diff mbox series

[3/5] std::__atomic_wait: don't use __detail::__waiter with futex

Message ID 20210226155937.621324-3-thiago.macieira@intel.com
State New
Headers show
Series [1/5] std::latch: reduce internal implementation from ptrdiff_t to int | expand

Commit Message

Thiago Macieira Feb. 26, 2021, 3:59 p.m. UTC 
That violates "don't pay for what you don't need" rule of C++. Users of
std::atomic<T>::wait will often have some bit in their atomic indicate
whether the value is contended or not, so we don't need libstdc++ to do
double book-keeping for us.
---
 libstdc++-v3/include/bits/atomic_wait.h | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)
diff mbox series

Patch

diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index 1c6bda2e2b6..4d240f44faf 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -258,14 +258,14 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       if (std::__atomic_spin(__pred))
 	return;
 
-      __waiter __w(__addr);
-      while (!__pred())
+      if constexpr (__platform_wait_uses_type<_Tp>)
 	{
-	  if constexpr (__platform_wait_uses_type<_Tp>)
-	    {
-	      __platform_wait(__addr, __old);
-	    }
-	  else
+	  __platform_wait(__addr, __old);
+	}
+      else
+	{
+	  __waiter __w(__addr);
+	  while (!__pred())
 	    {
 	      // TODO support timed backoff when this can be moved into the lib
 	      __w._M_do_wait();
@@ -274,13 +274,10 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _Tp>
-    void
+    inline void
     __atomic_notify(const _Tp* __addr, bool __all) noexcept
     {
       using namespace __detail;
-      auto& __w = __waiters::_S_for((void*)__addr);
-      if (!__w._M_waiting())
-	return;
 
 #ifdef _GLIBCXX_HAVE_LINUX_FUTEX
       if constexpr (__platform_wait_uses_type<_Tp>)
@@ -290,6 +287,9 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       else
 #endif
 	{
+	  auto& __w = __waiters::_S_for((void*)__addr);
+	  if (!__w._M_waiting())
+	    return;
 	  __w._M_notify(__all);
 	}
     }