fix breakage from "libstdc++: Remove unnecessary uses of <utility>"

Commit r12-2534 was incomplete and (by inspection derived from
an MMIX build) failing for targets without an insn for
compare_and_swap for pointer-size objects, IOW for targets for
which "ATOMIC_POINTER_LOCK_FREE != 2" is true:

x/gcc/libstdc++-v3/src/c++17/memory_resource.cc: In member function
 'std::pmr::memory_resource*
 std::pmr::{anonymous}::atomic_mem_res::exchange(std::pmr::memory_resource*)':
x/gcc/libstdc++-v3/src/c++17/memory_resource.cc:140:21: error:
 'exchange' is not a member of 'std'
  140 |         return std::exchange(val, r);
      |                     ^~~~~~~~
make[5]: *** [Makefile:577: memory_resource.lo] Error 1
make[5]: Leaving directory
 '/home/hp/tmp/newmmix-r12-2579-p3/gccobj/mmix/libstdc++-v3/src/c++17'

This fix was derived from edits elsewhere in that patch.

Tested mmix-knuth-mmixware, restoring build (together with
target-reviving patches as MMIX is currently and at that commit
broken for target-specific reasons).

Ok to commit?

libstdc++-v3/:
	* src/c++17/memory_resource.cc: Use __exchange instead
	of std::exchange.
---
 libstdc++-v3/src/c++17/memory_resource.cc | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

On Thu, 29 Jul 2021, 23:44 Hans-Peter Nilsson, <hp@bitrange.com> wrote:

> Commit r12-2534 was incomplete and (by inspection derived from
> an MMIX build) failing for targets without an insn for
> compare_and_swap for pointer-size objects, IOW for targets for
> which "ATOMIC_POINTER_LOCK_FREE != 2" is true:
>
> x/gcc/libstdc++-v3/src/c++17/memory_resource.cc: In member function
>  'std::pmr::memory_resource*
>
>  std::pmr::{anonymous}::atomic_mem_res::exchange(std::pmr::memory_resource*)':
> x/gcc/libstdc++-v3/src/c++17/memory_resource.cc:140:21: error:
>  'exchange' is not a member of 'std'
>   140 |         return std::exchange(val, r);
>       |                     ^~~~~~~~
> make[5]: *** [Makefile:577: memory_resource.lo] Error 1
> make[5]: Leaving directory
>  '/home/hp/tmp/newmmix-r12-2579-p3/gccobj/mmix/libstdc++-v3/src/c++17'
>
> This fix was derived from edits elsewhere in that patch.
>
> Tested mmix-knuth-mmixware, restoring build (together with
> target-reviving patches as MMIX is currently and at that commit
> broken for target-specific reasons).
>
> Ok to commit?
>

Yes, thanks.

(We could also just include <utility> to get the declaration of
std::exchange, since this isn't a header so we can include whatever we
want, but this is fine.)



> libstdc++-v3/:
>         * src/c++17/memory_resource.cc: Use __exchange instead
>         of std::exchange.
> ---
>  libstdc++-v3/src/c++17/memory_resource.cc | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++17/memory_resource.cc
> b/libstdc++-v3/src/c++17/memory_resource.cc
> index 5dfc29fc0ec8..1ba79903f870 100644
> --- a/libstdc++-v3/src/c++17/memory_resource.cc
> +++ b/libstdc++-v3/src/c++17/memory_resource.cc
> @@ -29,7 +29,7 @@
>  #include <new>
>  #if ATOMIC_POINTER_LOCK_FREE != 2
>  # include <bits/std_mutex.h>   // std::mutex, std::lock_guard
> -# include <bits/move.h>                // std::exchange
> +# include <bits/move.h>                // std::__exchange
>  #endif
>
>  namespace std _GLIBCXX_VISIBILITY(default)
> @@ -117,7 +117,7 @@ namespace pmr
>        memory_resource* exchange(memory_resource* r)
>        {
>         lock_guard<mutex> lock(mx);
> -       return std::exchange(val, r);
> +       return std::__exchange(val, r);
>        }
>      };
>  #else
> @@ -137,7 +137,7 @@ namespace pmr
>
>        memory_resource* exchange(memory_resource* r)
>        {
> -       return std::exchange(val, r);
> +       return std::__exchange(val, r);
>        }
>      };
>  #endif // ATOMIC_POINTER_LOCK_FREE == 2
> --
> 2.20.1
>
>