[Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer

[Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995

            Bug ID: 105995
           Summary: QoI: constexpr basic_string variable must use all of
                    its SSO buffer
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

The current implementation in libstdc++ only supports constexpr
std::basic_string variable of static storage duration which uses all of its SSO
buffer:

#include <string>

constexpr std::string str15{"0123456789abcde"};
// constexpr std::string str14{"0123456789abcd"}; // error: should we support
this?
// constexpr std::string str16{"0123456789abcdef"}; // error: unsupportable

constexpr std::u16string u16str7{u"0123456"};
constexpr std::u32string u32str3{U"012"};

int main()
{
    // constexpr std::string strdyn{"0123456789abcde"}; // error: unsupportable
}

Compiler Explorer links:
https://gcc.godbolt.org/z/cvbY1hK3G
https://gcc.godbolt.org/z/634888Y96

I think constexpr string variable that requires heap allocation is currently
unimplementable, and constexpr string variable of dynamic storage duration
can't be supported by libstdc++-v3 without ABI breakage now.

However, should we allow shorter strings to be stored into constexpr
basic_string variables? We can do this by always fully initializing the SSO
buffer (if it's used) during constant evluation.

[Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
C++20 supports dynamic allocation for constexpr.

[Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995

--- Comment #2 from Jiang An <de34 at live dot cn> ---
(In reply to Andrew Pinski from comment #1)
> C++20 supports dynamic allocation for constexpr.

Yeah, but a constexpr variable can't hold dynamically allocated memory.
Dynamically allocated memory must be deallocated within its initialization.

[Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Right, you can only use std::string as a local variable in a constexpr
function. I don't think it's expected to work as a constexpr variable.

[Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-06-16
           Severity|normal                      |enhancement

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This makes it work:

--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -352,7 +352,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 #if __cpp_lib_is_constant_evaluated
        if (std::is_constant_evaluated())
-         _M_local_buf[0] = _CharT();
+         for (_CharT& __c : _M_local_buf)
+           __c = _CharT();
 #endif
        return _M_local_data();
       }

[Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995

--- Comment #5 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:98a0d72a610a87e8e383d366e50253ddcc9a51dd

commit r13-1139-g98a0d72a610a87e8e383d366e50253ddcc9a51dd
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jun 16 14:57:32 2022 +0100

    libstdc++: Support constexpr global std::string for size < 15 [PR105995]

    I don't think this is required by the standard, but it's easy to
    support.

    libstdc++-v3/ChangeLog:

            PR libstdc++/105995
            * include/bits/basic_string.h (_M_use_local_data): Initialize
            the entire SSO buffer.
            * testsuite/21_strings/basic_string/cons/char/105995.cc: New test.

[Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995

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

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

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for GCC 13, thanks for the report.

[Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995

--- Comment #7 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:e562236851e06091256593aa0d3fbda60a28e45b

commit r12-8657-ge562236851e06091256593aa0d3fbda60a28e45b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jun 16 14:57:32 2022 +0100

    libstdc++: Support constexpr global std::string for size < 15 [PR105995]

    I don't think this is required by the standard, but it's easy to
    support.

    libstdc++-v3/ChangeLog:

            PR libstdc++/105995
            * include/bits/basic_string.h (_M_use_local_data): Initialize
            the entire SSO buffer.
            * testsuite/21_strings/basic_string/cons/char/105995.cc: New test.

    (cherry picked from commit 98a0d72a610a87e8e383d366e50253ddcc9a51dd)

[Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|13.0                        |12.2

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Backported for 12.2