libstdc++: '_GLIBCXX_HAS_GTHREADS' -- '__GTHREADS' vs. '__GTHREADS_CXX0X'

[Thomas Schwinge]

Hi!

I'm still working on libstdc++ support for GCC's GPU targets: GCN, nvptx.
These configurations are (in typical use) multi-threaded, and support
mutexes etc., but they don't support dynamic spawning of threads etc.
Therefore, re 'libgcc/gthr.h', they define '__GTHREADS' but *not*
'__GTHREADS_CXX0X'.  (See 'libgcc/config/gcn/gthr-gcn.h'; nvptx still to
be done, currently 'Thread model: single', very likely bogus...)

Now, 'libstdc++-v3/acinclude.m4:GLIBCXX_CHECK_GTHREADS' does:

    [...]
      AC_MSG_CHECKING([for gthreads library])
    
      AC_TRY_COMPILE([#include "gthr.h"],
        [
          #ifndef __GTHREADS_CXX0X
          #error
          #endif
        ], [ac_has_gthreads=yes], [ac_has_gthreads=no])
      else
        ac_has_gthreads=no
      fi
    
      AC_MSG_RESULT([$ac_has_gthreads])
    
      if test x"$ac_has_gthreads" = x"yes"; then
        AC_DEFINE(_GLIBCXX_HAS_GTHREADS, 1,
                  [Define if gthreads library is available.])
    [...]

That is, it defines '_GLIBCXX_HAS_GTHREADS' per '__GTHREADS_CXX0X'.
Dependent on this '_GLIBCXX_HAS_GTHREADS',
'libstdc++-v3/include/bits/std_mutex.h' then enables 'class mutex'.  In
other words, in the current GCN configuration ('__GTHREADS', but not
'__GTHREADS_CXX0X'), 'class mutex' (and probably more) is not available,
which leads to build issues (and presumably a lot of noise in the test
suite).

Is '__GTHREADS != __GTHREADS_CXX0X' simply a configuration that libstdc++
so far has not attempted to support?  To make that work, do we need to
consider '__GTHREADS' vs. '__GTHREADS_CXX0X' individually in libstdc++,
for guarding the respective features?


Grüße
 Thomas