PALOpenBSD implement WaitOnAddress feature attempt. (#716)

src/snmalloc/pal/pal_openbsd.h

@@ -3,6 +3,8 @@
 #if defined(__OpenBSD__) && !defined(_KERNEL)
 #  include "pal_bsd.h"
 
+#  include <sys/futex.h>
+
 namespace snmalloc
 {
   /**
@@ -21,7 +23,50 @@ namespace snmalloc
      * declared explicitly to remind anyone who modifies this class that they
      * should add any required features.
      */
-    static constexpr uint64_t pal_features = PALBSD::pal_features;
+    static constexpr uint64_t pal_features =
+      PALBSD::pal_features | WaitOnAddress;
+
+    using WaitingWord = int;
+
+    template<class T>
+    static void wait_on_address(std::atomic<T>& addr, T expected)
+    {
+      int backup = errno;
+      static_assert(
+        sizeof(T) == sizeof(WaitingWord) && alignof(T) == alignof(WaitingWord),
+        "T must be the same size and alignment as WaitingWord");
+      while (addr.load(std::memory_order_relaxed) == expected)
+      {
+        long ret = futex(
+          (uint32_t*)&addr,
+          FUTEX_WAIT_PRIVATE,
+          static_cast<int>(expected),
+          nullptr,
+          nullptr);
+
+        if (ret == 0)
+          break;
+      }
+      errno = backup;
+    }
+
+    template<class T>
+    static void notify_one_on_address(std::atomic<T>& addr)
+    {
+      static_assert(
+        sizeof(T) == sizeof(WaitingWord) && alignof(T) == alignof(WaitingWord),
+        "T must be the same size and alignment as WaitingWord");
+      futex((uint32_t*)&addr, FUTEX_WAKE_PRIVATE, 1, nullptr, nullptr);
+    }
+
+    template<class T>
+    static void notify_all_on_address(std::atomic<T>& addr)
+    {
+      static_assert(
+        sizeof(T) == sizeof(WaitingWord) && alignof(T) == alignof(WaitingWord),
+        "T must be the same size and alignment as WaitingWord");
+      futex((uint32_t*)&addr, FUTEX_WAKE_PRIVATE, INT_MAX, nullptr, nullptr);
+    }
   };
 } // namespace snmalloc
 #endif