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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103281] [9/12 Regression] Dead Code
 Elimination Regression at -O3 (trunk vs 11.2.0)
Date: Tue, 16 Nov 2021 20:51:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103281

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0
           Keywords|                            |missed-optimization
     Ever confirmed|0                           |1
      Known to fail|                            |10.1.0, 12.0, 4.7.1, 4.8.1,
                   |                            |9.1.0
            Summary|[12 Regression] Dead Code   |[9/12 Regression] Dead Code
                   |Elimination Regression at   |Elimination Regression at
                   |-O3 (trunk vs 11.2.0)       |-O3 (trunk vs 11.2.0)
   Last reconfirmed|                            |2021-11-16
      Known to work|                            |11.1.0, 4.9.0, 4.9.4,
                   |                            |5.1.0, 6.1.0, 7.1.0, 8.1.0
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The only thing which PHIOPT does is change:
  if (c_15 =3D=3D 0)
    goto <bb 4>; [INV]
  else
    goto <bb 5>; [INV]

  <bb 4> :

  <bb 5> :
  # iftmp.1_10 =3D PHI <0(4), c_15(3)>

to be:
iftmp.1_10 =3D c_15;

This is a missed VRP:
  # RANGE [0, 2] NONZERO 3
  c_9 =3D (charD.10) b.4_5;
  _1 =3D c_9 <=3D 0;
  # RANGE [0, 1] NONZERO 1
  _2 =3D (unsigned intD.14) _1;
  if (_2 =3D=3D b.4_5)

_1/_2 is true/1 only when c_9 is 0 and only when b.4_5 is 0.

----- CUT -----


We should be able to optimize:
void foo(void);

static unsigned b;

int main() {
  for (; b < 3; b++) {
    char c =3D b;
    char a =3D c;
    if (!((a < 1) ^ b))
      foo();
  }
}
Too.

We also miss:
void foo(void);

static unsigned b;

int main() {
  for (; b < 3; b++) {
    if (!((b < 1) ^ b))
      foo();
  }
}

  _1 =3D b.2_6 =3D=3D 0;
  _2 =3D (unsigned int) _1;
  if (_2 =3D=3D b.2_6)

I want to say we should defer this until after GCC 12 because this was only
being optimized on accident.  phiopt1=