Add subtract to atomics.

BUG=skia:

Review URL: https://codereview.chromium.org/1348113004

4 files changed, 25 insertions, 0 deletions

diff --git a/include/core/SkAtomics.h b/include/core/SkAtomics.h
index d31d9c64fb..7c5294b76d 100644
--- a/include/core/SkAtomics.h
+++ b/include/core/SkAtomics.h
@@ -30,6 +30,9 @@ template <typename T>
 T sk_atomic_fetch_add(T*, T, sk_memory_order = sk_memory_order_seq_cst);
 
 template <typename T>
+T sk_atomic_fetch_sub(T*, T, sk_memory_order = sk_memory_order_seq_cst);
+
+template <typename T>
 bool sk_atomic_compare_exchange(T*, T* expected, T desired,
                                 sk_memory_order success = sk_memory_order_seq_cst,
                                 sk_memory_order failure = sk_memory_order_seq_cst);
@@ -58,6 +61,10 @@ public:
         return sk_atomic_fetch_add(&fVal, val, mo);
     }
 
+    T fetch_sub(const T& val, sk_memory_order mo = sk_memory_order_seq_cst) {
+        return sk_atomic_fetch_sub(&fVal, val, mo);
+    }
+
     bool compare_exchange(T* expected, const T& desired,
                           sk_memory_order success = sk_memory_order_seq_cst,
                           sk_memory_order failure = sk_memory_order_seq_cst) {
diff --git a/include/ports/SkAtomics_atomic.h b/include/ports/SkAtomics_atomic.h
index ddbf7c3f37..64ee823f90 100644
--- a/include/ports/SkAtomics_atomic.h
+++ b/include/ports/SkAtomics_atomic.h
@@ -32,6 +32,12 @@ T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order mo) {
 }
 
 template <typename T>
+T sk_atomic_fetch_sub(T* ptr, T val, sk_memory_order mo) {
+    // All values of mo are valid.
+    return __atomic_fetch_sub(ptr, val, mo);
+}
+
+template <typename T>
 bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired,
                                 sk_memory_order success,
                                 sk_memory_order failure) {
diff --git a/include/ports/SkAtomics_std.h b/include/ports/SkAtomics_std.h
index 4c26858dfd..163efb78c0 100644
--- a/include/ports/SkAtomics_std.h
+++ b/include/ports/SkAtomics_std.h
@@ -39,6 +39,13 @@ T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order mo) {
 }
 
 template <typename T>
+T sk_atomic_fetch_sub(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_fetch_sub_explicit(ap, val, (std::memory_order)mo);
+}
+
+template <typename T>
 bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired,
                                 sk_memory_order success,
                                 sk_memory_order failure) {
diff --git a/include/ports/SkAtomics_sync.h b/include/ports/SkAtomics_sync.h
index 7ca0b46a94..02b1e58072 100644
--- a/include/ports/SkAtomics_sync.h
+++ b/include/ports/SkAtomics_sync.h
@@ -46,6 +46,11 @@ T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order) {
 }
 
 template <typename T>
+T sk_atomic_fetch_sub(T* ptr, T val, sk_memory_order) {
+    return __sync_fetch_and_sub(ptr, val);
+}
+
+template <typename T>
 bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired, sk_memory_order, sk_memory_order) {
     T prev = __sync_val_compare_and_swap(ptr, *expected, desired);
     if (prev == *expected) {