The Tor Browser: comparison gfx/skia/trunk/include/core/SkOnce.h

--1:000000000000
+:5e3f3675b359
+/*
+* 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 SK_DECLARE_STATIC_ONCE and SkOnce(), which you can use
+// together to create a threadsafe way to call a function just once.  This
+// is particularly useful for lazy singleton initialization. E.g.
+//
+// static void set_up_my_singleton(Singleton** singleton) {
+//     *singleton = new Singleton(...);
+// }
+// ...
+// const Singleton& GetSingleton() {
+//     static Singleton* singleton = NULL;
+//     SK_DECLARE_STATIC_ONCE(once);
+//     SkOnce(&once, set_up_my_singleton, &singleton);
+//     SkASSERT(NULL != singleton);
+//     return *singleton;
+// }
+//
+// OnceTest.cpp also should serve as a few other simple examples.
+//
+// You may optionally pass SkOnce a second function to be called at exit for cleanup.
+#include "SkDynamicAnnotations.h"
+#include "SkThread.h"
+#include "SkTypes.h"
+#define SK_ONCE_INIT { false, { 0, SkDEBUGCODE(0) } }
+#define SK_DECLARE_STATIC_ONCE(name) static SkOnceFlag name = SK_ONCE_INIT
+struct SkOnceFlag;  // If manually created, initialize with SkOnceFlag once = SK_ONCE_INIT
+template <typename Func, typename Arg>
+inline void SkOnce(SkOnceFlag* once, Func f, Arg arg, void(*atExit)() = NULL);
+// If you've already got a lock and a flag to use, this variant lets you avoid an extra SkOnceFlag.
+template <typename Lock, typename Func, typename Arg>
+inline void SkOnce(bool* done, Lock* lock, Func f, Arg arg, void(*atExit)() = NULL);
+//  ----------------------  Implementation details below here. -----------------------------
+// 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;)
+};
+struct SkOnceFlag {
+bool done;
+SkSpinlock lock;
+};
+// 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();
+}
+#else
+inline static void compiler_barrier() {
+asm volatile("" : : : "memory");
+}
+#endif
+inline static void full_barrier_on_arm() {
+#ifdef SK_CPU_ARM
+#  if SK_ARM_ARCH >= 7
+asm volatile("dmb" : : : "memory");
+#  else
+asm volatile("mcr p15, 0, %0, c7, c10, 5" : : "r" (0) : "memory");
+#  endif
+#endif
+}
+// On every platform, we issue a compiler barrier to prevent it from reordering
+// code.  That's enough for platforms like x86 where release and acquire
+// barriers are no-ops.  On other platforms we may need to be more careful;
+// ARM, in particular, needs real code for both acquire and release.  We use a
+// full barrier, which acts as both, because that the finest precision ARM
+// provides.
+inline static void release_barrier() {
+compiler_barrier();
+full_barrier_on_arm();
+}
+inline static void acquire_barrier() {
+compiler_barrier();
+full_barrier_on_arm();
+}
+// Works with SkSpinlock or SkMutex.
+template <typename Lock>
+class SkAutoLockAcquire {
+public:
+explicit SkAutoLockAcquire(Lock* lock) : fLock(lock) { fLock->acquire(); }
+~SkAutoLockAcquire() { fLock->release(); }
+private:
+Lock* fLock;
+};
+// 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 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 Lock, typename Func, typename Arg>
+static void sk_once_slow(bool* done, Lock* lock, Func f, Arg arg, void (*atExit)()) {
+const SkAutoLockAcquire<Lock> locked(lock);
+if (!*done) {
+f(arg);
+if (atExit != NULL) {
+atexit(atExit);
+}
+// Also known as a store-store/load-store barrier, this makes sure that the writes
+// done before here---in particular, those done by calling f(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 f(arg), then check in *done=true as a subsequent change".
+//
+// We'll use this in the fast path to make sure f(arg)'s effects are
+// observable whenever we observe *done == true.
+release_barrier();
+*done = true;
+}
+}
+// This is our fast path, called all the time.  We do really want it to be inlined.
+template <typename Lock, typename Func, typename Arg>
+inline void SkOnce(bool* done, Lock* lock, Func f, Arg arg, void(*atExit)()) {
+if (!SK_ANNOTATE_UNPROTECTED_READ(*done)) {
+sk_once_slow(done, lock, f, arg, atExit);
+}
+// 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 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 once->done = true".
+//
+// 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();
+}
+template <typename Func, typename Arg>
+inline void SkOnce(SkOnceFlag* once, Func f, Arg arg, void(*atExit)()) {
+return SkOnce(&once->done, &once->lock, f, arg, atExit);
+}
+#undef SK_ANNOTATE_BENIGN_RACE
+#endif  // SkOnce_DEFINED

The Tor Browser / file comparison

comparison: gfx/skia/trunk/include/core/SkOnce.h

gfx/skia/trunk/include/core/SkOnce.h