Re: [PATCHv2, rs6000] Enable have_cbranchcc4 on rs6000

[David Edelsohn <dje.gcc@gmail.com>]

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On Fri, Nov 18, 2022 at 7:20 AM Segher Boessenkool <
segher@kernel.crashing.org> wrote:

> On Fri, Nov 18, 2022 at 02:35:30PM +0800, HAO CHEN GUI wrote:
> > 在 2022/11/17 21:24, David Edelsohn 写道:
> > > Why are you using zero_constant predicate instead of matching
> (const_int 0) for operand 2?
> > The "const_int 0" is an operand other than a predicate. We need a
> predicate here.
>
> Said differently, it is passed as an operand to this named pattern or
> optab, so you need a match_operand here.
>

Earlier versions of patterns for other targets used (const_int 0), but they
seem to have changed that, so match_operand is needed.

Thanks, David


>
> > > Why does this need the new all_branch_comparison_operator?  Can the
> ifcvt optimization correctly elide the 2 insn sequence?
> > Because rs6000 defines "*cbranch_2insn" insn, such insns are generated
> after expand.
> >
> > (jump_insn 50 47 51 11 (set (pc)
> >         (if_then_else (ge (reg:CCFP 156)
> >                 (const_int 0 [0]))
> >             (label_ref 53)
> >             (pc)))
> "/home/guihaoc/gcc/gcc-mainline-base/gmp/mpz/cmpabs_d.c":80:7 884
> {*cbranch_2insn}
> >      (expr_list:REG_DEAD (reg:CCFP 156)
> >         (int_list:REG_BR_PROB 633507684 (nil)))
> >  -> 53)
>
> But notice the cost of *cbranch_2insn -- ifcvt should never generate
> cbranchcc4 with such composite conditions!
>
> > In prepare_cmp_insn, the comparison is verified by insn_operand_matches.
> If
> > extra_insn_branch_comparison_operator is not included in "cbranchcc4"
> predicate,
> > it hits ICE here.
> >
> >   if (GET_MODE_CLASS (mode) == MODE_CC)
> >     {
> >       enum insn_code icode = optab_handler (cbranch_optab, CCmode);
> >       test = gen_rtx_fmt_ee (comparison, VOIDmode, x, y);
> >       gcc_assert (icode != CODE_FOR_nothing
> >                   && insn_operand_matches (icode, 0, test));
> >       *ptest = test;
> >       return;
> >     }
> >
> > The real conditional move is generated by emit_conditional_move_1.
> Commonly
> > "*cbranch_2insn" can't be optimized out and it returns NULL_RTX.
> >
> >       if (COMPARISON_P (comparison))
> >         {
> >           saved_pending_stack_adjust save;
> >           save_pending_stack_adjust (&save);
> >           last = get_last_insn ();
> >           do_pending_stack_adjust ();
> >           machine_mode cmpmode = comp.mode;
> >           prepare_cmp_insn (XEXP (comparison, 0), XEXP (comparison, 1),
> >                             GET_CODE (comparison), NULL_RTX, unsignedp,
> >                             OPTAB_WIDEN, &comparison, &cmpmode);
> >           if (comparison)
> >             {
> >                rtx res = emit_conditional_move_1 (target, comparison,
> >                                                   op2, op3, mode);
> >                if (res != NULL_RTX)
> >                  return res;
> >             }
> >           delete_insns_since (last);
> >           restore_pending_stack_adjust (&save);
> >
> > I think that extra_insn_branch_comparison_operator should be included in
> > "cbranchcc4" predicates as such insns exist. And leave it to
> > emit_conditional_move which decides whether it can be optimized or not.
>
> I don't think we should pretend we have any conditional jumps the
> machine does not actually have, in cbranchcc4.  When would this ever be
> useful?  cror;beq can be quite expensive, compared to the code it would
> replace anyway.
>
> If something generates those here (which then ICEs later), that is
> wrong, fix *that*?  Is it ifcvt doing it?
>
>
> Segher
>