[Bug libstdc++/101965] New: check in charconv is vacuously true

[Bug libstdc++/101965] New: check in charconv is vacuously true

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101965

            Bug ID: 101965
           Summary: check in charconv is vacuously true
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

When investigating PVS-Studio in our codebase, it flagged this check in
charconv:

   322    template<typename _Tp>
   323      __detail::__integer_to_chars_result_type<_Tp>
   324      __to_chars_i(char* __first, char* __last, _Tp __value, int __base =
10)
   325      {
   326        __glibcxx_assert(2 <= __base && __base <= 36);
   327
   328        using _Up = __detail::__unsigned_least_t<_Tp>;
   329        _Up __unsigned_val = __value;
   330
   331        if (__first == __last) [[__unlikely__]]
   332          return { __last, errc::value_too_large };
   333
   334        if (__value == 0)
   335          {
   336            *__first = '0';
   337            return { __first + 1, errc{} };
   338          }
   339
   340        if _GLIBCXX17_CONSTEXPR (std::is_signed<_Tp>::value)
   341          if (__value < 0)
   342            {
   343              if (__builtin_expect(__first != __last, 1))
   344                *__first++ = '-';
   345              __unsigned_val = _Up(~__value) + _Up(1);
   346            }
   347
   348        switch (__base)
   349        {
   350        case 16:

On line 343, __first is never equal to __last. If it were, we would've returned
on line 332.

[Bug libstdc++/101965] check in charconv is vacuously true

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101965

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-08-19

[Bug libstdc++/101965] check in charconv is vacuously true

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101965

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
At least I got the branch prediction right though.

[Bug libstdc++/101965] check in charconv is vacuously true

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101965

--- Comment #2 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:85a709595005b5df4b2ee9d81717a5df19c0023f

commit r12-3025-g85a709595005b5df4b2ee9d81717a5df19c0023f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Aug 19 13:05:54 2021 +0100

    libstdc++: Don't check always-true condition [PR101965]

    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

    libstdc++-v3/ChangeLog:

            PR libstdc++/101965
            * include/std/charconv (__to_chars_i): Remove redundant check.

[Bug libstdc++/101965] check in charconv is vacuously true

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101965

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.4
           Severity|normal                      |trivial

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk, but I plan to backport it.

[Bug libstdc++/101965] check in charconv is vacuously true

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101965

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:8334ee1922104b5379efe94a19e17bc630cae9d5

commit r11-8922-g8334ee1922104b5379efe94a19e17bc630cae9d5
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Aug 19 13:05:54 2021 +0100

    libstdc++: Don't check always-true condition [PR101965]

    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

    libstdc++-v3/ChangeLog:

            PR libstdc++/101965
            * include/std/charconv (__to_chars_i): Remove redundant check.

    (cherry picked from commit 85a709595005b5df4b2ee9d81717a5df19c0023f)

[Bug libstdc++/101965] check in charconv is vacuously true

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101965

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:30033d9bb9d5e5303fadf448999f4f27e2693ed6

commit r10-10309-g30033d9bb9d5e5303fadf448999f4f27e2693ed6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Aug 19 13:05:54 2021 +0100

    libstdc++: Don't check always-true condition [PR101965]

    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

    libstdc++-v3/ChangeLog:

            PR libstdc++/101965
            * include/std/charconv (__to_chars_i): Remove redundant check.

    (cherry picked from commit 85a709595005b5df4b2ee9d81717a5df19c0023f)

[Bug libstdc++/101965] check in charconv is vacuously true

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101965

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

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

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for 11.3 and 10.4