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Subject: [Bug target/106187] armhf: Miscompilation at O2 level (O0 / O1 are
 working)
Date: Mon, 18 Jul 2022 15:45:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106187

Richard Earnshaw <rearnsha at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #26 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
git bisect points to commit r11-9688 resolving the issue.  Before that comm=
it
the ivopts pass generates:

  ivtmp.761_217 =3D (unsigned int) &au;
  _222 =3D &bu + 4;
  ivtmp.767_220 =3D (unsigned int) _222;
  _225 =3D (unsigned int) &au;
  _228 =3D _225 + 16;

  <bb 9> [local count: 858993457]:
  # prephitmp_136 =3D PHI <pretmp_120(10), 1073741824(8)>
  # prephitmp_32 =3D PHI <pretmp_18(10), 2147483648(8)>
  # ivtmp.761_278 =3D PHI <ivtmp.761_216(10), ivtmp.761_217(8)>
  # ivtmp.767_218 =3D PHI <ivtmp.767_219(10), ivtmp.767_220(8)>
  _16 =3D prephitmp_32 ^ prephitmp_136;
  _223 =3D (void *) ivtmp.761_278;
  MEM[(unsigned int *)_223] =3D _16;
  ivtmp.761_216 =3D ivtmp.761_278 + 4;
  if (ivtmp.761_216 !=3D _228)
    goto <bb 10>; [75.00%]
  else
    goto <bb 11>; [25.00%]

  <bb 10> [local count: 644245086]:
  _230 =3D (void *) ivtmp.761_216;
  pretmp_120 =3D MEM[(unsigned int *)_230];
  _229 =3D (void *) ivtmp.767_218;
  pretmp_18 =3D MEM[(unsigned int *)_229];
  ivtmp.767_219 =3D ivtmp.767_218 + 4;
  goto <bb 9>; [100.00%]

And once that patch is applied we get:

  ivtmp.761_217 =3D (unsigned int) &au;
  ivtmp.766_220 =3D (unsigned int) &bu;
  _223 =3D (unsigned int) &au;
  _225 =3D _223 + 16;

  <bb 9> [local count: 858993457]:
  # prephitmp_136 =3D PHI <pretmp_120(10), 1073741824(8)>
  # prephitmp_32 =3D PHI <pretmp_18(10), 2147483648(8)>
  # ivtmp.761_278 =3D PHI <ivtmp.761_216(10), ivtmp.761_217(8)>
  # ivtmp.766_218 =3D PHI <ivtmp.766_219(10), ivtmp.766_220(8)>
  _16 =3D prephitmp_32 ^ prephitmp_136;
  _222 =3D (void *) ivtmp.761_278;
  MEM[(unsigned int *)_222] =3D _16;
  ivtmp.761_216 =3D ivtmp.761_278 + 4;
  if (ivtmp.761_216 !=3D _225)
    goto <bb 10>; [75.00%]
  else
    goto <bb 11>; [25.00%]

The main difference being that in the 'bad' code we start with &bu + 4, whi=
le
in the good code we start with &bu.

I'm afraid I don't know enough about this code to take this further.  Richi=
?=