Add an Atomic portability header

Summary: Because there are situations where we need to do an atomic write to a plain pointer, rather than an atomic value. Unfortunately, there is no support in the standard library for this, so different compilers implement the primitives differently. This specifically implements an `int64_t` overload for `__sync_fetch_and_add` for use in the atomic hash map test.

Reviewed By: yfeldblum

Differential Revision: D3571830

fbshipit-source-id: c27d8d2a5238bbc9aba6a9e48e4b3412a199288f

folly/Makefile.am

@@ -252,6 +252,7 @@ nobase_follyinclude_HEADERS = \
 	PicoSpinLock.h \
 	Portability.h \
 	portability/Asm.h \
+	portability/Atomic.h \
 	portability/Builtins.h \
 	portability/Config.h \
 	portability/Constexpr.h \

folly/portability/Atomic.h

+/*
+ * Copyright 2016 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#ifdef _WIN32
+
+#include <stdint.h>
+
+#include <folly/Portability.h>
+// The intrinsics we need are in Windows.h :(
+#include <folly/portability/Windows.h>
+
+FOLLY_ALWAYS_INLINE int64_t __sync_fetch_and_add(int64_t* ptr, int64_t value) {
+  return InterlockedExchangeAdd64(ptr, value);
+}
+
+#endif

folly/test/AtomicHashMapTest.cpp

@@ -23,6 +23,7 @@
 #include <memory>
 #include <folly/Benchmark.h>
 #include <folly/Conv.h>
+#include <folly/portability/Atomic.h>
 #include <folly/portability/SysTime.h>
 
 using std::vector;