[Bug libstdc++/64735] New: std::future broken on armel

[Bug libstdc++/64735] New: std::future broken on armel

[bastiaan at bjacques dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

            Bug ID: 64735
           Summary: std::future broken on armel
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bastiaan at bjacques dot org

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/armv5tel-unknown-linux-gnueabi/4.9.2/lto-wrapper
Target: armv5tel-unknown-linux-gnueabi
Configured with: /build/gcc/src/gcc-4.9-20141224/configure --prefix=/usr
--libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info
--with-bugurl=https://github.com/archlinuxarm/PKGBUILDs/issues
--enable-languages=c,c++,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch
--disable-libssp --enable-gnu-unique-object --enable-linker-build-id
--enable-cloog-backend=isl --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu --disable-multilib
--disable-werror --enable-checking=release
--host=armv5tel-unknown-linux-gnueabi --build=armv5tel-unknown-linux-gnueabi
--with-arch=armv5te --with-float=soft
Thread model: posix
gcc version 4.9.2 20141224 (prerelease) (GCC) 

Test case from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=727621 :

#include <future>

int main()
{
        std::future<bool> fb = std::async([]{ return false; });
        return 0;
}

yields:

future.cc: In function ‘int main()’:
future.cc:5:27: error: variable ‘std::future<bool> fb’ has initializer but
incomplete type
         std::future<bool> fb = std::async([]{ return false; });
                           ^
future.cc:5:62: error: invalid use of incomplete type ‘class std::future<bool>’
         std::future<bool> fb = std::async([]{ return false; });
                                                              ^
In file included from future.cc:1:0:
/usr/include/c++/4.9.2/future:114:11: error: declaration of ‘class
std::future<bool>’
     class future;
           ^
/usr/include/c++/4.9.2/future: At global scope:
/usr/include/c++/4.9.2/future:176:5: error: ‘std::future<typename
std::result_of<_Functor(_ArgTypes ...)>::type> std::async(_Fn&&, _Args&& ...)
[with _Fn = main()::<lambda()>; _Args = {}; typename
std::result_of<_Functor(_ArgTypes ...)>::type = bool]’, declared using local
type ‘main()::<lambda()>’, is used but never defined [-fpermissive]
     async(_Fn&& __fn, _Args&&... __args);
     ^
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From: "law at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/52076] bloated code for setting single bits in bitfields on m68k
Date: Thu, 22 Jan 2015 21:25:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idR076

--- Comment #2 from Jeffrey A. Law <law at gcc dot gnu.org> ---
Author: law
Date: Thu Jan 22 21:24:28 2015
New Revision: 220015

URL: https://gcc.gnu.org/viewcvs?rev"0015&root=gcc&view=rev
Log:
    PR target/52076
    * config/m68k/m68k.md (xorsi3_internal): Twiddle constraints to
    improve code density for small immediate to memory case.
    (insv): Better handle bitfield assignments when the field is
    being set to all ones.
    * config/m68k/predicates.md (reg_or_pow2_m1_operand): New
    operand predicate.

    PR target/52076
    * gcc.target/m68k/pr52076-1.c: New test.
    * gcc.target/m68k/pr52076-2.c: New test.

Added:
    trunk/gcc/config/m68k/.m68k.md.swp   (with props)
    trunk/gcc/testsuite/gcc.target/m68k/pr52076-1.c
    trunk/gcc/testsuite/gcc.target/m68k/pr52076-2.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/m68k/m68k.md
    trunk/gcc/config/m68k/predicates.md
    trunk/gcc/testsuite/ChangeLog

Propchange: trunk/gcc/config/m68k/.m68k.md.swp
            ('svn:executable' added)

[Bug libstdc++/64735] std::future broken on armel

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think this is expected, the std:future type and std::async function are
declared unconditionally, but the definitions are guarded by:

#if defined(_GLIBCXX_HAS_GTHREADS) && defined(_GLIBCXX_USE_C99_STDINT_TR1) \
  && (ATOMIC_INT_LOCK_FREE > 1)

If your target doesn't meet those conditions then you don't get to use
std::future

[Bug libstdc++/64735] std::future broken on armel

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
i.e. it's not broken, it's missing, and that's by design.

[Bug c++/64735] std::future broken on armel

[ramana at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

Ramana Radhakrishnan <ramana at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|libstdc++                   |c++

--- Comment #4 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #3)
> i.e. it's not broken, it's missing, and that's by design.

Right so if I parse that correctly, it boils down to implementation of atomics
with calls to kernel helper function vs the lack of it IIUC. Not really a
libstdc++ issue but more of an issue with the way in which atomics are
implemented on arch's that don't have the actual instruction support but are
implemented through kernel helper functions.

It appears as though this breaks quite a few packages on older versions of the
architecture so that's interesting.

Ramana

[Bug c++/64735] std::future broken on armel

[bastiaan at bjacques dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

--- Comment #5 from Bastiaan Jacques <bastiaan at bjacques dot org> ---
(In reply to Jonathan Wakely from comment #3)
> i.e. it's not broken, it's missing, and that's by design.

So is it the intention of the GCC developers that program writers targeting
such platforms simply avoid these facilities and use std::thread/mutex instead?

[Bug c++/64735] std::future broken on armel

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Bastiaan Jacques from comment #5)
> (In reply to Jonathan Wakely from comment #3)
> > i.e. it's not broken, it's missing, and that's by design.
> 
> So is it the intention of the GCC developers that program writers targeting
> such platforms simply avoid these facilities and use std::thread/mutex
> instead?

Or they could contribute an implementation that works for their target.

The intention of the developers is that the feature should work correctly, and
if that isn't possible with the current implementation for some target, then
the feature should be disabled on that target. And that's what you see.

[Bug libstdc++/64735] std::future broken on armel

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

--- Comment #14 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:6c0c7fc6236470a533675cd3cd1ebb1cc3dd112c

commit r11-8198-g6c0c7fc6236470a533675cd3cd1ebb1cc3dd112c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Apr 14 20:48:54 2021 +0100

    libstdc++: Move atomic functions to libsupc++ [PR 96657]

    The changes for PR libstdc++/64735 mean that libsupc++ function might
    now depend on the __exchange_and_add and __atomic_add functions defined
    in config/cpu/*/atomicity.h which is not compiled into libsupc++. This
    causes a link failure for some targets when trying to use libsupc++
    without the rest of libstdc++.

    This patch simply moves the definitions of those functions into
    libsupc++ so that they are available there.

    libstdc++-v3/ChangeLog:

            PR libstdc++/96657
            * libsupc++/Makefile.am: Add atomicity.cc here.
            * src/c++98/Makefile.am: Remove it from here.
            * libsupc++/Makefile.in: Regenerate.
            * src/c++98/Makefile.in: Regenerate.
            * testsuite/18_support/exception_ptr/96657.cc: New test.

[Bug libstdc++/64735] std::future broken on armel

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

--- Comment #15 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:f2074277aa3ce0848429e34d6149ba26ff3b708e

commit r10-9997-gf2074277aa3ce0848429e34d6149ba26ff3b708e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Apr 14 20:48:54 2021 +0100

    libstdc++: Move atomic functions to libsupc++ [PR 96657]

    The changes for PR libstdc++/64735 mean that libsupc++ function might
    now depend on the __exchange_and_add and __atomic_add functions defined
    in config/cpu/*/atomicity.h which is not compiled into libsupc++. This
    causes a link failure for some targets when trying to use libsupc++
    without the rest of libstdc++.

    This patch simply moves the definitions of those functions into
    libsupc++ so that they are available there.

    libstdc++-v3/ChangeLog:

            PR libstdc++/96657
            * libsupc++/Makefile.am: Add atomicity.cc here.
            * src/c++98/Makefile.am: Remove it from here.
            * libsupc++/Makefile.in: Regenerate.
            * src/c++98/Makefile.in: Regenerate.
            * testsuite/18_support/exception_ptr/96657.cc: New test.

    (cherry picked from commit 6c0c7fc6236470a533675cd3cd1ebb1cc3dd112c)

[Bug libstdc++/64735] std::future broken on armel

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

--- Comment #16 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:ba5e10a8c84f56bb2838adb6d1cc9b74741ac4f1

commit r9-9640-gba5e10a8c84f56bb2838adb6d1cc9b74741ac4f1
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Apr 14 20:48:54 2021 +0100

    libstdc++: Move atomic functions to libsupc++ [PR 96657]

    The changes for PR libstdc++/64735 mean that libsupc++ function might
    now depend on the __exchange_and_add and __atomic_add functions defined
    in config/cpu/*/atomicity.h which is not compiled into libsupc++. This
    causes a link failure for some targets when trying to use libsupc++
    without the rest of libstdc++.

    This patch simply moves the definitions of those functions into
    libsupc++ so that they are available there.

    libstdc++-v3/ChangeLog:

            PR libstdc++/96657
            * libsupc++/Makefile.am: Add atomicity.cc here.
            * src/c++98/Makefile.am: Remove it from here.
            * libsupc++/Makefile.in: Regenerate.
            * src/c++98/Makefile.in: Regenerate.
            * testsuite/18_support/exception_ptr/96657.cc: New test.

    (cherry picked from commit 6c0c7fc6236470a533675cd3cd1ebb1cc3dd112c)