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, 70 insertions, 1 deletions

diff --git a/include/core/SkAtomics.h b/include/core/SkAtomics.h
index ed19533163..1a84049970 100644
--- a/include/core/SkAtomics.h
+++ b/include/core/SkAtomics.h
@@ -4,10 +4,79 @@
 // This file is not part of the public Skia API.
 #include "SkTypes.h"
 
+enum sk_memory_order {
+    sk_memory_order_relaxed,
+    sk_memory_order_consume,
+    sk_memory_order_acquire,
+    sk_memory_order_release,
+    sk_memory_order_acq_rel,
+    sk_memory_order_seq_cst,
+};
+
+template <typename T>
+T sk_atomic_load(const T*, sk_memory_order = sk_memory_order_seq_cst);
+
+template <typename T>
+void sk_atomic_store(T*, T, sk_memory_order = sk_memory_order_seq_cst);
+
+template <typename T>
+T sk_atomic_fetch_add(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);
 #if defined(_MSC_VER)
-    #include "../ports/SkAtomics_win.h"
+    #include "../ports/SkAtomics_std.h"
+#elif !defined(SK_BUILD_FOR_IOS) && defined(__ATOMIC_RELAXED)
+    #include "../ports/SkAtomics_atomic.h"
 #else
     #include "../ports/SkAtomics_sync.h"
 #endif
 
+// From here down we have shims for our old atomics API, to be weaned off of.
+// We use the default sequentially-consistent memory order to make things simple
+// and to match the practical reality of our old _sync and _win implementations.
+
+inline int32_t sk_atomic_inc(int32_t* ptr)            { return sk_atomic_fetch_add(ptr, +1); }
+inline int32_t sk_atomic_dec(int32_t* ptr)            { return sk_atomic_fetch_add(ptr, -1); }
+inline int32_t sk_atomic_add(int32_t* ptr, int32_t v) { return sk_atomic_fetch_add(ptr,  v); }
+
+inline int64_t sk_atomic_inc(int64_t* ptr) { return sk_atomic_fetch_add<int64_t>(ptr, +1); }
+
+inline bool sk_atomic_cas(int32_t* ptr, int32_t expected, int32_t desired) {
+    return sk_atomic_compare_exchange(ptr, &expected, desired);
+}
+
+inline void* sk_atomic_cas(void** ptr, void* expected, void* desired) {
+    (void)sk_atomic_compare_exchange(ptr, &expected, desired);
+    return expected;
+}
+
+inline int32_t sk_atomic_conditional_inc(int32_t* ptr) {
+    int32_t prev = sk_atomic_load(ptr);
+    do {
+        if (0 == prev) {
+            break;
+        }
+    } while(!sk_atomic_compare_exchange(ptr, &prev, prev+1));
+    return prev;
+}
+
+template <typename T>
+T sk_acquire_load(T* ptr) { return sk_atomic_load(ptr, sk_memory_order_acquire); }
+
+template <typename T>
+T sk_consume_load(T* ptr) {
+    // On every platform we care about, consume is the same as relaxed.
+    // If we pass consume here, some compilers turn that into acquire, which is overkill.
+    return sk_atomic_load(ptr, sk_memory_order_relaxed);
+}
+
+template <typename T>
+void sk_release_store(T* ptr, T val) { sk_atomic_store(ptr, val, sk_memory_order_release); }
+
+inline void sk_membar_acquire__after_atomic_dec() {}
+inline void sk_membar_acquire__after_atomic_conditional_inc() {}
+
 #endif//SkAtomics_DEFINED