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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug ipa/103875] Dead writes are not optimized out
Date: Fri, 31 Dec 2021 14:14:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103875

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
          Component|middle-end                  |ipa
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-12-31
                 CC|                            |marxin at gcc dot gnu.org

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The inliner removes the clobber in this case it seems:
We have:

void foo::pop (struct foo * const this)
{
  long unsigned int _1;
  long unsigned int _2;
  long unsigned int _3;
  value_type * _4;

  <bb 2> :
  _1 =3D this_6(D)->size;
  _2 =3D _1 + 18446744073709551615;
  this_6(D)->size =3D _2;
  _3 =3D this_6(D)->size;
  _4 =3D &this_6(D)->data[_3];
  *_4 =3D{v} {CLOBBER};
  return;

}
void use_foo (struct foo & f)
{
  int _1;
  int _2;
  value_type & _6;

  <bb 2> :
  _6 =3D foo::back (f_4(D));
  _1 =3D *_6;
  _2 =3D _1 * 42;
  *_6 =3D _2;
  foo::pop (f_4(D));
  return;

}

And then foo::pop (and back) gets inlined we get:

void use_foo (struct foo & f)
{
  value_type & D.2143;
  int _1;
  int _2;
  long unsigned int _5;
  value_type & _6;
  long unsigned int _9;
  long unsigned int _10;
  long unsigned int _11;
  value_type & _12;
  value_type & _13;

  <bb 2> :
  _10 =3D f_4(D)->size;
  _11 =3D _10 + 18446744073709551615;
  _12 =3D &f_4(D)->data[_11];
  _13 =3D _12;
  _6 =3D _13;
  _1 =3D *_6;
  _2 =3D _1 * 42;
  *_6 =3D _2;
  _5 =3D f_4(D)->size;
  _9 =3D _5 + 18446744073709551615;
  f_4(D)->size =3D _9;
  return;

}=