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SDL_atomic.c
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/*
SDL - Simple DirectMedia Layer
Copyright (C) 1997-2010 Sam Lantinga
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
Sam Lantinga
slouken@libsdl.org
*/
#include "SDL_stdinc.h"
#include "SDL_atomic.h"
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/* Note that we undefine the atomic operations here, in case they are
defined as compiler intrinsics while building SDL but the library user
doesn't have that compiler. That way we always have a working set of
atomic operations built into the library.
*/
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/*
If any of the operations are not provided then we must emulate some
of them. That means we need a nice implementation of spin locks
that avoids the "one big lock" problem. We use a vector of spin
locks and pick which one to use based on the address of the operand
of the function.
To generate the index of the lock we first shift by 3 bits to get
rid on the zero bits that result from 32 and 64 bit allignment of
data. We then mask off all but 5 bits and use those 5 bits as an
index into the table.
Picking the lock this way insures that accesses to the same data at
the same time will go to the same lock. OTOH, accesses to different
data have only a 1/32 chance of hitting the same lock. That should
pretty much eliminate the chances of several atomic operations on
different data from waiting on the same "big lock". If it isn't
then the table of locks can be expanded to a new size so long as
the new size is a power of two.
Contributed by Bob Pendleton, bob@pendleton.com
*/
static SDL_SpinLock locks[32];
static __inline__ void
enterLock(void *a)
{
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uintptr_t index = ((((uintptr_t)a) >> 3) & 0x1f);
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SDL_AtomicLock(&locks[index]);
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}
static __inline__ void
leaveLock(void *a)
{
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uintptr_t index = ((((uintptr_t)a) >> 3) & 0x1f);
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SDL_AtomicUnlock(&locks[index]);
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#undef SDL_AtomicSet
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int
SDL_AtomicSet(SDL_atomic_t *a, int value)
{
int oldvalue;
enterLock(a);
oldvalue = a->value;
a->value = value;
leaveLock(a);
return oldvalue;
}
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#undef SDL_AtomicGet
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int
SDL_AtomicGet(SDL_atomic_t *a)
{
/* Assuming integral reads on this platform, we're safe here since the
functions that set the variable have the necessary memory barriers.
*/
return a->value;
}
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#undef SDL_AtomicAdd
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int
SDL_AtomicAdd(SDL_atomic_t *a, int value)
{
int oldvalue;
enterLock(a);
oldvalue = a->value;
a->value += value;
leaveLock(a);
return oldvalue;
}
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#undef SDL_AtomicIncRef
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void
SDL_AtomicIncRef(SDL_atomic_t *a)
{
SDL_AtomicAdd(a, 1);
}
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#undef SDL_AtomicDecRef
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SDL_bool
SDL_AtomicDecRef(SDL_atomic_t *a)
{
return SDL_AtomicAdd(a, -1) == 1;
}
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#undef SDL_AtomicCAS
SDL_bool
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SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval)
{
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SDL_bool retval = SDL_FALSE;
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enterLock(a);
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if (a->value == oldval) {
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a->value = newval;
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retval = SDL_TRUE;
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}
leaveLock(a);
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return retval;
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#undef SDL_AtomicSetPtr
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void
SDL_AtomicSetPtr(void** a, void* value)
{
void *prevval;
do {
prevval = *a;
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} while (!SDL_AtomicCASPtr(a, prevval, value));
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#undef SDL_AtomicGetPtr
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void*
SDL_AtomicGetPtr(void** a)
{
/* Assuming integral reads on this platform, we're safe here since the
functions that set the pointer have the necessary memory barriers.
*/
return *a;
}
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#undef SDL_AtomicCASPtr
SDL_bool SDL_AtomicCASPtr(void **a, void *oldval, void *newval)
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SDL_bool retval = SDL_FALSE;
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enterLock(a);
if (*a == oldval) {
*a = newval;
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retval = SDL_TRUE;
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}
leaveLock(a);
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return retval;
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}
/* vi: set ts=4 sw=4 expandtab: */