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/**
* 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 <pthread.h>
/**
* 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 = (*guard_object) >> 56;
if (1 == first_byte) { return 0; }
int32_t *lock = low_32_bits(guard_object);
// 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))
{
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 = (*guard_object) >> 56;
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(guard_object);
*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) << 56;
__cxa_guard_abort(guard_object);
}
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