[Bug libstdc++/62187] New: std::string==const char* could compare sizes first

[Bug libstdc++/62187] New: std::string==const char* could compare sizes first

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62187

            Bug ID: 62187
           Summary: std::string==const char* could compare sizes first
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org

Hello,

when I compare 2 basic_string with ==, libstdc++ only uses the optimization of
first checking that the sizes are the same (before calling compare) if
__is_char<_CharT> and the traits and allocator are the default ones. I don't
understand why, but assuming there is a good reason, I believe the optimization
should still apply when comparing std::string and const char*.

(this applies to __vstring as well)

This was noticed in PR 62156, where we also see that std::string("foo") does a
memcpy of size 3 then sets the 4th char to '\0', where a single memcpy of size
4 would make sense.

[Bug libstdc++/62187] std::string==const char* could compare sizes first

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62187

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|NEW                         |ASSIGNED

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #5)
> I've also created an LWG issue about this,

Rather than a new issue, this was added to https://wg21.link/lwg2852 

The resolution was to confirm that operator== doesn't need to call compare if
it can determine the result another way. That means we can do the length check
unconditionally.

[Bug libstdc++/62187] std::string==const char* could compare sizes first

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62187

--- Comment #8 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:6abe341558abec40c9c44d76e7fb4fb3978e894b

commit r13-1096-g6abe341558abec40c9c44d76e7fb4fb3978e894b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jun 14 16:19:32 2022 +0100

    libstdc++: Check lengths first in operator== for basic_string [PR62187]

    As confirmed by LWG 2852, the calls to traits_type::compare do not need
    to be obsvervable, so we can make operator== compare string lengths
    first and return immediately for non-equal lengths. This avoids doing a
    slow string comparison for "abc...xyz" == "abc...xy". Previously we only
    did this optimization for std::char_traits<char>, but we can enable it
    unconditionally thanks to LWG 2852.

    For comparisons with a const char* we can call traits_type::length right
    away to do the same optimization. That strlen call can be folded away
    for constant arguments, making it very efficient.

    For the pre-C++20 operator== and operator!= overloads we can swap the
    order of the arguments to take advantage of the operator== improvements.

    libstdc++-v3/ChangeLog:

            PR libstdc++/62187
            * include/bits/basic_string.h (operator==): Always compare
            lengths before checking string contents.
            [!__cpp_lib_three_way_comparison] (operator==, operator!=):
            Reorder arguments.

[Bug libstdc++/62187] std::string==const char* could compare sizes first

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62187

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED
   Target Milestone|---                         |13.0

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for GCC 13.