Re: [PATCH] [libstdc++] Refactor/cleanup of atomic wait implementation

[Jonathan Wakely via Gcc-patches]

On 23/02/21 13:57 -0800, Thomas Rodgers wrote:

diff --git a/libstdc++-v3/include/bits/atomic_wait.h 
b/libstdc++-v3/include/bits/atomic_wait.h
index 1a0f0943ebd..fa83ef6c231 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -39,17 +39,16 @@
#include <ext/numeric_traits.h>

#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+#define _GLIBCXX_HAVE_PLATFORM_WAIT 1

This is defined here (to 1) and then ...

# include <cerrno>
# include <climits>
# include <unistd.h>
# include <syscall.h>
# include <bits/functexcept.h>
-// TODO get this from Autoconf
-# define _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE 1
-#else
-# include <bits/std_mutex.h>  // std::mutex, std::__condvar
#endif

+# include <bits/std_mutex.h>  // std::mutex, std::__condvar
+
#define __cpp_lib_atomic_wait 201907L

namespace std _GLIBCXX_VISIBILITY(default)
@@ -57,20 +56,27 @@ namespace std _GLIBCXX_VISIBILITY(default)
_GLIBCXX_BEGIN_NAMESPACE_VERSION
  namespace __detail
  {
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
    using __platform_wait_t = int;
+#else
+    using __platform_wait_t = uint64_t;
+#endif
+  } // namespace __detail

-    constexpr auto __atomic_spin_count_1 = 16;
-    constexpr auto __atomic_spin_count_2 = 12;
-
-    template<typename _Tp>
-      inline constexpr bool __platform_wait_uses_type
-#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
-       = is_same_v<remove_cv_t<_Tp>, __platform_wait_t>;
+  template<typename _Tp>
+    inline constexpr bool __platform_wait_uses_type
+#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
+      = is_same_v<remove_cv_t<_Tp>, __detail::__platform_wait_t>
+       || ((sizeof(_Tp) == sizeof(__detail::__platform_wait_t))
+           && (alignof(_Tp*) == alignof(__detail::__platform_wait_t)));
#else
-       = false;
+      = false;
#endif

+  namespace __detail
+  {
#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+#define _GLIBCXX_HAVE_PLATFORM_WAIT

Redefined here (to empty), after it's already been tested.

Presumably this redefinition shouldn't be here.

Also the HAVE_PLATFORM_TIMED_WAIT macro is defined to empty. I think
they should both be defined to 1 (or both empty, but not
inconsistently).

I'm still going through the rest of the patch.