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From: "shin.david at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/108230] New: assert() spuriously activates
 maybe-initialized warning
Date: Mon, 26 Dec 2022 19:02:32 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.2.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108230

            Bug ID: 108230
           Summary: assert() spuriously activates maybe-initialized
                    warning
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: shin.david at gmail dot com
  Target Milestone: ---

gcc-12.2 reports a maybe-uninitialized warning for the last line of the bel=
ow
code if-and-only-if the macro ADD_ASSERT is defined:

#include <bitset>
#include <Eigen/Core>

using Array =3D Eigen::Array<float, -1, 1, 0, 3, 1>;

struct Tree {
  Array array;
  std::bitset<8> bitset;
};

auto func(Tree* tree) {
  int c =3D tree->array.rows();
  int d =3D tree->bitset.count();
#ifdef ADD_ASSERT
  assert(c=3D=3Dd);
#endif  // ADD_ASSERT
  Array e(c);
  for (int k =3D 0; k < d; ++k) e(k) =3D k;
  return e.sum() / (e + 1);
}

int main() {
  Tree tree;
  func(&tree);
}

This is a bug because adding this assert() should have no bearing on whethe=
r or
not the return statement might entail uninitialized memory usage.

gcc cmd1, generates warning: -Wmaybe-uninitialized -O3 -DADD_ASSERT
gcc cmd2, no warning:        -Wmaybe-uninitialized -O3

Godbolt link, which includes full gcc warning output:
https://godbolt.org/z/nv11aaKb6

This example, convoluted as it seems, appears to be a minimal reproducible
example. For example:

- Removing the for-loop makes the warning appear with both gcc cmds
- Replacing the return expression with something simpler like (e + 1) makes=
 the
warning disappear with both gcc cmds
- Replacing std::bitset with something else also makes the warning disappear
with both gcc cmds

Eigen library version is 3.4.0.=