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/*
* Copyright 2010-2012 PathScale, 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:
*
* 1. Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* 2. 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.
*
* 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 HOLDER 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.
*/
/**
* guard.cc: Functions for thread-safe static initialisation.
*
* Static values in C++ can be initialised lazily their first use. This file
* contains functions that are used to ensure that two threads attempting to
* initialize the same static do not call the constructor twice. This is
* important because constructors can have side effects, so calling the
* constructor twice may be very bad.
*
* Statics that require initialisation are protected by a 64-bit value. Any
* platform that can do 32-bit atomic test and set operations can use this
* value as a low-overhead lock. Because statics (in most sane code) are
* accessed far more times than they are initialised, this lock implementation
* is heavily optimised towards the case where the static has already been
* initialised.
*/
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#include <assert.h>
#include "atomic.h"
// Older GCC doesn't define __LITTLE_ENDIAN__
#ifndef __LITTLE_ENDIAN__
// If __BYTE_ORDER__ is defined, use that instead
# ifdef __BYTE_ORDER__
# if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
# define __LITTLE_ENDIAN__
# endif
// x86 and ARM are the most common little-endian CPUs, so let's have a
// special case for them (ARM is already special cased). Assume everything
// else is big endian.
# elif defined(__x86_64) || defined(__i386)
# define __LITTLE_ENDIAN__
# endif
#endif
/*
* The least significant bit of the guard variable indicates that the object
* has been initialised, the most significant bit is used for a spinlock.
*/
#ifdef __arm__
// ARM ABI - 32-bit guards.
typedef uint32_t guard_t;
static const uint32_t LOCKED = ((guard_t)1) << 31;
static const uint32_t INITIALISED = 1;
#else
typedef uint64_t guard_t;
# if defined(__LITTLE_ENDIAN__)
static const guard_t LOCKED = ((guard_t)1) << 63;
static const guard_t INITIALISED = 1;
# else
static const guard_t LOCKED = 1;
static const guard_t INITIALISED = ((guard_t)1) << 56;
# endif
#endif
/**
* Acquires a lock on a guard, returning 0 if the object has already been
* initialised, and 1 if it has not. If the object is already constructed then
* this function just needs to read a byte from memory and return.
*/
extern "C" int __cxa_guard_acquire(volatile guard_t *guard_object)
{
// Not an atomic read, doesn't establish a happens-before relationship, but
// if one is already established and we end up seeing an initialised state
// then it's a fast path, otherwise we'll do something more expensive than
// this test anyway...
if ((INITIALISED == *guard_object)) { return 0; }
// Spin trying to do the initialisation
while (1)
{
// Loop trying to move the value of the guard from 0 (not
// locked, not initialised) to the locked-uninitialised
// position.
switch (__sync_val_compare_and_swap(guard_object, 0, LOCKED))
{
// If the old value was 0, we succeeded, so continue
// initialising
case 0:
return 1;
// If this was already initialised, return and let the caller skip
// initialising it again.
case INITIALISED:
return 0;
// If it is locked by another thread, relinquish the CPU and try
// again later.
case LOCKED:
case LOCKED | INITIALISED:
sched_yield();
break;
// If it is some other value, then something has gone badly wrong.
// Give up.
default:
fprintf(stderr, "Invalid state detected attempting to lock static initialiser.\n");
abort();
}
}
//__builtin_unreachable();
return 0;
}
/**
* Releases the lock without marking the object as initialised. This function
* is called if initialising a static causes an exception to be thrown.
*/
extern "C" void __cxa_guard_abort(volatile guard_t *guard_object)
{
__attribute__((unused))
bool reset = __sync_bool_compare_and_swap(guard_object, LOCKED, 0);
assert(reset);
}
/**
* Releases the guard and marks the object as initialised. This function is
* called after successful initialisation of a static.
*/
extern "C" void __cxa_guard_release(volatile guard_t *guard_object)
{
__attribute__((unused))
bool reset = __sync_bool_compare_and_swap(guard_object, LOCKED, INITIALISED);
assert(reset);
}
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