Atomics overhaul.

This merges and refactors SkAtomics.h and SkBarriers.h into SkAtomics.h and
some ports/ implementations. The major new feature is that we can express
memory orders explicitly rather than only through comments.

The porting layer is reduced to four template functions:
  - sk_atomic_load
  - sk_atomic_store
  - sk_atomic_fetch_add
  - sk_atomic_compare_exchange
From those four we can reconstruct all our previous sk_atomic_foo.

There are three ports:
  - SkAtomics_std:    uses C++11 <atomic>,             used with MSVC
  - SkAtomics_atomic: uses newer GCC/Clang intrinsics, used on not-MSVC where possible
  - SkAtomics_sync:   uses older GCC/Clang intrinsics, used where SkAtomics_atomic not supported

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

BUG=skia:

Review URL: https://codereview.chromium.org/896553002

1 files changed, 40 insertions, 0 deletions

diff --git a/include/core/SkMutex.h b/include/core/SkMutex.h
index 7dbe957d8b..ea7e81726b 100644
--- a/include/core/SkMutex.h
+++ b/include/core/SkMutex.h
@@ -10,4 +10,44 @@
     #include "../ports/SkMutex_pthread.h"
 #endif
 
+class SkAutoMutexAcquire : SkNoncopyable {
+public:
+    explicit SkAutoMutexAcquire(SkBaseMutex& mutex) : fMutex(&mutex) {
+        SkASSERT(fMutex != NULL);
+        mutex.acquire();
+    }
+
+    explicit SkAutoMutexAcquire(SkBaseMutex* mutex) : fMutex(mutex) {
+        if (mutex) {
+            mutex->acquire();
+        }
+    }
+
+    /** If the mutex has not been released, release it now. */
+    ~SkAutoMutexAcquire() {
+        if (fMutex) {
+            fMutex->release();
+        }
+    }
+
+    /** If the mutex has not been released, release it now. */
+    void release() {
+        if (fMutex) {
+            fMutex->release();
+            fMutex = NULL;
+        }
+    }
+
+    /** Assert that we're holding the mutex. */
+    void assertHeld() {
+        SkASSERT(fMutex);
+        fMutex->assertHeld();
+    }
+
+private:
+    SkBaseMutex* fMutex;
+};
+#define SkAutoMutexAcquire(...) SK_REQUIRE_LOCAL_VAR(SkAutoMutexAcquire)
+
+
 #endif//SkMutex_DEFINED