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

[Uros Bizjak <ubizjak@gmail.com>]

On Thu, Mar 7, 2024 at 11:07 PM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Thu, Mar 7, 2024 at 10:37 PM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> >
> > On Thu, Mar 07, 2024 at 10:04:32PM +0100, Uros Bizjak wrote:
> >
> > [snip]
> >
> > > The part we want to fix deals with the *user* of the CC register. It
> > > is not true that this is always COMPARISON_P, so EQ, NE, GE, LT, ...
> > > in the form of
> > >
> > > (LT:CCGC (reg:CCGC 17 flags) (const_int 0))
> > >
> > > but can be something else, such as the above noted
> > >
> > >  (unspec:DI [
> > >          (reg:CC 17 flags)
> > >      ] UNSPEC_PUSHFL)
> >
> > But that is invalid RTL?  The only valid use of a CC is written as
> > cc-compared-to-0.  It means "the result of something compared to 0,
> > stored in that cc reg".
> >
> > (And you can copy a CC reg around, but that is not a use ;-) )

Hm... under this premise, we can also say that any form that is not
cc-compared-to-0 is not a use. Consequently, if it is not a use, then
the CC reg should not be updated at its use location, so my v1 patch,
where we simply skip the update (but retain the combined insn),
actually makes sense.

In this concrete situation, we don't care about CC register mode in
the PUSHFL insn. And we should not change CC reg mode of the use,
because any other mode than the generic CCmode won't be recognized by
the insn pattern.

Uros.