Add atomics for Skia in preperation for landing http://codereview.appspot.com/5649046/

skia/ext/SkThread_chrome.cc

Index: skia/ext/SkThread_chrome.cc
===================================================================
--- skia/ext/SkThread_chrome.cc	(revision 137400)
+++ skia/ext/SkThread_chrome.cc	(working copy)
@@ -11,18 +11,61 @@
 #include "base/logging.h"
 #include "base/synchronization/lock.h"
 
+/** Adds one to the int specified by the address (in a thread-safe manner), and
+    returns the previous value.
+    No additional memory barrier is required.
+    This must act as a compiler barrier.
+*/
 int32_t sk_atomic_inc(int32_t* addr) {
-  // sk_atomic_inc is expected to return the old value, Barrier_AtomicIncrement
-  // returns the new value.
-  return base::subtle::Barrier_AtomicIncrement(addr, 1) - 1;
+  // sk_atomic_inc is expected to return the old value,
+  // Barrier_AtomicIncrement returns the new value.
+  return base::subtle::NoBarrier_AtomicIncrement(addr, 1) - 1;
 }
 
+/*  Subtracts one from the int specified by the address (in a thread-safe
+    manner), and returns the previous value.
+    Expected to act as a release (SL/S) memory barrier and a compiler barrier.
+*/
 int32_t sk_atomic_dec(int32_t* addr) {
-  // sk_atomic_dec is expected to return the old value, Barrier_AtomicIncrement
-  // returns the new value.
+  // sk_atomic_dec is expected to return the old value,
+  // Barrier_AtomicIncrement returns the new value.
   return base::subtle::Barrier_AtomicIncrement(addr, -1) + 1;
 }
+/** If sk_atomic_dec does not act as an aquire (L/SL) barrier, this is expected
+    to act as an aquire (L/SL) memory barrier and as a compiler barrier.
+*/
+void sk_membar_aquire__after_atomic_dec() { }
 
+/** Adds one to the int specified by the address iff the int specified by the
+    address is not zero (in a thread-safe manner), and returns the previous
+    value.
+    No additional memory barrier is required.
+    This must act as a compiler barrier.
+*/
+int32_t sk_atomic_conditional_inc(int32_t* addr) {
+    int32_t value = *addr;
+
+    while (true) {
+        if (value == 0) {
+            return 0;
+        }
+
+        int32_t before;
+        before = base::subtle::Aquire_CompareAndSwap(addr, value, value + 1);
+
+        if (before == value) {
+            return value;
+        } else {
+            value = before;
+        }
+    }
+}
+/** If sk_atomic_conditional_inc does not act as an aquire (L/SL) barrier, this
+    is expected to act as an aquire (L/SL) memory barrier and as a compiler
+    barrier.
+*/
+void sk_membar_aquire__after_atomic_conditional_inc() { }
+
 SkMutex::SkMutex() {
   COMPILE_ASSERT(sizeof(base::Lock) <= sizeof(fStorage), Lock_is_too_big_for_SkMutex);
   base::Lock* lock = reinterpret_cast<base::Lock*>(fStorage);