[Bug libstdc++/105258] New: std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[Bug libstdc++/105258] New: std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[wielkiegie at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

            Bug ID: 105258
           Summary: std::get_temporary_buffer() does not respect alignment
                    (affects std::stable_sort())
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wielkiegie at gmail dot com
  Target Milestone: ---

Created attachment 52795
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52795&action=edit
The reproducer

The `std::get_temporary_buffer()` function uses raw `operator new()` to
allocate enough memory for the `sizeof(T) * count` bytes. However, it does not
take `alignof(T)` into account.

While `std::get_temporary_buffer()` is deprecated in C++17 [1], it is still
used internally by `std::stable_sort()`.

The issue is reproducible on the gcc HEAD [2]. Please note that the alignment
issues might not be deterministic as the allocator can return an allocation of
correct alignment by accident.

For convenience, I have also attached the reproducer to this bug report.

[1] https://en.cppreference.com/w/cpp/memory/get_temporary_buffer
[2] https://wandbox.org/permlink/zZt6Weh3V1XWmUWt

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-04-13

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The simplest solution would be to just use std::allocator<T> which already does
the right thing, but that would break if there is a program-defined
specialization of std::allocator<T>.

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 52799
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52799&action=edit
Patch to support extended alignment

This needs a testsuite addition, and should probably wait for GCC 13 now.

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

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

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

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|13.0                        |13.2

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 13.1 is being released, retargeting bugs to GCC 13.2.

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|13.2                        |13.3

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 13.2 is being released, retargeting bugs to GCC 13.3.

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|13.3                        |13.4

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 13.3 is being released, retargeting bugs to GCC 13.4.

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[Marco.Porsch at intenta dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

Marco Porsch <Marco.Porsch at intenta dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Marco.Porsch at intenta dot de

--- Comment #6 from Marco Porsch <Marco.Porsch at intenta dot de> ---
When compiled to ARM NEON vector operations, which have strict alignment
requirements, this issue causes a kernel alignment trap and SIGBUS.

See godbolt [1] for an example using Eigen matrix library. Compiled with clang,
the instruction `vst1.64 {d16, d17}, [r0:128]` is generated which crashes if
the pointer value is not aligned properly.

I created a corresponding ticket with Eigen [2] which points to this issue and
has extra info.

Looking at the stdlib implementation of libc++ [3] and MSVC [4] I see a similar
approach as proposed in the attached patch.

[1] https://godbolt.org/z/1TxcTzz53
[2] https://gitlab.com/libeigen/eigen/-/issues/2826
[3]
https://github.com/llvm/llvm-project/blob/main/libcxx/include/__memory/temporary_buffer.h#L26
[4] https://github.com/microsoft/STL/blob/main/stl/inc/xmemory#L1541

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The patch needs a little refactoring to share code with std::stacktrace, but it
will be fixed soon.

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch
                URL|                            |https://gcc.gnu.org/piperma
                   |                            |il/gcc-patches/2024-June/65
                   |                            |3296.html

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Patch posted: https://gcc.gnu.org/pipermail/gcc-patches/2024-June/653296.html

[Bug libstdc++/105258] std::get_temporary_buffer() does not respect alignment (affects std::stable_sort())

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105258

--- Comment #9 from GCC 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:b0efdcbf58a76af3b8fff75f1d53d334fb5b46ee

commit r15-997-gb0efdcbf58a76af3b8fff75f1d53d334fb5b46ee
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Apr 13 13:03:44 2022 +0100

    libstdc++: Handle extended alignment in std::get_temporary_buffer
[PR105258]

    This adds extended alignment support to std::get_temporary_buffer etc.
    so that when std::stable_sort uses a temporary buffer it works for
    overaligned types.

    Also simplify the _Temporary_buffer type by using RAII for the
    allocation, via a new data member. This simplifies the _Temporary_buffer
    constructor and destructor by makingthem only responsible for
    constructing and destroying the elements, not managing the memory.

    libstdc++-v3/ChangeLog:

            PR libstdc++/105258
            * include/bits/stl_tempbuf.h (__detail::__get_temporary_buffer):
            New function to do allocation for get_temporary_buffer, with
            extended alignment support.
            (__detail::__return_temporary_buffer): Support extended
            alignment.
            (get_temporary_buffer): Use __get_temporary_buffer.
            (return_temporary_buffer): Support extended alignment. Add
            deprecated attribute.
            (_Temporary_buffer): Move allocation and deallocation into a
            subobject and remove try-catch block in constructor.
            (__uninitialized_construct_buf): Use argument deduction for
            value type.
            * testsuite/20_util/temporary_buffer.cc: Add dg-warning for new
            deprecated warning.
            * testsuite/25_algorithms/stable_sort/overaligned.cc: New test.