Use a spinlock in SkOnce.

SkOnceFlag is now statically initializable on all platforms.

Also adds sk_atomic_cas, used to implement new SkSpinlock.

Going to punt on making SkOnceFlag any smaller (for now, it's 8 bytes).  We could conceivably get it down to two bits, one for done and one for a one-bit spinlock (we'd need atomic-& and atomic-| to make that work, but they appear to be available everywhere).

BUG=skia:1929
R=bungeman@google.com, reed@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/123093002

git-svn-id: http://skia.googlecode.com/svn/trunk@12968 2bbb7eff-a529-9590-31e7-b0007b416f81

1 files changed, 34 insertions, 0 deletions

diff --git a/include/core/SkThread.h b/include/core/SkThread.h
index 412ace31eb..160b40ce29 100644
--- a/include/core/SkThread.h
+++ b/include/core/SkThread.h
@@ -33,6 +33,12 @@ static int32_t sk_atomic_dec(int32_t* addr);
  */
 static int32_t sk_atomic_conditional_inc(int32_t* addr);
 
+/** Atomic compare and set.
+ *  If *addr == before, set *addr to after and return true, otherwise return false.
+ *  This must act as a release (SL/S) memory barrier and as a compiler barrier.
+ */
+static bool sk_atomic_cas(int32_t* addr, int32_t before, int32_t after);
+
 /** If sk_atomic_dec does not act as an acquire (L/SL) barrier,
  *  this must act as an acquire (L/SL) memory barrier and as a compiler barrier.
  */
@@ -45,6 +51,34 @@ static void sk_membar_acquire__after_atomic_conditional_inc();
 
 #include SK_ATOMICS_PLATFORM_H
 
+// This is POD and must be zero-initialized.
+struct SkSpinlock {
+    void acquire() {
+        SkASSERT(shouldBeZero == 0);
+        // No memory barrier needed, but sk_atomic_cas gives us at least release anyway.
+        while (!sk_atomic_cas(&thisIsPrivate, 0, 1)) {
+            // spin
+        }
+    }
+
+    void release() {
+        SkASSERT(shouldBeZero == 0);
+        // This requires a release memory barrier before storing, which sk_atomic_cas guarantees.
+        SkAssertResult(sk_atomic_cas(&thisIsPrivate, 1, 0));
+    }
+
+    int32_t thisIsPrivate;
+    SkDEBUGCODE(int32_t shouldBeZero;)
+};
+
+class SkAutoSpinlock : SkNoncopyable {
+public:
+    explicit SkAutoSpinlock(SkSpinlock* lock) : fLock(lock) { fLock->acquire(); }
+    ~SkAutoSpinlock() { fLock->release(); }
+private:
+    SkSpinlock* fLock;
+};
+#define SkAutoSpinlock(...) SK_REQUIRE_LOCAL_VAR(SkAutoSpinlock)
 
 /** SK_MUTEX_PLATFORM_H must provide the following (or equivalent) declarations.