Extract the spinlock from SkOnce as SkSpinlock.

This uses slightly newer APIs from SkAtomics.h to make it a bit more efficient.

No public API changes.
TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1039323002

3 files changed, 44 insertions, 0 deletions

diff --git a/include/ports/SkAtomics_atomic.h b/include/ports/SkAtomics_atomic.h
index 6ae78a89a9..ddbf7c3f37 100644
--- a/include/ports/SkAtomics_atomic.h
+++ b/include/ports/SkAtomics_atomic.h
@@ -1,3 +1,10 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
 #ifndef SkAtomics_atomic_DEFINED
 #define SkAtomics_atomic_DEFINED
 
@@ -37,4 +44,10 @@ bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired,
     return __atomic_compare_exchange_n(ptr, expected, desired, false/*weak?*/, success, failure);
 }
 
+template <typename T>
+T sk_atomic_exchange(T* ptr, T val, sk_memory_order mo) {
+    // All values of mo are valid.
+    return __atomic_exchange_n(ptr, val, mo);
+}
+
 #endif//SkAtomics_atomic_DEFINED
diff --git a/include/ports/SkAtomics_std.h b/include/ports/SkAtomics_std.h
index e89d74981c..4c26858dfd 100644
--- a/include/ports/SkAtomics_std.h
+++ b/include/ports/SkAtomics_std.h
@@ -1,3 +1,10 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
 #ifndef SkAtomics_std_DEFINED
 #define SkAtomics_std_DEFINED
 
@@ -47,4 +54,11 @@ bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired,
                                                         (std::memory_order)failure);
 }
 
+template <typename T>
+T sk_atomic_exchange(T* ptr, T val, sk_memory_order mo) {
+    // All values of mo are valid.
+    std::atomic<T>* ap = reinterpret_cast<std::atomic<T>*>(ptr);
+    return std::atomic_exchange_explicit(ap, val, (std::memory_order)mo);
+}
+
 #endif//SkAtomics_std_DEFINED
diff --git a/include/ports/SkAtomics_sync.h b/include/ports/SkAtomics_sync.h
index 66da4d35ee..7ca0b46a94 100644
--- a/include/ports/SkAtomics_sync.h
+++ b/include/ports/SkAtomics_sync.h
@@ -1,3 +1,10 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
 #ifndef SkAtomics_sync_DEFINED
 #define SkAtomics_sync_DEFINED
 
@@ -48,4 +55,14 @@ bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired, sk_memory_order,
     return false;
 }
 
+template <typename T>
+T sk_atomic_exchange(T* ptr, T val, sk_memory_order) {
+    // There is no __sync exchange.  Emulate it with a CAS loop.
+    T prev;
+    do {
+        prev = sk_atomic_load(ptr);
+    } while(!sk_atomic_compare_exchange(ptr, &prev, val));
+    return prev;
+}
+
 #endif//SkAtomics_sync_DEFINED