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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109119] New: missing loading fre with branch
 comparing two pointers
Date: Tue, 14 Mar 2023 04:58:30 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: pinskia at gcc dot gnu.org
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109119

            Bug ID: 109119
           Summary: missing loading fre with branch comparing two pointers
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
void fun (unsigned *x, unsigned *b) {
   asm volatile(""::"r"(*x));
  unsigned *a =3D b;//malloc (*x);
  if (a =3D=3D 0)
    return;
  if (a !=3D x)            // (A)
    *a =3D *x;
  *x =3D *a;
}
```
GCC currently produces:
  <bb 2> [local count: 1073741824]:
  _1 =3D *x_6(D);
  __asm__ __volatile__("" :  : "r" _1);
  if (b_7(D) =3D=3D 0B)
    goto <bb 7>; [18.09%]
  else
    goto <bb 3>; [81.91%]

  <bb 3> [local count: 879501929]:
  if (x_6(D) !=3D b_7(D))
    goto <bb 5>; [70.00%]
  else
    goto <bb 4>; [30.00%]

  <bb 4> [local count: 263850576]:
  pretmp_10 =3D *b_7(D);
  goto <bb 6>; [100.00%]

  <bb 5> [local count: 615651353]:
  *b_7(D) =3D _1;

  <bb 6> [local count: 879501929]:
  # prephitmp_11 =3D PHI <pretmp_10(4), _1(5)>
  *x_6(D) =3D prephitmp_11;

But pretmp_10 should be the same as _1 as at that point b_7 =3D=3D x_6.

Note I noticed this while looking into PR 96564.=