[libc++] Enable <atomic> when threads are disabled

std::atomic is, for the most part, just a thin veneer on top of compiler
builtins. Hence, it should be available even when threads are not available
on the system, and in fact there has been requests for such support.

This patch:
- Moves __libcpp_thread_poll_with_backoff to its own header so it can
  be used in <atomic> when threads are disabled.
- Adds a dummy backoff policy for atomic polling that doesn't know about
  threads.
- Adjusts the <atomic> feature-test macros so they are provided even when
  threads are disabled.
- Runs the <atomic> tests when threads are disabled.

rdar://77873569

Differential Revision: https://reviews.llvm.org/D114109

1 files changed, 7 insertions, 7 deletions

diff --git a/libcxx/src/memory.cpp b/libcxx/src/memory.cpp
index 981ab5571edd..4c9bf9f073c8 100644
--- a/libcxx/src/memory.cpp
+++ b/libcxx/src/memory.cpp
@@ -8,11 +8,11 @@
 
 #include "memory"
 #ifndef _LIBCPP_HAS_NO_THREADS
-#include "mutex"
-#include "thread"
-#if defined(__ELF__) && defined(_LIBCPP_LINK_PTHREAD_LIB)
-#pragma comment(lib, "pthread")
-#endif
+#   include "mutex"
+#   include "thread"
+#   if defined(__ELF__) && defined(_LIBCPP_LINK_PTHREAD_LIB)
+#       pragma comment(lib, "pthread")
+#   endif
 #endif
 #include "include/atomic_support.h"
 
@@ -130,7 +130,7 @@ __shared_weak_count::__get_deleter(const type_info&) const noexcept
     return nullptr;
 }
 
-#if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
+#if !defined(_LIBCPP_HAS_NO_THREADS)
 
 _LIBCPP_SAFE_STATIC static const std::size_t __sp_mut_count = 16;
 _LIBCPP_SAFE_STATIC static __libcpp_mutex_t mut_back[__sp_mut_count] =
@@ -181,7 +181,7 @@ __get_sp_mut(const void* p)
     return muts[hash<const void*>()(p) & (__sp_mut_count-1)];
 }
 
-#endif // !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
+#endif // !defined(_LIBCPP_HAS_NO_THREADS)
 
 void*
 align(size_t alignment, size_t size, void*& ptr, size_t& space)