https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109849
--- Comment #34 from Martin Jambor <jamborm at gcc dot gnu.org> ---
(In reply to Jan Hubicka from comment #32)
> > /tmp/build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/stl_algobase.h:437:
> > warning: 'void* __builtin_memcpy(void*, const void*, long unsigned int)'
> > writing between 2 and 9223372036854775806 bytes into a region of size 0
> > overflows the destination [-Wstringop-overflow=]
>
> It warns on:
>
> template<bool _IsMove>
> struct __copy_move<_IsMove, true, random_access_iterator_tag>
> {
> template<typename _Tp, typename _Up>
> _GLIBCXX20_CONSTEXPR
> static _Up*
> __copy_m(_Tp* __first, _Tp* __last, _Up* __result)
> {
> const ptrdiff_t _Num = __last - __first;
> if (__builtin_expect(_Num > 1, true))
> __builtin_memmove(__result, __first, sizeof(_Tp) * _Num);
> else if (_Num == 1)
> std::__copy_move<_IsMove, false, random_access_iterator_tag>::
> __assign_one(__result, __first);
> return __result + _Num;
> }
> };
>
> It is likely false positive on a code path that never happens in real
> code, but we now optimize it better.
>
We end up with:
<bb 16> [local count: 64736968]:
__builtin_memcpy (1B, v$_M_impl$D10203$_M_start_448, _354);
IIRC the statement variant is created by jump threading (specifically
thread2).
Moreover, if I understand the comment in compute_objsize_r about the
INTEGER_CST case correctly, small integers are considered potential
"result of erroneous null pointer addition/subtraction." So not
warning on a constant 1 destination does not seem to be desirable.
