On Wed, Apr 21, 2021 at 3:16 PM Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Wed, Apr 21, 2021 at 08:28:55PM +0200, Jakub Jelinek via Gcc-patches wrote:
> > > There's a patch attempt for the problem with
> > > std::thread::hardware_concurrency where
> > > it's used only if _GLIBCXX_HAS_GTHREADS is set.
> > >
> > > Does it help?
> > > Thanks,
> > > Martin
> > >
> > > gcc/ChangeLog:
> > >
> > > PR bootstrap/100186
> > > * lto-wrapper.c: Use hardware_concurrency only if
> > > _GLIBCXX_HAS_GTHREADS.
> > > ---
> > > gcc/lto-wrapper.c | 6 +++++-
> > > 1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/gcc/lto-wrapper.c b/gcc/lto-wrapper.c
> > > index 6ba401007f6..8a85b3e93a8 100644
> > > --- a/gcc/lto-wrapper.c
> > > +++ b/gcc/lto-wrapper.c
> > > @@ -1285,7 +1285,11 @@ run_gcc (unsigned argc, char *argv[])
> > > static char current_dir[] = { '.', DIR_SEPARATOR, '\0' };
> > >
> > > /* Number of CPUs that can be used for parallel LTRANS phase. */
> > > - unsigned long nthreads_var = std::thread::hardware_concurrency ();
> > > + unsigned long nthreads_var = 0;
> > > +
> > > +#ifdef _GLIBCXX_HAS_GTHREADS
> > > + nthreads_var = std::thread::hardware_concurrency ();
> > > +#endif
> >
> > _GLIBCXX_HAS_GTHREADS is a libstdc++ internal macro, it shouldn't be used
> > outside of libstdc++.
> > And, when using some other compiler or standard C++ library, nthreads_var
> > will be always 0. That isn't an improvement.
>
> What would be IMHO a good idea would be to use configure test for
> #include <thread>
> int t = std::thread::hardware_concurrency ();
> and in that case use that as a fallback to the previous implementation,
> that will be strictly an improvement.
Would it be good enough to add a compile-time test for GCC VERSION >=
10 and fallback to the value 1 otherwise? If one performs the
recommended bootstrap of the compiler, stage2 and stage3 will pick up
<thread> from the newly built libstdc++ that always defines
std::thread and will provide the expected feature. The problem is the
stage1 compiler that may be built with an older compiler or building
GCC without performing a three-stage bootstrap.
Thanks, David
