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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/37810] Bad store sinking job
Date: Tue, 27 Jul 2021 07:03:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D37810

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #2)
> The original testcase (from an IRC discussion) reduced to a C testcase is:
>=20
> struct A {
>   int n;
>   int m;
> };
>=20
> void g();
>=20
> void test (struct A* iter)
> {
>   struct A end =3D { 0, 0 };
>   while (iter->n !=3D end.n)
>     {
>       iter->n =3D iter->n + 1;
>       if (iter->n =3D=3D iter->m)
>         g();
>     }
> }
>=20
> where there is an optimization possibility to sink the store to iter->n to
> before the call and apply load-store motion to iter->n for the remaining
> loop.

So to clarify the desired code would look like

void test (struct A* iter)
{
  struct A end =3D { 0, 0 };
  int tem =3D iter->n;
  while (tem !=3D end.n)
    {
      tem =3D tem + 1;
      if (tem =3D=3D iter->m)
        {
          iter->n =3D tem;
          g();
          tem =3D iter->n;
        }
    }
  iter->n =3D tem;
}

PRE does half of the job, but then sinking would need to duplicate the store
on the loop exit, something it currently cannot do (the logic is to duplica=
te
to two places less frequently executed).=