[Bug libstdc++/109299] New: wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[Bug libstdc++/109299] New: wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[benni.buch at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

            Bug ID: 109299
           Summary: wrong warning on std::wstring with -O2 -std=c++20
                    -D_FORTIFY_SOURCE=2
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: benni.buch at gmail dot com
  Target Milestone: ---

I run into this by using googletest with CMake in release mode.

```
#include <string>

static std::wstring foo(std::wstring text = {}) {
    text.resize(42);
    return text;
}

int main() {
    foo();
}
```

```
$ g++ -O2 -std=c++20 -D_FORTIFY_SOURCE=2 main.cpp 
In file included from /usr/include/features.h:486,
                 from
/usr/include/x86_64-linux-gnu/c++/12/bits/os_defines.h:39,
                 from
/usr/include/x86_64-linux-gnu/c++/12/bits/c++config.h:655,
                 from /usr/include/c++/12/string:38,
                 from main.cpp:1:
In function ‘wchar_t* wmemcpy(wchar_t*, const wchar_t*, size_t)’,
    inlined from ‘static constexpr std::char_traits<wchar_t>::char_type*
std::char_traits<wchar_t>::copy(char_type*, const char_type*, std::size_t)’ at
/usr/include/c++/12/bits/char_traits.h:558:16,
    inlined from ‘constexpr std::__cxx11::basic_string<_CharT, _Traits,
_Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&)
[with _CharT = wchar_t; _Traits = std::char_traits<wchar_t>; _Alloc =
std::allocator<wchar_t>]’ at /usr/include/c++/12/bits/basic_string.h:675:23,
    inlined from ‘std::wstring foo(std::wstring)’ at main.cpp:5:12,
    inlined from ‘int main()’ at main.cpp:9:8:
/usr/include/x86_64-linux-gnu/bits/wchar2.h:42:10: warning: call to
‘__wmemcpy_chk_warn’ declared with attribute warning: wmemcpy called with
length bigger than size of destination buffer [-Wattribute-warning]
   42 |   return __glibc_fortify_n (wmemcpy, __n, sizeof (wchar_t),
      |          ^~~~~~~~~~~~~~~~~
```

On stackoverflow.com you can already find a few discussion about it:

- https://stackoverflow.com/questions/75689057

user17732522 reduced it to:

```
#include <wchar.h>

wchar_t* _M_dataplus;
int _M_string_length;

wchar_t _M_local_buf[4];
wchar_t _M_local_buf2[4];

void g();

void f() {
    g();
    _M_string_length = 4;
    if (_M_dataplus == _M_local_buf)
        wmemcpy(_M_local_buf2, _M_local_buf, _M_string_length + 1);
}
```

```
$ g++ -O1 -D_FORTIFY_SOURCE=2 -c main.cpp 
In file included from /usr/include/features.h:486,
                 from
/usr/include/x86_64-linux-gnu/bits/libc-header-start.h:33,
                 from /usr/include/wchar.h:27,
                 from main.cpp:1:
In function ‘wchar_t* wmemcpy(wchar_t*, const wchar_t*, size_t)’,
    inlined from ‘void f()’ at main.cpp:15:16:
/usr/include/x86_64-linux-gnu/bits/wchar2.h:42:10: warning: call to
‘__wmemcpy_chk_warn’ declared with attribute warning: wmemcpy called with
length bigger than size of destination buffer [-Wattribute-warning]
   42 |   return __glibc_fortify_n (wmemcpy, __n, sizeof (wchar_t),
      |          ^~~~~~~~~~~~~~~~~
```

[Bug tree-optimization/109299] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |xry111 at gcc dot gnu.org

--- Comment #1 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
You are reducing it wrong.  The reduced example should indeed trigger the
warning, as it's copying 5 wchar's into a buffer with length 4.

[Bug tree-optimization/109299] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Are you really using GCC 12.0? If not, please fix the Version field.

[Bug tree-optimization/109299] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[benni.buch at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

Benjamin Buch <benni.buch at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|12.0                        |12.1.0

--- Comment #3 from Benjamin Buch <benni.buch at gmail dot com> ---
You are right, I use GCC 12.1.0, sorry!

I also ran reproduce it via godbolt.org starting from version 12.1 until
current trunk.

https://godbolt.org/z/Y9frhGr86

[Bug libstdc++/109299] [12/13 Regression] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
      Known to fail|                            |12.2.0, 13.0
   Target Milestone|---                         |12.3
           Priority|P3                          |P2
   Last reconfirmed|                            |2023-03-28
      Known to work|                            |11.3.0
            Version|12.1.0                      |12.2.0
            Summary|wrong warning on            |[12/13 Regression] wrong
                   |std::wstring with -O2       |warning on std::wstring
                   |-std=c++20                  |with -O2 -std=c++20
                   |-D_FORTIFY_SOURCE=2         |-D_FORTIFY_SOURCE=2
          Component|tree-optimization           |libstdc++
             Status|UNCONFIRMED                 |NEW

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed for the unreduced testcase.  Works for GCC 11.

[Bug libstdc++/109299] [12/13 Regression] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is not a library bug. The library code is:

      basic_string(basic_string&& __str) noexcept
      : _M_dataplus(_M_local_data(), std::move(__str._M_get_allocator()))
      {
        if (__str._M_is_local())
          {
            traits_type::copy(_M_local_buf, __str._M_local_buf,
                              __str.length() + 1);
          }
        else
          {
            _M_data(__str._M_data());
            _M_capacity(__str._M_allocated_capacity);
          }


The warning is coming from the call to traits_type::copy which only happens if
the string fits in the local buffer. Warning that it overflows the buffer is
not helpful when we don't take that branch BECAUSE IT WOULD OVERFLOW THE
BUFFER.

These warnings need to be removed from the compiler.

[Bug libstdc++/109299] [12/13 Regression] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This change to bits/basic_string.h seems to shut the compiler up:

--- include/bits/basic_string.h 2023-02-27 13:42:21.295513537 +0000
@@ -271,7 +271,15 @@
       _GLIBCXX20_CONSTEXPR
       bool
       _M_is_local() const
-      { return _M_data() == _M_local_data(); }
+      {
+        if (_M_data() == _M_local_data())
+          {
+            if (_M_string_length > _S_local_capacity)
+              __builtin_unreachable();
+            return true;
+          }
+        return false;
+      }

       // Create & Destroy
       _GLIBCXX20_CONSTEXPR

[Bug libstdc++/109299] [12/13 Regression] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Do we also get better generated code with your above patch?  If so, we should
go with it.

[Bug libstdc++/109299] [12/13 Regression] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, it looks better:

--- 109299-orig.s       2023-03-28 11:01:09.886119878 +0100
+++ 109299-fixed.s      2023-03-28 11:01:21.361187289 +0100
@@ -235,21 +235,14 @@
        xorl    %esi, %esi
        call    wmemset
        movq    (%rsp), %rax
-       leaq    48(%rsp), %rdi
-       movq    $42, 8(%rsp)
-       movq    %rdi, 32(%rsp)
+       leaq    32(%rsp), %rdi
+       movq    %rbx, (%rsp)
+       movq    $42, 40(%rsp)
        movl    $0, 168(%rax)
-       cmpq    %rbx, %rax
-       je      .L35
        movq    %rax, 32(%rsp)
        movq    16(%rsp), %rax
-       movq    %rax, 48(%rsp)
-       movl    $42, %eax
-.L36:
-       leaq    32(%rsp), %rdi
-       movq    %rax, 40(%rsp)
-       movq    %rbx, (%rsp)
        movq    $0, 8(%rsp)
+       movq    %rax, 48(%rsp)
        movl    $0, 16(%rsp)
        call   
_ZNSt7__cxx1112basic_stringIwSt11char_traitsIwESaIwEE10_M_disposeEv
        movq    %rsp, %rdi
@@ -261,17 +254,10 @@
        popq    %rbx
        .cfi_def_cfa_offset 8
        ret
-.L35:
+.L36:
        .cfi_restore_state
-       movl    $4, %ecx
-       movl    $43, %edx
-       movq    %rbx, %rsi
-       call    __wmemcpy_chk
-       movq    8(%rsp), %rax
-       jmp     .L36
-.L38:
        movq    %rax, %rbx
-       jmp     .L37
+       jmp     .L35
        .globl  __gxx_personality_v0
        .section        .gcc_except_table,"a",@progbits
 .LLSDA1811:
@@ -282,7 +268,7 @@
 .LLSDACSB1811:
        .uleb128 .LEHB0-.LFB1811
        .uleb128 .LEHE0-.LEHB0
-       .uleb128 .L38-.LFB1811
+       .uleb128 .L36-.LFB1811
        .uleb128 0
 .LLSDACSE1811:
        .section        .text.startup
@@ -294,7 +280,7 @@
        .type   main.cold, @function
 main.cold:
 .LFSB1811:
-.L37:
+.L35:
        .cfi_def_cfa_offset 80
        .cfi_offset 3, -16
        movq    %rsp, %rdi

[Bug libstdc++/109299] [12/13 Regression] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And the reason I put the hint in _M_is_local() not in the function giving the
warning is that it might help in other functions too.

I haven't tested it yet though.

[Bug libstdc++/109299] [12/13 Regression] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:bf78b43873b0b7e8f9a430df38749b8b61f9c9b8

commit r13-6915-gbf78b43873b0b7e8f9a430df38749b8b61f9c9b8
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Mar 28 10:50:40 2023 +0100

    libstdc++: Tell GCC what basic_string::_M_is_local() means [PR109299]

    This avoids a bogus warning about overflowing a buffer, because GCC
    can't tell that we don't copy into the buffer unless it fits. By adding
    a __builtin_unreachable() hint we inform the compiler about the
    invariant that the buffer is only used when it's big enough.

    This can also improve codegen, by eliminating dead code that GCC
    couldn't tell was unreachable.

    libstdc++-v3/ChangeLog:

            PR libstdc++/109299
            * include/bits/basic_string.h (basic_string::_M_is_local()): Add
            hint for compiler that local strings fit in the local buffer.

[Bug libstdc++/109299] [12 Regression] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:00ac6fa3f2a54fb9cc17b7b7f51eae3c6bf7a1bd

commit r12-9330-g00ac6fa3f2a54fb9cc17b7b7f51eae3c6bf7a1bd
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Mar 28 10:50:40 2023 +0100

    libstdc++: Tell GCC what basic_string::_M_is_local() means [PR109299]

    This avoids a bogus warning about overflowing a buffer, because GCC
    can't tell that we don't copy into the buffer unless it fits. By adding
    a __builtin_unreachable() hint we inform the compiler about the
    invariant that the buffer is only used when it's big enough.

    This can also improve codegen, by eliminating dead code that GCC
    couldn't tell was unreachable.

    libstdc++-v3/ChangeLog:

            PR libstdc++/109299
            * include/bits/basic_string.h (basic_string::_M_is_local()): Add
            hint for compiler that local strings fit in the local buffer.

    (cherry picked from commit bf78b43873b0b7e8f9a430df38749b8b61f9c9b8)

[Bug libstdc++/109299] [12 Regression] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for 12.3