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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/103006] [9/10/11/12 Regression] wrong code at
 -O1 or -O2 on x86_64-linux-gnu by r7-7101
Date: Mon, 01 Nov 2021 11:13:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103006

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |matz at gcc dot gnu.org

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Doesn't look to me like RA issue, but rather incorrect coalescing of tempor=
ary
vars.
Optimized dump has:
  <bb 2> [local count: 1652516]:
  ivtmp.40_47 =3D (unsigned long) &h;
  _30 =3D ivtmp.40_47 + 96;

  <bb 3> [local count: 13370357]:
  # ivtmp.40_38 =3D PHI <ivtmp.40_47(2), ivtmp.40_44(3)>
  _21 =3D (void *) ivtmp.40_38;
  MEM[(long int *)_21] =3D 1;
  MEM[(long int *)_21 + 8B] =3D 1;
  ivtmp.40_44 =3D ivtmp.40_38 + 16;
  if (_30 !=3D ivtmp.40_44)
    goto <bb 3>; [89.00%]
  else
    goto <bb 4>; [11.00%]

  <bb 4> [local count: 1652516]:
  h =3D{v} {CLOBBER};

  <bb 5> [local count: 13370357]:
  # ivtmp.30_29 =3D PHI <ivtmp.30_48(5), ivtmp.40_47(4)>
  _31 =3D (void *) ivtmp.30_29;
  MEM[(long int *)_31] =3D 1;
  MEM[(long int *)_31 + 8B] =3D 1;
  ivtmp.30_48 =3D ivtmp.30_29 + 16;
  if (_30 !=3D ivtmp.30_48)
    goto <bb 5>; [89.00%]
  else
    goto <bb 6>; [11.00%]

  <bb 6> [local count: 1652516]:
  f =3D 2;
  e =3D 6;
  c =3D 1;
  b =3D &d;
  d[6] =3D &d;
  ff ();
  d =3D{v} {CLOBBER};
  _5 =3D h[3][0];
  _18 =3D (int) _5;
  h =3D{v} {CLOBBER};
  return _18;

So, the code initializes the whole h array, then has h =3D{v} {CLOBBER};, t=
hen
initializes it again, but unfortunately without mentioning the var in the I=
L -
it reuses ivtmp.40_47 for that, then sets various vars, including d[6]
initialization with &d escaping, clobbers d, reads from h and finally clobb=
ers
h again.  I guess the var partitioning code from the above thinks h isn't
really live in between the first h clobber and h[3][0] load and so decides:
Partition 0: size 96 align 16
        h       d
and places both h and d into the same stack slot.=