1 files changed, 27 insertions, 9 deletions

diff --git a/libstdc++-v3/include/std/latch b/libstdc++-v3/include/std/latch
index cf64854..9504df0 100644
--- a/libstdc++-v3/include/std/latch
+++ b/libstdc++-v3/include/std/latch
@@ -62,7 +62,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     constexpr explicit
     latch(ptrdiff_t __expected) noexcept
-    : _M_a(__expected)
+    : _M_counter(__expected)
     { __glibcxx_assert(__expected >= 0 && __expected <= max()); }
 
     ~latch() = default;
@@ -74,35 +74,53 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     count_down(ptrdiff_t __update = 1)
     {
       __glibcxx_assert(__update >= 0 && __update <= max());
-      auto const __old = __atomic_impl::fetch_sub(&_M_a, __update,
+      auto const __old = __atomic_impl::fetch_sub(&_M_counter, __update,
 						  memory_order::release);
       if (std::cmp_equal(__old, __update))
-	__atomic_impl::notify_all(&_M_a);
+	__atomic_impl::notify_all(&_M_counter);
       else
 	__glibcxx_assert(std::cmp_less(__update, __old));
     }
 
     _GLIBCXX_ALWAYS_INLINE bool
     try_wait() const noexcept
-    { return __atomic_impl::load(&_M_a, memory_order::acquire) == 0; }
+    { return __atomic_impl::load(&_M_counter, memory_order::acquire) == 0; }
 
     _GLIBCXX_ALWAYS_INLINE void
     wait() const noexcept
     {
-      auto const __pred = [this] { return this->try_wait(); };
-      std::__atomic_wait_address(&_M_a, __pred);
+      auto const __vfn = [this] {
+	return __atomic_impl::load(&_M_counter, memory_order::acquire);
+      };
+      auto const __pred = [](__detail::__platform_wait_t __v) {
+	return __v == 0;
+      };
+      std::__atomic_wait_address(&_M_counter, __pred, __vfn);
     }
 
     _GLIBCXX_ALWAYS_INLINE void
     arrive_and_wait(ptrdiff_t __update = 1) noexcept
     {
-      count_down(__update);
-      wait();
+      // The standard specifies this functions as count_down(update); wait();
+      // but we combine those two calls into one and avoid the wait() if we
+      // know the counter reached zero.
+
+      __glibcxx_assert(__update >= 0 && __update <= max());
+      // Use acq_rel here because an omitted wait() would have used acquire:
+      auto const __old = __atomic_impl::fetch_sub(&_M_counter, __update,
+						  memory_order::acq_rel);
+      if (std::cmp_equal(__old, __update))
+	__atomic_impl::notify_all(&_M_counter);
+      else
+	{
+	  __glibcxx_assert(std::cmp_less(__update, __old));
+	  wait();
+	}
     }
 
   private:
     alignas(__detail::__platform_wait_alignment)
-      __detail::__platform_wait_t _M_a;
+      __detail::__platform_wait_t _M_counter;
   };
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace