Initial C++ runtime work. Much more to do.
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runtime/guard.cc
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193
runtime/guard.cc
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/*
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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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typedef uint32_t guard_lock_t;
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static const uint32_t LOCKED = static_cast<guard_t>(1) << 31;
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static const uint32_t INITIALISED = 1;
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#define LOCK_PART(guard) (guard)
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#define INIT_PART(guard) (guard)
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#elif defined(_LP64)
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typedef uint64_t guard_t;
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typedef uint64_t guard_lock_t;
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# if defined(__LITTLE_ENDIAN__)
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static const guard_t LOCKED = static_cast<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 = static_cast<guard_t>(1) << 56;
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# endif
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#define LOCK_PART(guard) (guard)
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#define INIT_PART(guard) (guard)
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#else
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typedef uint32_t guard_lock_t;
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# if defined(__LITTLE_ENDIAN__)
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typedef struct {
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uint32_t init_half;
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uint32_t lock_half;
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} guard_t;
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static const uint32_t LOCKED = static_cast<guard_lock_t>(1) << 31;
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static const uint32_t INITIALISED = 1;
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# else
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typedef struct {
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uint32_t init_half;
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uint32_t lock_half;
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} guard_t;
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_Static_assert(sizeof(guard_t) == sizeof(uint64_t), "");
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static const uint32_t LOCKED = 1;
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static const uint32_t INITIALISED = static_cast<guard_lock_t>(1) << 24;
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# endif
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#define LOCK_PART(guard) (&(guard)->lock_half)
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#define INIT_PART(guard) (&(guard)->init_half)
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#endif
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static const guard_lock_t INITIAL = 0;
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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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guard_lock_t old;
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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 == *INIT_PART(guard_object))
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return 0;
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// Spin trying to do the initialisation
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for (;;)
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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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old = __sync_val_compare_and_swap(LOCK_PART(guard_object),
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INITIAL, LOCKED);
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if (old == INITIAL) {
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// Lock obtained. If lock and init bit are
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// in separate words, check for init race.
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if (INIT_PART(guard_object) == LOCK_PART(guard_object))
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return 1;
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if (INITIALISED != *INIT_PART(guard_object))
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return 1;
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// No need for a memory barrier here,
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// see first comment.
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*LOCK_PART(guard_object) = INITIAL;
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return 0;
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}
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// If lock and init bit are in the same word, check again
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// if we are done.
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if (INIT_PART(guard_object) == LOCK_PART(guard_object) &&
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old == INITIALISED)
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return 0;
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assert(old == LOCKED);
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// Another thread holds the lock.
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// If lock and init bit are in different words, check
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// if we are done before yielding and looping.
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if (INIT_PART(guard_object) != LOCK_PART(guard_object) &&
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INITIALISED == *INIT_PART(guard_object))
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return 0;
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sched_yield();
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}
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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(LOCK_PART(guard_object),
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LOCKED, INITIAL);
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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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guard_lock_t old;
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if (INIT_PART(guard_object) == LOCK_PART(guard_object))
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old = LOCKED;
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else
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old = INITIAL;
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__attribute__((unused))
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bool reset = __sync_bool_compare_and_swap(INIT_PART(guard_object),
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old, INITIALISED);
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assert(reset);
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if (INIT_PART(guard_object) != LOCK_PART(guard_object))
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*LOCK_PART(guard_object) = INITIAL;
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}
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