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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/94905] [10/11 Regression] Bogus warning
 -Werror=maybe-uninitialized
Date: Fri, 21 Jan 2022 13:41:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94905

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
      Known to fail|                            |11.2.1
      Known to work|                            |12.0
           Keywords|                            |needs-bisection
   Last reconfirmed|2020-09-03 00:00:00         |2022-1-21
            Summary|[10/11/12 Regression] Bogus |[10/11 Regression] Bogus
                   |warning                     |warning
                   |-Werror=3Dmaybe-uninitialized |-Werror=3Dmaybe-uninitia=
lized

--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
The diagnostic is gone with GCC 12 as we there pattern-recognize a memcpy:

  <bb 2> [local count: 119292720]:
  insert_axis_len.0_1 =3D insert_axis_len;
  ret =3D{v} {CLOBBER};
  ret.rho[0] =3D insert_axis_len.0_1;
  __builtin_memcpy (_3(D), &ret, 32);
  ret =3D{v} {CLOBBER};
  return _3(D);

and the partial uninitialized read from it is not what the uninit pass can
diagnose (it's memory walking stops at the first may-def, it does not
prune must-def ranges).  But even with -fno-tree-loop-distribute-patterns
-fno-tree-vectorize where we get similar IL as with GCC 11 we do not warn s=
o I
wonder
what fixed it on trunk (when you add those options).=