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

[Segher Boessenkool <segher@kernel.crashing.org>]

On Thu, Mar 07, 2024 at 10:55:12AM +0100, Richard Biener wrote:
> On Thu, 7 Mar 2024, Uros Bizjak wrote:
> > This is
> > 
> > 3236              /* Just replace the CC reg with a new mode.  */
> > 3237              SUBST (XEXP (*cc_use_loc, 0), newpat_dest);
> > 3238              undobuf.other_insn = cc_use_insn;
> > 
> > in combine.cc, where *cc_use_loc is
> > 
> > (unspec:DI [
> >         (reg:CC 17 flags)
> >     ] UNSPEC_PUSHFL)
> > 
> > combine assumes CC must be used inside of a comparison and uses XEXP (..., 0)

No.  It has established *this is the case* some time earlier.  Lines\
3155 and on, what begins with
  /* Many machines have insns that can both perform an
     arithmetic operation and set the condition code.

> > OK for trunk?
> 
> Since you CCed me - looking at the code I wonder why we fatally fail.

I did not get this email btw.  Some blip in email (on the sender's side)
I guess?

> The following might also fix the issue and preserve more of the
> rest of the flow of the function.

> --- a/gcc/combine.cc
> +++ b/gcc/combine.cc
> @@ -3182,7 +3182,8 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn 
> *i1, rtx_insn *i0,
>  
>        if (undobuf.other_insn == 0
>           && (cc_use_loc = find_single_use (SET_DEST (newpat), i3,
> -                                           &cc_use_insn)))
> +                                           &cc_use_insn))
> +         && COMPARISON_P (*cc_use_loc))

Line 3167 already is
      && GET_CODE (SET_SRC (PATTERN (i3))) == COMPARE
so what in your backend is unusual?


Segher