accesses the later one.  */
> >                && ((MEM_ALIAS_SET (mem) == MEM_ALIAS_SET (s_info->mem)
> >                     || alias_set_subset_of (MEM_ALIAS_SET (mem),
> >                                             MEM_ALIAS_SET (s_info->mem)))
> >                    && (!MEM_EXPR (s_info->mem)
> >                        || refs_same_for_tbaa_p (MEM_EXPR (s_info->mem),
> >                                                 MEM_EXPR (mem)))))
> >
>
> OK, that's an easy enough change.
>
> > +  /* We failed to find a recorded value in the cselib history, so try the
> > +     source of this set.  */
> > +  rtx src = SET_SRC (set);
> > +  while (GET_CODE (src) == SUBREG)
> > +    src = XEXP (src, 0);
> > +
> > +  if (MEM_P (src) && rtx_equal_for_cselib_1 (dest_addr, XEXP (src, 0),
> > +                                            GET_MODE (dest), 0))
> > +    return alias_sets_conflict_p (MEM_ALIAS_SET (dest),
> > +                                 MEM_ALIAS_SET (src));
> >
> > this looks like an odd case to me - wouldn't that only catch things
> > like self-assignments, aka *p = *p?  So I'd simply drop this fallback.
>
> It catches the case of *p = *q when p and q have the same value.  It did
> come up in testing on x86 (when previously I was aborting to make sure
> I'd caught everything).  We could leave it out as the fallback case in
> this instance is to record a conflict, but it's not a path that's likely
> to be performance critical and the probability of this being a redundant
> store is quite high.  I'll update the comment to make this clearer.

Ah OK - if it did actually catch cases then it's fine to keep.  Note the
alias check needs to be updated the same as above.

Richard.

>
>
> R.
>
> >
> > Otherwise it looks OK to me.
> >
> > Thanks,
> > Richard.
> >
> >> gcc/ChangeLog:
> >>          * cselib.h (cselib_redundant_set_p): Declare.
> >>          * cselib.cc: Include alias.h
> >>          (cselib_redundant_set_p): New function.
> >>          * cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead
> >>          of rtx_equal_for_cselib_p.
> >>          * postreload.c (reload_cse_simplify): Use cselib_redundant_set_p.
> >>          (reload_cse_noop_set_p): Delete.