d9187c085b
This reverts commit 3b347232bc.
Reason for revert: tree done gone red.
Original change's description:
> SkMath takes some functions from from SkTypes
>
> Moved to include/core/SkMath.h: Sk{Is|}Align{2|4|8|Ptr}, SkLeftShift,
> SkAbs{32|}, SkM{ax|in}32 SkTM{in|ax}, SkTClamp, SkFastMin32, SkTPin.
>
> Change-Id: Ibcc07be0fc3677731048e7cc86006e7aa493cb92
> Reviewed-on: https://skia-review.googlesource.com/133381
> Auto-Submit: Hal Canary <halcanary@google.com>
> Reviewed-by: Ben Wagner <bungeman@google.com>
> Commit-Queue: Hal Canary <halcanary@google.com>
TBR=mtklein@google.com,halcanary@google.com,bungeman@google.com,reed@google.com
Change-Id: I44073cf397e2a3a6a941a90f0aa63c6396d4c742
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/152587
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
87 lines
2.7 KiB
C++
87 lines
2.7 KiB
C++
/*
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* Copyright 2015 Google Inc.
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*
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* Use of this source code is governed by a BSD-style license that can be
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* found in the LICENSE file.
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*/
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#ifndef SkSemaphore_DEFINED
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#define SkSemaphore_DEFINED
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#include "../private/SkOnce.h"
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#include "SkTypes.h"
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#include <atomic>
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class SkBaseSemaphore {
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public:
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constexpr SkBaseSemaphore(int count = 0)
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: fCount(count), fOSSemaphore(nullptr) {}
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// Increment the counter n times.
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// Generally it's better to call signal(n) instead of signal() n times.
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void signal(int n = 1);
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// Decrement the counter by 1,
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// then if the counter is < 0, sleep this thread until the counter is >= 0.
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void wait();
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// If the counter is positive, decrement it by 1 and return true, otherwise return false.
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bool try_wait();
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// SkBaseSemaphore has no destructor. Call this to clean it up.
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void cleanup();
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private:
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// This implementation follows the general strategy of
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// 'A Lightweight Semaphore with Partial Spinning'
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// found here
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// http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
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// That article (and entire blog) are very much worth reading.
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//
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// We wrap an OS-provided semaphore with a user-space atomic counter that
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// lets us avoid interacting with the OS semaphore unless strictly required:
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// moving the count from >=0 to <0 or vice-versa, i.e. sleeping or waking threads.
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struct OSSemaphore;
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void osSignal(int n);
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void osWait();
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std::atomic<int> fCount;
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SkOnce fOSSemaphoreOnce;
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OSSemaphore* fOSSemaphore;
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};
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class SkSemaphore : public SkBaseSemaphore {
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public:
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using SkBaseSemaphore::SkBaseSemaphore;
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~SkSemaphore() { this->cleanup(); }
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};
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inline void SkBaseSemaphore::signal(int n) {
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int prev = fCount.fetch_add(n, std::memory_order_release);
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// We only want to call the OS semaphore when our logical count crosses
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// from <0 to >=0 (when we need to wake sleeping threads).
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//
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// This is easiest to think about with specific examples of prev and n.
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// If n == 5 and prev == -3, there are 3 threads sleeping and we signal
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// SkTMin(-(-3), 5) == 3 times on the OS semaphore, leaving the count at 2.
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//
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// If prev >= 0, no threads are waiting, SkTMin(-prev, n) is always <= 0,
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// so we don't call the OS semaphore, leaving the count at (prev + n).
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int toSignal = SkTMin(-prev, n);
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if (toSignal > 0) {
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this->osSignal(toSignal);
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}
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}
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inline void SkBaseSemaphore::wait() {
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// Since this fetches the value before the subtract, zero and below means that there are no
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// resources left, so the thread needs to wait.
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if (fCount.fetch_sub(1, std::memory_order_acquire) <= 0) {
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this->osWait();
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}
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}
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#endif//SkSemaphore_DEFINED
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