Re: GCC 11.1 Release Candidate available from gcc.gnu.org

[Jonathan Wakely via Gcc-patches]

On 21/04/21 15:30 +0100, Jonathan Wakely wrote:

On 21/04/21 13:12 +0100, Jonathan Wakely wrote:

On 21/04/21 12:38 +0100, Jonathan Wakely wrote:

On 20/04/21 22:12 -0700, Thomas Rodgers wrote:

@@ -86,6 +88,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        }
 }

+    _GLIBCXX_ALWAYS_INLINE bool
+    _M_try_acquire() noexcept
+    {
+      for (;;)
+       {
+         auto __err = sem_trywait(&_M_semaphore);
+         if (__err && (errno == EINTR))
+           continue;
+         else if (__err && (errno == EAGAIN))
+           return false;
+         else if (__err)
+           std::terminate();
+         else
+           break;
+       }
+      return true;
+    }
+
 _GLIBCXX_ALWAYS_INLINE void
 _M_release(std::ptrdiff_t __update) noexcept
 {

Please just commit this part to trunk and gcc-11, not the macro
renaming (as that's been fixed by Jakub already).

I think on trunk I'd prefer to do the attached. WDYT?

In fact I think something like this is neded even on gcc-11 branch,
otherwise anything that tries to include <semaphore> without atomics
or sem_t gets hard errors:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100179

And <thread> includes <stop_token> which includes <semaphore>, meaning
<thread> is unusable on those targets.

So I think removing this #error is essential:

-// Note: the _GLIBCXX_REQUIRE_POSIX_SEMAPHORE macro can be used to force the
-// use of Posix semaphores (sem_t). Doing so however, alters the ABI.
-#if defined __cpp_lib_atomic_wait && !_GLIBCXX_REQUIRE_POSIX_SEMAPHORE
 using __semaphore_impl = __atomic_semaphore;
-#elif _GLIBCXX_HAVE_POSIX_SEMAPHORE
-  using __semaphore_impl = __platform_semaphore;
-#else
-#  error "No suitable semaphore implementation available"
-#endif
+#endif // __cpp_lib_atomic_wait

Here's a simpler patch which just removes the #error and renames the
REQUIRE macro to USE. This still dumps the whole of <semaphore.h> and
<limits.h> in the global namespace when <semaphore> is included, but
we'll have to live with that for the 11.1 release.

I'm just finishing testing this on various targets and will push to
trunk. I'd like to backport it to gcc-11 too, to fix PR100179.


commit 50070d8602a07160cece5890899929e9f210244d
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu Apr 22 11:10:06 2021

    libstdc++: Remove #error from <semaphore> implementation [PR 100179]
    
    This removes the #error from <bits/semaphore_base.h> for the case where
    neither __atomic_semaphore nor __platform_semaphore is defined.
    
    Also rename the _GLIBCXX_REQUIRE_POSIX_SEMAPHORE macro to
    _GLIBCXX_USE_POSIX_SEMAPHORE for consistency with the similar
    _GLIBCXX_USE_CXX11_ABI macro that can be used to request an alternative
    (ABI-changing) implementation.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/100179
            * include/bits/semaphore_base.h: Remove #error.
            * include/std/semaphore: Do not define anything unless one of
            the implementations is available.

diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h
index 84b33423fff..4948f0fd0bc 100644
--- a/libstdc++-v3/include/bits/semaphore_base.h
+++ b/libstdc++-v3/include/bits/semaphore_base.h
@@ -263,14 +263,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   };
 #endif // __cpp_lib_atomic_wait
 
-// Note: the _GLIBCXX_REQUIRE_POSIX_SEMAPHORE macro can be used to force the
+// Note: the _GLIBCXX_USE_POSIX_SEMAPHORE macro can be used to force the
 // use of Posix semaphores (sem_t). Doing so however, alters the ABI.
-#if defined __cpp_lib_atomic_wait && !_GLIBCXX_REQUIRE_POSIX_SEMAPHORE
+#if defined __cpp_lib_atomic_wait && !_GLIBCXX_USE_POSIX_SEMAPHORE
   using __semaphore_impl = __atomic_semaphore;
 #elif _GLIBCXX_HAVE_POSIX_SEMAPHORE
   using __semaphore_impl = __platform_semaphore;
-#else
-#  error "No suitable semaphore implementation available"
 #endif
 
 _GLIBCXX_END_NAMESPACE_VERSION
diff --git a/libstdc++-v3/include/std/semaphore b/libstdc++-v3/include/std/semaphore
index a1560915d83..52addca742c 100644
--- a/libstdc++-v3/include/std/semaphore
+++ b/libstdc++-v3/include/std/semaphore
@@ -34,6 +34,7 @@
 #if __cplusplus > 201703L
 #include <bits/semaphore_base.h>
 
+#if __cpp_lib_atomic_wait || _GLIBCXX_HAVE_POSIX_SEMAPHORE
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
@@ -89,5 +90,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
+#endif // cpp_lib_atomic_wait || _GLIBCXX_HAVE_POSIX_SEMAPHORE
 #endif // C++20
 #endif // _GLIBCXX_SEMAPHORE