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/* Copyright (C) 2012-2024 Free Software Foundation, Inc.
Contributed by Richard Henderson <rth@redhat.com>.
This file is part of the GNU Atomic Library (libatomic).
Libatomic is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
Libatomic 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 General Public License for
more details.
Under Section 7 of GPL version 3, you are granted additional
permissions described in the GCC Runtime Library Exception, version
3.1, as published by the Free Software Foundation.
You should have received a copy of the GNU General Public License and
a copy of the GCC Runtime Library Exception along with this program;
see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
<http://www.gnu.org/licenses/>. */
#include "libatomic_i.h"
/* If we support the builtin, just use it. */
#if !DONE && defined(atomic_compare_exchange_n)
bool
SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
int smodel, int fmodel UNUSED)
{
if (maybe_specialcase_relaxed(smodel))
return atomic_compare_exchange_n (mptr, eptr, newval, false,
__ATOMIC_RELAXED, __ATOMIC_RELAXED);
else if (maybe_specialcase_acqrel(smodel))
return atomic_compare_exchange_n (mptr, eptr, newval, false,
__ATOMIC_ACQ_REL, __ATOMIC_RELAXED);
else
return atomic_compare_exchange_n (mptr, eptr, newval, false,
__ATOMIC_SEQ_CST, __ATOMIC_RELAXED);
}
#define DONE 1
#endif /* HAVE_ATOMIC_CAS */
/* If this type is not larger than word-sized, fall back to a word-sized
compare-and-swap loop, possibly assisted by the OS. */
#if !DONE && N <= WORDSIZE && defined(atomic_compare_exchange_w)
bool
SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
int smodel, int fmodel)
{
UWORD mask, shift, weval, woldval, wnewval, t, *wptr;
pre_barrier (smodel);
if (N < WORDSIZE)
{
wptr = (UWORD *)((uintptr_t)mptr & -WORDSIZE);
shift = (((uintptr_t)mptr % WORDSIZE) * CHAR_BIT) ^ SIZE(INVERT_MASK);
mask = SIZE(MASK) << shift;
}
else
{
wptr = (UWORD *)mptr;
shift = 0;
mask = -1;
}
weval = (UWORD)*eptr << shift;
wnewval = (UWORD)newval << shift;
woldval = __atomic_load_n (wptr, __ATOMIC_RELAXED);
do
{
if ((woldval & mask) != weval)
goto failure;
t = (woldval & ~mask) | wnewval;
}
while (!atomic_compare_exchange_w (wptr, &woldval, t, true,
__ATOMIC_RELAXED, __ATOMIC_RELAXED));
post_barrier (smodel);
return true;
failure:
*eptr = woldval >> shift;
post_barrier (fmodel);
return false;
}
#define DONE 1
#endif /* HAVE_ATOMIC_CAS && N <= WORDSIZE */
/* Otherwise, fall back to some sort of protection mechanism. */
#if !DONE
bool
SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
int smodel, int fmodel UNUSED)
{
UTYPE oldval;
UWORD magic;
bool ret;
pre_seq_barrier (smodel);
magic = protect_start (mptr);
oldval = *mptr;
ret = (oldval == *eptr);
if (ret)
*mptr = newval;
else
*eptr = oldval;
protect_end (mptr, magic);
post_seq_barrier (smodel);
return ret;
}
#endif
EXPORT_ALIAS (SIZE(compare_exchange));
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