153 lines
5.4 KiB
C++
153 lines
5.4 KiB
C++
/*
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* Copyright 2010-2012 PathScale, Inc. All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions are met:
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*
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* 1. Redistributions of source code must retain the above copyright notice,
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* this list of conditions and the following disclaimer.
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*
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* 2. Redistributions in binary form must reproduce the above copyright notice,
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* this list of conditions and the following disclaimer in the documentation
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* and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS
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* IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
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* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
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* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
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* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
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* OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
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* WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
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* OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
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* ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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/**
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* guard.cc: Functions for thread-safe static initialisation.
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*
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* Static values in C++ can be initialised lazily their first use. This file
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* contains functions that are used to ensure that two threads attempting to
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* initialize the same static do not call the constructor twice. This is
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* important because constructors can have side effects, so calling the
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* constructor twice may be very bad.
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*
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* Statics that require initialisation are protected by a 64-bit value. Any
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* platform that can do 32-bit atomic test and set operations can use this
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* value as a low-overhead lock. Because statics (in most sane code) are
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* accessed far more times than they are initialised, this lock implementation
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* is heavily optimised towards the case where the static has already been
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* initialised.
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*/
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#include <stdint.h>
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#include <stdlib.h>
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#include <stdio.h>
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#include <pthread.h>
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#include <assert.h>
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#include "atomic.h"
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// Older GCC doesn't define __LITTLE_ENDIAN__
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#ifndef __LITTLE_ENDIAN__
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// If __BYTE_ORDER__ is defined, use that instead
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# ifdef __BYTE_ORDER__
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# if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
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# define __LITTLE_ENDIAN__
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# endif
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// x86 and ARM are the most common little-endian CPUs, so let's have a
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// special case for them (ARM is already special cased). Assume everything
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// else is big endian.
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# elif defined(__x86_64) || defined(__i386)
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# define __LITTLE_ENDIAN__
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# endif
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#endif
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/*
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* The least significant bit of the guard variable indicates that the object
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* has been initialised, the most significant bit is used for a spinlock.
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*/
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#ifdef __arm__
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// ARM ABI - 32-bit guards.
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typedef uint32_t guard_t;
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static const uint32_t LOCKED = ((guard_t)1) << 31;
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static const uint32_t INITIALISED = 1;
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#else
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typedef uint64_t guard_t;
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# if defined(__LITTLE_ENDIAN__)
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static const guard_t LOCKED = ((guard_t)1) << 63;
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static const guard_t INITIALISED = 1;
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# else
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static const guard_t LOCKED = 1;
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static const guard_t INITIALISED = ((guard_t)1) << 56;
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# endif
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#endif
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/**
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* Acquires a lock on a guard, returning 0 if the object has already been
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* initialised, and 1 if it has not. If the object is already constructed then
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* this function just needs to read a byte from memory and return.
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*/
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extern "C" int __cxa_guard_acquire(volatile guard_t *guard_object)
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{
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// Not an atomic read, doesn't establish a happens-before relationship, but
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// if one is already established and we end up seeing an initialised state
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// then it's a fast path, otherwise we'll do something more expensive than
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// this test anyway...
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if ((INITIALISED == *guard_object)) { return 0; }
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// Spin trying to do the initialisation
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while (1)
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{
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// Loop trying to move the value of the guard from 0 (not
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// locked, not initialised) to the locked-uninitialised
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// position.
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switch (__sync_val_compare_and_swap(guard_object, 0, LOCKED))
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{
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// If the old value was 0, we succeeded, so continue
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// initialising
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case 0:
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return 1;
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// If this was already initialised, return and let the caller skip
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// initialising it again.
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case INITIALISED:
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return 0;
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// If it is locked by another thread, relinquish the CPU and try
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// again later.
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case LOCKED:
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case LOCKED | INITIALISED:
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sched_yield();
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break;
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// If it is some other value, then something has gone badly wrong.
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// Give up.
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default:
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fprintf(stderr, "Invalid state detected attempting to lock static initialiser.\n");
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abort();
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}
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}
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//__builtin_unreachable();
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return 0;
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}
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/**
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* Releases the lock without marking the object as initialised. This function
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* is called if initialising a static causes an exception to be thrown.
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*/
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extern "C" void __cxa_guard_abort(volatile guard_t *guard_object)
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{
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__attribute__((unused))
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bool reset = __sync_bool_compare_and_swap(guard_object, LOCKED, 0);
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assert(reset);
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}
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/**
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* Releases the guard and marks the object as initialised. This function is
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* called after successful initialisation of a static.
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*/
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extern "C" void __cxa_guard_release(volatile guard_t *guard_object)
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{
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__attribute__((unused))
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bool reset = __sync_bool_compare_and_swap(guard_object, LOCKED, INITIALISED);
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assert(reset);
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}
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