Revert of SkOncePtr -> SkOnce (patchset #1 id:1 of https://codereview.chromium.org/1949313003/ )

Reason for revert:
ok, I have no idea what's going on

Original issue's description:
> Reland of SkOncePtr -> SkOnce (patchset #1 id:1 of https://codereview.chromium.org/1945293004/ )
>
> Reason for revert:
> previous revert unneeded (I think)
>
> Original issue's description:
> > Revert of SkOncePtr -> SkOnce (patchset #4 id:60001 of https://codereview.chromium.org/1953533002/ )
> >
> > Reason for revert:
> > broken the Mac and Linux builders, e.g.:
> >
> > https://build.chromium.org/p/chromium/builders/Mac/builds/15151
> > https://build.chromium.org/p/chromium/builders/Linux%20x64/builds/19052
> >
> > Original issue's description:
> > > SkOncePtr -> SkOnce
> > >
> > > It's always nice to kill off a synchronization primitive.
> > > And while less terse, I think this new code reads more clearly.
> > >
> > > ... and, SkOncePtr's tests were the only thing now using sk_num_cores()
> > > outside of SkTaskGroup, so I've hidden it as static inside SkTaskGroup.cpp.
> > >
> > > BUG=skia:
> > > GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1953533002
> > > CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot
> > >
> > > Committed: https://skia.googlesource.com/skia/+/9bd3fc7fa9bb7cad050bd619aa93d4c48ebb5c02
> >
> > TBR=herb@google.com,mtklein@chromium.org
> > # Skipping CQ checks because original CL landed less than 1 days ago.
> > NOPRESUBMIT=true
> > NOTREECHECKS=true
> > NOTRY=true
> > BUG=skia:
> >
> > Committed: https://skia.googlesource.com/skia/+/7eb33da7eede34c050b865dbb1b60c3dcea7191b
>
> TBR=herb@google.com,mtklein@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/82da9a8aa0ce648f36882830765b42e0ada6c0fa

TBR=herb@google.com,mtklein@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review-Url: https://codereview.chromium.org/1948313002

1 files changed, 65 insertions, 0 deletions

diff --git a/include/private/SkOncePtr.h b/include/private/SkOncePtr.h
new file mode 100644
index 0000000000..921c6a6534
--- /dev/null
+++ b/include/private/SkOncePtr.h
@@ -0,0 +1,65 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkOncePtr_DEFINED
+#define SkOncePtr_DEFINED
+
+#include "../private/SkAtomics.h"
+#include <memory>
+
+// Use this to create a global static pointer that's intialized exactly once when you call get().
+#define SK_DECLARE_STATIC_ONCE_PTR(type, name) namespace {} static SkBaseOncePtr<type> name;
+
+template <typename T>
+class SkBaseOncePtr {
+public:
+    template <typename F>
+    T* get(const F& f) const {
+        uintptr_t state = sk_atomic_load(&fState, sk_memory_order_acquire);
+        if (state < 2) {
+            if (state == 0) {
+                // It looks like no one has tried to create our pointer yet.
+                // We try to claim that task by atomically swapping our state from '0' to '1'.
+                // See SkOnce.h for why we use an acquire memory order here rather than relaxed.
+                if (sk_atomic_compare_exchange(
+                    &fState, &state, (uintptr_t)1, sk_memory_order_acquire, sk_memory_order_acquire)) {
+                    // We've claimed it.  Create our pointer and store it into fState.
+                    state = (uintptr_t)f();
+                    SkASSERT(state > 1);
+                    sk_atomic_store(&fState, state, sk_memory_order_release);
+                } else {
+                    // Someone else claimed it.
+                    // We fall through to the spin loop just below to wait for them to finish.
+                }
+            }
+
+            while (state == 1) {
+                // State '1' is our busy-but-not-done state.
+                // Some other thread has claimed the job of creating our pointer.
+                // We just need to wait for it to finish.
+                state = sk_atomic_load(&fState, sk_memory_order_acquire);
+            }
+
+            // We shouldn't be able to get here without having created our pointer.
+            SkASSERT(state > 1);
+        }
+        return (T*)state;
+    }
+
+    operator T*() const {
+        auto state = sk_atomic_load(&fState, sk_memory_order_acquire);
+        return state < 2 ? nullptr : (T*)state;
+        // TODO: If state == 1 spin until it's not?
+    }
+
+    // fState == 0 --> we have not created our ptr yet
+    // fState == 1 --> someone is in the middle of creating our ptr
+    // else        --> (T*)fState is our ptr
+    mutable uintptr_t fState;
+};
+
+#endif//SkOncePtr_DEFINED