[Bug target/114615] spurious warning on mingw-w64: 'memcpy' reading 4 or more bytes from a region of size 2 with std::wstring{L""} and -flto -O1 [Wstringop-overread]

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114615

--- Comment #4 from Andrew Pinski  ---
(In reply to Jonathan Wakely from comment #3)
> The dumb part is that __n here comes from wcslen(__s2), so the compiler is
> able to track that __s2 is only two bytes, but not capable of tracking that
> __n == 0.
> 
> Specifically, __n is (__s2 + wcslen(__s2)) - __s2 which is just wcslen(L"")
> which is 0.

I dont think we fold/track wide_t builtins at all ...

[Bug target/114615] spurious warning on mingw-w64: 'memcpy' reading 4 or more bytes from a region of size 2 with std::wstring{L""} and -flto -O1 [Wstringop-overread]

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114615

--- Comment #3 from Jonathan Wakely  ---
The dumb part is that __n here comes from wcslen(__s2), so the compiler is able
to track that __s2 is only two bytes, but not capable of tracking that __n ==
0.

Specifically, __n is (__s2 + wcslen(__s2)) - __s2 which is just wcslen(L"")
which is 0.

[Bug target/114615] spurious warning on mingw-w64: 'memcpy' reading 4 or more bytes from a region of size 2 with std::wstring{L""} and -flto -O1 [Wstringop-overread]

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114615

Jonathan Wakely  changed:

   What|Removed |Added

   Keywords||diagnostic

--- Comment #2 from Jonathan Wakely  ---
Or jump threading is splitting the code into two branches for N <= 1 and N >=
2, and then warning that the N >= 2 case would read past the end of the source
buffer. But that case never actually happens.

The constructor calls _M_construct which goes to:

  static void
  _S_copy(_CharT* __d, const _CharT* __s, size_type __n)
  {
if (__n == 1)
  traits_type::assign(*__d, *__s);
else
  traits_type::copy(__d, __s, __n);
  }

The N == 1 case is handled here, then char_traits::copy does:

  static _GLIBCXX20_CONSTEXPR char_type*
  copy(char_type* __s1, const char_type* __s2, size_t __n)
  {
if (__n == 0)
  return __s1;
#if __cplusplus >= 202002L
if (std::__is_constant_evaluated())
  return __gnu_cxx::char_traits::copy(__s1, __s2, __n);
#endif
return wmemcpy(__s1, __s2, __n);
  }

So the N == 0 case is also handled here, so we only use wmemcpy for N >= 2. And
that would indeed read N * sizeof(wchar_t), i.e. 4 or more bytes, from L""
which is only 2 bytes.

But it's unreachable, because we take the if (__n == 0) branch.

[Bug target/114615] spurious warning on mingw-w64: 'memcpy' reading 4 or more bytes from a region of size 2 with std::wstring{L""} and -flto -O1 [Wstringop-overread]

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114615

Andrew Pinski  changed:

   What|Removed |Added

 Target||x86_64-w64-mingw32
  Component|tree-optimization   |target

--- Comment #1 from Andrew Pinski  ---
-municode defines UNICODE .

Maybe there is some mismatching of wchar_t somewhere ...