tree 05443d78fb7d26fefeddd0efa5919b3b962553fe
parent 9134686fd9ce384e9163b44942538d033b5bff11
author mtklein <mtklein@chromium.org> 1461185355 -0700
committer Commit bot <commit-bot@chromium.org> 1461185355 -0700

SkOnce: 2 bytes -> 1 byte

This uses the same logic we worked out for SkOncePtr to reduce
the memory footprint of SkOnce from a done byte and lock byte
to a single 3-state byte:

  - NotStarted: no thread has tried to run fn() yet
  - Active:     a thread is running fn()
  - Done:       fn() is complete

Threads which see Done return immediately.
Threads which see NotStarted try to move to Active, run fn(), then move to Done.
Threads which see Active spin until the active thread moves to Done.

This additionally fixes a too-weak memory order bug in SkOncePtr,
and adds a big note to explain.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1904483003

Committed: https://skia.googlesource.com/skia/+/df02d338be8e3c1c50b48a3a9faa582703a39c07

Review URL: https://codereview.chromium.org/1904483003