linux-sgx-trts-modified/sdk/cpprt/linux/guard.cc

/* 
 * 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 <assert.h>

#include "se_cdefs.h"

SGX_ACCESS_VERSION(tstdcxx, 3)

#ifdef __arm__
// ARM ABI - 32-bit guards.

/**
 * 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 int32_t *guard_object)
{
	if ((1<<31) == *guard_object) { return 0; }
	// If we can atomically move the value from 0 -> 1, then this is
	// uninitialised.
	if (__sync_bool_compare_and_swap(guard_object, 0, 1))
	{
		return 1;
	}
	// If the value is not 0, some other thread was initialising this.  Spin
	// until it's finished.
	while (__sync_bool_compare_and_swap(guard_object, (1<<31), (1<<31)))
	{
		// If the other thread aborted, then we grab the lock
		if (__sync_bool_compare_and_swap(guard_object, 0, 1))
		{
			return 1;
		}
		sched_yield();
	}
	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(int32_t *guard_object)
{
	assert(__sync_bool_compare_and_swap(guard_object, 1, 0));
}
/**
 * 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(int32_t *guard_object)
{
	assert(__sync_bool_compare_and_swap(guard_object, 1, (1<<31)));
}


#else
// Itanium ABI: 64-bit guards

/**
 * Returns a pointer to the low 32 bits in a 64-bit value, respecting the
 * platform's byte order.
 */
static int32_t *low_32_bits(volatile int64_t *ptr)
{
	int32_t *low= (int32_t*)ptr;
	// Test if the machine is big endian - constant propagation at compile time
	// should eliminate this completely.
	int one = 1;
	if (*(char*)&one != 1)
	{
		low++;
	}
	return low;
}

/**
 * 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 int64_t *guard_object)
{
    char first_byte = (char) (*guard_object);
    if (1 == first_byte) { return 0; }
    int32_t *lock = low_32_bits((int64_t *)((char*)guard_object + 1));
    // Simple spin lock using the low 32 bits.  We assume that concurrent
    // attempts to initialize statics are very rare, so we don't need to
    // optimise for the case where we have lots of threads trying to acquire
    // the lock at the same time.    
    while (!__sync_bool_compare_and_swap_4(lock, 0, 1))
    {
        if (1 == ((char) (*guard_object)))
        {
             break;
        }
       // sched_yield need to be implemented as a OCALL, 
       // while we try to remove OCALLs in this library.
       //sched_yield();        
    }
    // We have to test the guard again, in case another thread has performed
    // the initialisation while we were trying to acquire the lock.
    first_byte = (char) (*guard_object);
    return (1 != first_byte);    
}

/**
 * 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(int64_t *guard_object)
{
    int32_t *lock = low_32_bits((int64_t *)((char*)guard_object + 1));
    *lock = 0;
}
/**
 * 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(int64_t *guard_object)
{
    // Set the first byte to 1
    *guard_object |= ((int64_t)1);
    __cxa_guard_abort(guard_object);
}

#endif