[RFA] choosing __platform_wait_t on targets without lock-free 64 atomics

[Jonathan Wakely]

How about this?

I don't think we should worry about targets without atomic int, so don't
bother using types smaller than int.


-- >8 --

For non-futex targets the __platform_wait_t type is currently uint64_t,
but that requires a lock in libatomic for some 32-bit targets. We don't
really need a 64-bit type, so use unsigned long if that is lock-free,
and int otherwise. This should mean it's lock-free on a wider set of
targets.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_wait.h (__detail::__platform_wait_t):
	Define as unsigned long if always lock-free, and unsigned int
	otherwise.
---
 libstdc++-v3/include/bits/atomic_wait.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index bd1ed56d157..46f39f10cbc 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -64,7 +64,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 // and __platform_notify() if there is a more efficient primitive supported
 // by the platform (e.g. __ulock_wait()/__ulock_wake()) which is better than
 // a mutex/condvar based wait.
-    using __platform_wait_t = uint64_t;
+# if  ATOMIC_LONG_LOCK_FREE == 2
+    using __platform_wait_t = unsigned long;
+# else
+    using __platform_wait_t = unsigned int;
+# endif
     inline constexpr size_t __platform_wait_alignment
       = __alignof__(__platform_wait_t);
 #endif
-- 
2.39.0