Re: [PATCH v2] combine: Fix ICE in try_combine on pr112494.c [PR112560]

[Uros Bizjak <ubizjak@gmail.com>]

On Thu, Mar 7, 2024 at 12:11 PM Richard Biener <rguenther@suse.de> wrote:
>
> On Thu, 7 Mar 2024, Jakub Jelinek wrote:
>
> > On Thu, Mar 07, 2024 at 11:11:35AM +0100, Uros Bizjak wrote:
> > > > Since you CCed me - looking at the code I wonder why we fatally fail.
> > > > The following might also fix the issue and preserve more of the
> > > > rest of the flow of the function.
> > > >
> > > > If that works I'd prefer it.  But I'll defer approval to the combine
> > > > maintainer which is Segher.
> > >
> > > Your patch is basically what v1 did [1], but it was suggested (in a
> > > reply by you ;) ) that we should stop the attempt to combine if we
> > > can't handle the use. So, the v2 patch undoes the combine and records
> > > a nice message in this case.
> >
> > My understanding of Richi's patch is that it it treats the non-COMPARISON_P
> > the same as if find_single_use fails, which is a common case that certainly
> > has to be handled right and it doesn't seem that we are giving up completely
> > for that case.  So, I think it is reasonable to treat the non-COMPARISON_P
> > *cc_use_loc as NULL cc_use_loc.
>
> The question is, whether a NULL cc_use_loc (find_single_use returning
> NULL) means "there is no use" or it can mean "huh, don't know, maybe
> more than one, maybe I was too stupid to indentify the single use".
> The implementation suggests it's all broken ;)

As I understood find_single_use, it is returning RTX iff DEST is used
only a single time in an insn sequence following INSN.
find_single_use_1 returns RTX iff argument is used exactly once in
DEST. So, find_single_use returns RTX only when DEST is used exactly
once in a sequence following INSN.

We can reject the combination without worries of multiple uses.

Uros,