SK_ONCE for SkData

src/core/SkOnce.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkOnce_DEFINED
 #define SkOnce_DEFINED
 
-// SkOnce.h defines two macros, DEF_SK_ONCE and SK_ONCE.
-// You can use these macros together to create a threadsafe block of code that
-// runs at most once, no matter how many times you call it.  This is
-// particularly useful for lazy singleton initialization.  E.g.
-//
-// DEF_SK_ONCE(set_up_my_singleton, SingletonType* singleton) {
-//   // Code in this block will run at most once.
-//   *singleton = new Singleton(...);
-// }
-// ...
-// const Singleton& getSingleton() {
-//   static Singleton* singleton = NULL;
-//   // Always call SK_ONCE.  It's very cheap to call after the first time.
-//   SK_ONCE(set_up_my_singleton, singleton);
-//   SkASSERT(NULL != singleton);
-//   return *singleton;
-// }
-//
-// OnceTest.cpp also should serve as another simple example.
+// TODO(mtklein): update
 
 #include "SkThread.h"
 #include "SkTypes.h"
 
-
-// Pass a unique name (at least in this scope) for name, and a type and name
-// for arg (as if writing a function declaration).
-// E.g.
-//   DEF_SK_ONCE(my_onetime_setup, int* foo) {
-//     *foo += 5;
-//   }
-#define DEF_SK_ONCE(name, arg)                       \
-    static bool sk_once_##name##_done = false;       \
-    SK_DECLARE_STATIC_MUTEX(sk_once_##name##_mutex); \
-    static void sk_once_##name##_function(arg)
-
-// Call this anywhere you need to guarantee that the corresponding DEF_SK_ONCE
-// block of code has run.  name should match the DEF_SK_ONCE, and here you pass
-// the actual value of the argument.
-// E.g
-//   int foo = 0;
-//   SK_ONCE(my_onetime_setup, &foo);
-//   SkASSERT(5 == foo);
-#define SK_ONCE(name, arg) \
-    sk_once(&sk_once_##name##_done, &sk_once_##name##_mutex, sk_once_##name##_function, arg)
-
+struct SkOnceFlag {
+    SkBaseMutex mutex;
+    bool done;
+};
+#define SK_DECLARE_STATIC_ONCE(name) \
+    static SkOnceFlag name = { { PTHREAD_MUTEX_INITIALIZER }, false }

[bungeman-skia, 2013/10/22 21:06:11]

This doesn't work on Windows?

[mtklein, 2013/10/22 21:08:24]

Does SK_DECLARE_STATIC_MUTEX not work on Windows either?  This just inlines it.

[mtklein, 2013/10/22 21:22:08]

Figure this is the way to go now?  How does the mutex get intitialized if you
leave off its initializer?  zero?

 
 //  ----------------------  Implementation details below here. -----------------------------
 
 
 // TODO(bungeman, mtklein): move all these *barrier* functions to SkThread when refactoring lands.
 
 #ifdef SK_BUILD_FOR_WIN
 #include <intrin.h>
 inline static void compiler_barrier() {
     _ReadWriteBarrier();
@@ skipping 25 matching lines @@
 inline static void acquire_barrier() {
     compiler_barrier();
     full_barrier_on_arm();
 }
 
 // We've pulled a pretty standard double-checked locking implementation apart
 // into its main fast path and a slow path that's called when we suspect the
 // one-time code hasn't run yet.
 
 // This is the guts of the code, called when we suspect the one-time code hasn't been run yet.
-// This should be rarely called, so we separate it from sk_once and don't mark it as inline.
+// This should be rarely called, so we separate it from SkOnce and don't mark it as inline.
 // (We don't mind if this is an actual function call, but odds are it'll be inlined anyway.)
 template <typename Arg>
-static void sk_once_slow(bool* done, SkBaseMutex* mutex, void (*once)(Arg), Arg arg) {
-    const SkAutoMutexAcquire lock(*mutex);
-    if (!*done) {
-        once(arg);
+static void sk_once_slow(SkOnceFlag* once, void (*f)(Arg), Arg arg) {
+    const SkAutoMutexAcquire lock(once->mutex);
+    if (!once->done) {
+        f(arg);
         // Also known as a store-store/load-store barrier, this makes sure that the writes
         // done before here---in particular, those done by calling once(arg)---are observable
         // before the writes after the line, *done = true.
         //
         // In version control terms this is like saying, "check in the work up
         // to and including once(arg), then check in *done=true as a subsequent change".
         //
         // We'll use this in the fast path to make sure once(arg)'s effects are
         // observable whenever we observe *done == true.
         release_barrier();
-        *done = true;
+        once->done = true;
     }
 }
 
 // We nabbed this code from the dynamic_annotations library, and in their honor
 // we check the same define.  If you find yourself wanting more than just
 // ANNOTATE_BENIGN_RACE, it might make sense to pull that in as a dependency
 // rather than continue to reproduce it here.
 
 #if DYNAMIC_ANNOTATIONS_ENABLED
 // TSAN provides this hook to supress a known-safe apparent race.
 extern "C" {
 void AnnotateBenignRace(const char* file, int line, const volatile void* mem, const char* desc);
 }
 #define ANNOTATE_BENIGN_RACE(mem, desc) AnnotateBenignRace(__FILE__, __LINE__, mem, desc)
 #else
 #define ANNOTATE_BENIGN_RACE(mem, desc)
 #endif
 
 // This is our fast path, called all the time.  We do really want it to be inlined.
 template <typename Arg>
-inline static void sk_once(bool* done, SkBaseMutex* mutex, void (*once)(Arg), Arg arg) {
-    ANNOTATE_BENIGN_RACE(done, "Don't worry TSAN, we're sure this is safe.");
-    if (!*done) {
-        sk_once_slow(done, mutex, once, arg);
+inline static void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg) {
+    ANNOTATE_BENIGN_RACE(once->done, "Don't worry TSAN, we're sure this is safe.");
+    if (!once->done) {
+        sk_once_slow(once, f, arg);
     }
     // Also known as a load-load/load-store barrier, this acquire barrier makes
     // sure that anything we read from memory---in particular, memory written by
-    // calling once(arg)---is at least as current as the value we read from done.
+    // calling f(arg)---is at least as current as the value we read from once->done.
     //
     // In version control terms, this is a lot like saying "sync up to the
-    // commit where we wrote *done = true".
+    // commit where we wrote once->done = true".
     //
-    // The release barrier in sk_once_slow guaranteed that *done = true
-    // happens after once(arg), so by syncing to *done = true here we're
-    // forcing ourselves to also wait until the effects of once(arg) are readble.
+    // The release barrier in sk_once_slow guaranteed that once->done = true
+    // happens after f(arg), so by syncing to once->done = true here we're
+    // forcing ourselves to also wait until the effects of f(arg) are readble.
     acquire_barrier();
 }
 
 #undef ANNOTATE_BENIGN_RACE
 
-
 #endif  // SkOnce_DEFINED