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From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Fri, 28 Oct 2022 15:28:16 -0700
Message-ID: <CAMe9rOq9HyTv6WsJoXHyw859xJvABZWQmvdTQ_dYjZJX_b-YFQ@mail.gmail.com>
Subject: Re: [PATCH] x86: Replace ne:CCC/ne:CCO with UNSPEC_CC_NE in neg patterns
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: gcc-patches@gcc.gnu.org, Uros Bizjak <ubizjak@gmail.com>
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On Fri, Oct 28, 2022 at 2:34 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Wed, Oct 26, 2022 at 11:58:57AM -0700, H.J. Lu via Gcc-patches wrote:
> > In i386.md, neg patterns which set MODE_CC register like
> >
> > (set (reg:CCC FLAGS_REG)
> >      (ne:CCC (match_operand:SWI48 1 "general_reg_operand") (const_int 0)))
> >
> > can lead to errors when operand 1 is a constant value.  If FLAGS_REG in
>
> But it cannot be.  general_reg_operand will not allow that:
> ===
> (define_predicate "general_reg_operand"
>   (and (match_code "reg")
>        (match_test "GENERAL_REGNO_P (REGNO (op))")))
> ===
>
> > (set (reg:CCC FLAGS_REG)
> >      (ne:CCC (const_int 2) (const_int 0)))
> >
> > is set to 1, RTX simplifiers may simplify
>

Here is another example:

(define_insn "*neg<mode>_ccc_1"
  [(set (reg:CCC FLAGS_REG)
    (ne:CCC
      (match_operand:SWI 1 "nonimmediate_operand" "0")
      (const_int 0)))
   (set (match_operand:SWI 0 "nonimmediate_operand" "=<r>m")
    (neg:SWI (match_dup 1)))]
  ""
  "neg{<imodesuffix>}\t%0"
  [(set_attr "type" "negnot")
   (set_attr "mode" "<MODE>")])

Operand 1 can be a known value.

H.J.