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- // -*- Mode: C++; c-basic-offset: 2; indent-tabs-mode: nil -*-
- /* Copyright (c) 2010, Google Inc.
- * All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions are
- * met:
- *
- * * Redistributions of source code must retain the above copyright
- * notice, this list of conditions and the following disclaimer.
- * * Redistributions in binary form must reproduce the above
- * copyright notice, this list of conditions and the following disclaimer
- * in the documentation and/or other materials provided with the
- * distribution.
- * * Neither the name of Google Inc. nor the names of its
- * contributors may be used to endorse or promote products derived from
- * this software without specific prior written permission.
- *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
- * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
- * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
- * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
- * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
- * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
- * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
- * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
- * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
- * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
- * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- */
- // The OS-specific header included below must provide two calls:
- // base::internal::SpinLockDelay() and base::internal::SpinLockWake().
- // See spinlock_internal.h for the spec of SpinLockWake().
- // void SpinLockDelay(volatile Atomic32 *w, int32 value, int loop)
- // SpinLockDelay() generates an apprproate spin delay on iteration "loop" of a
- // spin loop on location *w, whose previously observed value was "value".
- // SpinLockDelay() may do nothing, may yield the CPU, may sleep a clock tick,
- // or may wait for a delay that can be truncated by a call to SpinlockWake(w).
- // In all cases, it must return in bounded time even if SpinlockWake() is not
- // called.
- #include "base/spinlock_internal.h"
- // forward declaration for use by spinlock_*-inl.h
- namespace base { namespace internal { static int SuggestedDelayNS(int loop); }}
- #if defined(_WIN32)
- #include "base/spinlock_win32-inl.h"
- #elif defined(__linux__)
- #include "base/spinlock_linux-inl.h"
- #else
- #include "base/spinlock_posix-inl.h"
- #endif
- namespace base {
- namespace internal {
- // Return a suggested delay in nanoseconds for iteration number "loop"
- static int SuggestedDelayNS(int loop) {
- // Weak pseudo-random number generator to get some spread between threads
- // when many are spinning.
- #ifdef BASE_HAS_ATOMIC64
- static base::subtle::Atomic64 rand;
- uint64 r = base::subtle::NoBarrier_Load(&rand);
- r = 0x5deece66dLL * r + 0xb; // numbers from nrand48()
- base::subtle::NoBarrier_Store(&rand, r);
- r <<= 16; // 48-bit random number now in top 48-bits.
- if (loop < 0 || loop > 32) { // limit loop to 0..32
- loop = 32;
- }
- // loop>>3 cannot exceed 4 because loop cannot exceed 32.
- // Select top 20..24 bits of lower 48 bits,
- // giving approximately 0ms to 16ms.
- // Mean is exponential in loop for first 32 iterations, then 8ms.
- // The futex path multiplies this by 16, since we expect explicit wakeups
- // almost always on that path.
- return r >> (44 - (loop >> 3));
- #else
- static Atomic32 rand;
- uint32 r = base::subtle::NoBarrier_Load(&rand);
- r = 0x343fd * r + 0x269ec3; // numbers from MSVC++
- base::subtle::NoBarrier_Store(&rand, r);
- r <<= 1; // 31-bit random number now in top 31-bits.
- if (loop < 0 || loop > 32) { // limit loop to 0..32
- loop = 32;
- }
- // loop>>3 cannot exceed 4 because loop cannot exceed 32.
- // Select top 20..24 bits of lower 31 bits,
- // giving approximately 0ms to 16ms.
- // Mean is exponential in loop for first 32 iterations, then 8ms.
- // The futex path multiplies this by 16, since we expect explicit wakeups
- // almost always on that path.
- return r >> (12 - (loop >> 3));
- #endif
- }
- } // namespace internal
- } // namespace base
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