AtomicStruct<T> provides atomic ops on small trivial classes

Summary:
AtomicStruct<T> acts like std::atomic<T>, but it supports any
trivial or trivially-copyable type up to 8 bytes in size.  I'm not sure
why these types weren't included in the original std::atomic standard,
since this construct removes a lot of boilerplate for some uses of
std::atomic.

Test Plan:
1. unit tests
2. this code is adapted from production use in tao/queues/LifoSem

Reviewed By: davejwatson@fb.com

FB internal diff: D1085106

diff --git a/folly/AtomicStruct.h b/folly/AtomicStruct.h
new file mode 100644 (file)
index 0000000..ed4b507
--- /dev/null
+++ b/folly/AtomicStruct.h
@@ -0,0 +1,139 @@
+/*
+ * Copyright 2013 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.
+ */
+
+#ifndef FOLLY_ATOMIC_STRUCT_H_
+#define FOLLY_ATOMIC_STRUCT_H_
+
+#include <atomic>
+#include <type_traits>
+#include <folly/Traits.h>
+#include <string.h>
+#include <stdint.h>
+
+namespace folly {
+
+namespace detail {
+template <int N> struct AtomicStructIntPick {};
+}
+
+/// AtomicStruct<T> work like C++ atomics, but can be used on any POD
+/// type <= 8 bytes.
+template <
+    typename T,
+    template<typename> class Atom = std::atomic,
+    typename Raw = typename detail::AtomicStructIntPick<sizeof(T)>::type>
+class AtomicStruct {
+  static_assert(alignof(T) <= alignof(Raw),
+      "target type can't have stricter alignment than matching int");
+  static_assert(sizeof(T) <= sizeof(Raw),
+      "underlying type isn't big enough");
+  static_assert(std::is_trivial<T>::value ||
+                folly::IsTriviallyCopyable<T>::value,
+      "target type must be trivially copyable");
+
+  Atom<Raw> data;
+
+  static Raw encode(T v) noexcept {
+    // we expect the compiler to optimize away the memcpy, but without
+    // it we would violate strict aliasing rules
+    Raw d = 0;
+    memcpy(&d, &v, sizeof(T));
+    return d;
+  }
+
+  static T decode(Raw d) noexcept {
+    T v;
+    memcpy(&v, &d, sizeof(T));
+    return v;
+  }
+
+ public:
+  AtomicStruct() = default;
+  ~AtomicStruct() = default;
+  AtomicStruct(AtomicStruct<T> const &) = delete;
+  AtomicStruct<T>& operator= (AtomicStruct<T> const &) = delete;
+
+  constexpr /* implicit */ AtomicStruct(T v) noexcept : data(encode(v)) {}
+
+  bool is_lock_free() const noexcept {
+    return data.is_lock_free();
+  }
+
+  bool compare_exchange_strong(
+          T& v0, T v1,
+          std::memory_order mo = std::memory_order_seq_cst) noexcept {
+    Raw d0 = encode(v0);
+    bool rv = data.compare_exchange_strong(d0, encode(v1), mo);
+    if (!rv) {
+      v0 = decode(d0);
+    }
+    return rv;
+  }
+
+  bool compare_exchange_weak(
+          T& v0, T v1,
+          std::memory_order mo = std::memory_order_seq_cst) noexcept {
+    Raw d0 = encode(v0);
+    bool rv = data.compare_exchange_weak(d0, encode(v1), mo);
+    if (!rv) {
+      v0 = decode(d0);
+    }
+    return rv;
+  }
+
+  T exchange(T v, std::memory_order mo = std::memory_order_seq_cst) noexcept {
+    return decode(data.exchange(encode(v), mo));
+  }
+
+  /* implicit */ operator T () const noexcept {
+    return decode(data);
+  }
+
+  T load(std::memory_order mo = std::memory_order_seq_cst) const noexcept {
+    return decode(data.load(mo));
+  }
+
+  T operator= (T v) noexcept {
+    return decode(data = encode(v));
+  }
+
+  void store(T v, std::memory_order mo = std::memory_order_seq_cst) noexcept {
+    data.store(encode(v), mo);
+  }
+
+  // std::atomic also provides volatile versions of all of the access
+  // methods.  These are callable on volatile objects, and also can
+  // theoretically have different implementations than their non-volatile
+  // counterpart.  If someone wants them here they can easily be added
+  // by duplicating the above code and the corresponding unit tests.
+};
+
+namespace detail {
+
+template <> struct AtomicStructIntPick<1> { typedef uint8_t type; };
+template <> struct AtomicStructIntPick<2> { typedef uint16_t type; };
+template <> struct AtomicStructIntPick<3> { typedef uint32_t type; };
+template <> struct AtomicStructIntPick<4> { typedef uint32_t type; };
+template <> struct AtomicStructIntPick<5> { typedef uint64_t type; };
+template <> struct AtomicStructIntPick<6> { typedef uint64_t type; };
+template <> struct AtomicStructIntPick<7> { typedef uint64_t type; };
+template <> struct AtomicStructIntPick<8> { typedef uint64_t type; };
+
+} // namespace detail
+
+} // namespace folly
+
+#endif

diff --git a/folly/Makefile.am b/folly/Makefile.am
index a9647533b1d994d528516f91207f18b7ea21d65b..45c5371679ef26fc38a7c018b7855162f73e0808 100644 (file)
--- a/folly/Makefile.am
+++ b/folly/Makefile.am
@@ -25,6 +25,7 @@ nobase_follyinclude_HEADERS = \
        AtomicHashArray-inl.h \
        AtomicHashMap.h \
        AtomicHashMap-inl.h \
+       AtomicStruct.h \
        Benchmark.h \
        Bits.h \
        Chrono.h \

diff --git a/folly/test/AtomicStructTest.cpp b/folly/test/AtomicStructTest.cpp
new file mode 100644 (file)
index 0000000..9e2f879
--- /dev/null
+++ b/folly/test/AtomicStructTest.cpp
@@ -0,0 +1,76 @@
+/*
+ * Copyright 2013 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.
+ */
+
+#include "folly/AtomicStruct.h"
+
+#include <gflags/gflags.h>
+#include <gtest/gtest.h>
+
+using namespace folly;
+
+struct TwoBy32 {
+  uint32_t left;
+  uint32_t right;
+};
+
+TEST(AtomicStruct, two_by_32) {
+  AtomicStruct<TwoBy32> a(TwoBy32{ 10, 20 });
+  TwoBy32 av = a;
+  EXPECT_EQ(av.left, 10);
+  EXPECT_EQ(av.right, 20);
+  EXPECT_TRUE(a.compare_exchange_strong(av, TwoBy32{ 30, 40 }));
+  EXPECT_FALSE(a.compare_exchange_weak(av, TwoBy32{ 31, 41 }));
+  EXPECT_EQ(av.left, 30);
+  EXPECT_TRUE(a.is_lock_free());
+  auto b = a.exchange(TwoBy32{ 50, 60 });
+  EXPECT_EQ(b.left, 30);
+  EXPECT_EQ(b.right, 40);
+  EXPECT_EQ(a.load().left, 50);
+  a = TwoBy32{ 70, 80 };
+  EXPECT_EQ(a.load().right, 80);
+  a.store(TwoBy32{ 90, 100 });
+  av = a;
+  EXPECT_EQ(av.left, 90);
+  AtomicStruct<TwoBy32> c;
+  c = b;
+  EXPECT_EQ(c.load().right, 40);
+}
+
+TEST(AtomicStruct, size_selection) {
+  struct S1 { char x[1]; };
+  struct S2 { char x[2]; };
+  struct S3 { char x[3]; };
+  struct S4 { char x[4]; };
+  struct S5 { char x[5]; };
+  struct S6 { char x[6]; };
+  struct S7 { char x[7]; };
+  struct S8 { char x[8]; };
+
+  EXPECT_EQ(sizeof(AtomicStruct<S1>), 1);
+  EXPECT_EQ(sizeof(AtomicStruct<S2>), 2);
+  EXPECT_EQ(sizeof(AtomicStruct<S3>), 4);
+  EXPECT_EQ(sizeof(AtomicStruct<S4>), 4);
+  EXPECT_EQ(sizeof(AtomicStruct<S5>), 8);
+  EXPECT_EQ(sizeof(AtomicStruct<S6>), 8);
+  EXPECT_EQ(sizeof(AtomicStruct<S7>), 8);
+  EXPECT_EQ(sizeof(AtomicStruct<S8>), 8);
+}
+
+int main(int argc, char** argv) {
+  testing::InitGoogleTest(&argc, argv);
+  google::ParseCommandLineFlags(&argc, &argv, true);
+  return RUN_ALL_TESTS();
+}