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From: "mrsam@courier-mta.com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/100366] New: spurious warning - std::vector::clear followed
 by std::vector::insert(vec.end(), ...) with -O2
Date: Sat, 01 May 2021 02:36:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100366

            Bug ID: 100366
           Summary: spurious warning - std::vector::clear followed by
                    std::vector::insert(vec.end(), ...) with -O2
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mrsam@courier-mta.com
  Target Milestone: ---

I'm seeing this with both gcc 11.1 on: https://godbolt.org/z/1Wv9jj3r1

and gcc (GCC) 11.0.1 20210324 (Red Hat 11.0.1-0), below:

Compiling the following with -O2 -Wall -Werror:

#include <vector>
#include <memory>

static char UTC[4];

void func(std::vector<char> &vec)
{
        vec.clear();
        vec.insert(vec.end(), UTC, UTC+4);
}

This generates a massive complaint that boils down to:

/usr/include/c++/11/bits/stl_algobase.h:431:30: error: =E2=80=98void*
__builtin_memcpy(v
oid*, const void*, long unsigned int)=E2=80=99 writing 1 or more bytes into=
 a region of=20
size 0 overflows the destination [-Werror=3Dstringop-overflow=3D]
  431 |             __builtin_memmove(__result, __first, sizeof(_Tp) * _Num=
);
      |             ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from
/usr/include/c++/11/x86_64-redhat-linux/bits/c++allocator.

...skipping 1 line
                 from /usr/include/c++/11/bits/allocator.h:46,
                 from /usr/include/c++/11/vector:64,
                 from t.C:1:

Removing vec.clear() makes this go away.

Adding vec.reserve(4) after vec.clear() makes this go away.=