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From: Andrew Pinski <pinskia@gmail.com>
Date: Fri, 26 Aug 2022 10:40:31 -0700
Message-ID: <CA+=Sn1n3R4Phg4STrdBx7D2FSSZsXYUP1O=oVxgA1Ra_N=tTBg@mail.gmail.com>
Subject: Re: [PATCH] Add support for floating point endpoints to frange.
To: Aldy Hernandez <aldyh@redhat.com>
Cc: GCC patches <gcc-patches@gcc.gnu.org>, Andrew MacLeod <amacleod@redhat.com>, 
	Jakub Jelinek <jakub@redhat.com>
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On Fri, Aug 26, 2022 at 8:55 AM Aldy Hernandez <aldyh@redhat.com> wrote:
>
> [pinskia: I'm CCing you as the author of the match.pd pattern.]
>
> So, as I wrap up the work here (latest patch attached), I see there's
> another phiopt regression (not spaceship related).  I was hoping
> someone could either give me a hand, or offer some guidance.
>
> The failure is in gcc.dg/tree-ssa/phi-opt-24.c.
>
> We fail to transform the following into -A:
>
> /* { dg-options "-O2 -fno-signed-zeros -fdump-tree-phiopt" } */
>
> float f0(float A)
> {
>   //     A == 0? A : -A    same as -A
>   if (A == 0)  return A;
>   return -A;
> }
>
> This is because the abs/negative match.pd pattern here:
>
> /* abs/negative simplifications moved from fold_cond_expr_with_comparison,
>    Need to handle (A - B) case as fold_cond_expr_with_comparison does.
>    Need to handle UN* comparisons.
>    ...
>    ...
>
> Sees IL that has the 0.0 propagated.
>
> Instead of:
>
>   <bb 2> [local count: 1073741824]:
>   if (A_2(D) == 0.0)
>     goto <bb 4>; [34.00%]
>   else
>     goto <bb 3>; [66.00%]
>
>   <bb 3> [local count: 708669601]:
>   _3 = -A_2(D);
>
>   <bb 4> [local count: 1073741824]:
>   # _1 = PHI <A_2(D)(2), _3(3)>
>
> It now sees:
>
>   <bb 4> [local count: 1073741824]:
>   # _1 = PHI <0.0(2), _3(3)>
>
> which it leaves untouched, causing the if conditional to survive.
>
> Is this something that can be done by improving the match.pd pattern,
> or should be done elsewhere?

Oh the pattern which is supposed to catch this does:
  (simplify
   (cnd (cmp @0 zerop) integer_zerop (negate@1 @0))
    (if (!HONOR_SIGNED_ZEROS (type))
     @1))

Notice the integer_zerop here.
fold_cond_expr_with_comparison has integer_zerop there too.
I am not 100% sure you can replace A_2 with 0.0 where you are doing it
as mentioned in another thread.

Thanks,
Andrew Pinski


>
> Thanks.
> Aldy