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/* Monotonically increasing wide counters (at least 62 bits).
Copyright (C) 2016-2024 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C 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.
The GNU C 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 the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <atomic_wide_counter.h>
#if !__HAVE_64B_ATOMICS
/* Values we add or xor are less than or equal to 1<<31, so we only
have to make overflow-and-addition atomic wrt. to concurrent load
operations and xor operations. To do that, we split each counter
into two 32b values of which we reserve the MSB of each to
represent an overflow from the lower-order half to the higher-order
half.
In the common case, the state is (higher-order / lower-order half, and . is
basically concatenation of the bits):
0.h / 0.l = h.l
When we add a value of x that overflows (i.e., 0.l + x == 1.L), we run the
following steps S1-S4 (the values these represent are on the right-hand
side):
S1: 0.h / 1.L == (h+1).L
S2: 1.(h+1) / 1.L == (h+1).L
S3: 1.(h+1) / 0.L == (h+1).L
S4: 0.(h+1) / 0.L == (h+1).L
If the LSB of the higher-order half is set, readers will ignore the
overflow bit in the lower-order half.
To get an atomic snapshot in load operations, we exploit that the
higher-order half is monotonically increasing; if we load a value V from
it, then read the lower-order half, and then read the higher-order half
again and see the same value V, we know that both halves have existed in
the sequence of values the full counter had. This is similar to the
validated reads in the time-based STMs in GCC's libitm (e.g.,
method_ml_wt).
One benefit of this scheme is that this makes load operations
obstruction-free because unlike if we would just lock the counter, readers
can almost always interpret a snapshot of each halves. Readers can be
forced to read a new snapshot when the read is concurrent with an overflow.
However, overflows will happen infrequently, so load operations are
practically lock-free. */
uint64_t
__atomic_wide_counter_fetch_add_relaxed (__atomic_wide_counter *c,
unsigned int op)
{
/* S1. Note that this is an atomic read-modify-write so it extends the
release sequence of release MO store at S3. */
unsigned int l = atomic_fetch_add_relaxed (&c->__value32.__low, op);
unsigned int h = atomic_load_relaxed (&c->__value32.__high);
uint64_t result = ((uint64_t) h << 31) | l;
l += op;
if ((l >> 31) > 0)
{
/* Overflow. Need to increment higher-order half. Note that all
add operations are ordered in happens-before. */
h++;
/* S2. Release MO to synchronize with the loads of the higher-order half
in the load operation. See __atomic_wide_counter_load_relaxed. */
atomic_store_release (&c->__value32.__high,
h | ((unsigned int) 1 << 31));
l ^= (unsigned int) 1 << 31;
/* S3. See __atomic_wide_counter_load_relaxed. */
atomic_store_release (&c->__value32.__low, l);
/* S4. Likewise. */
atomic_store_release (&c->__value32.__high, h);
}
return result;
}
uint64_t
__atomic_wide_counter_load_relaxed (__atomic_wide_counter *c)
{
unsigned int h, l, h2;
do
{
/* This load and the second one below to the same location read from the
stores in the overflow handling of the add operation or the
initializing stores (which is a simple special case because
initialization always completely happens before further use).
Because no two stores to the higher-order half write the same value,
the loop ensures that if we continue to use the snapshot, this load
and the second one read from the same store operation. All candidate
store operations have release MO.
If we read from S2 in the first load, then we will see the value of
S1 on the next load (because we synchronize with S2), or a value
later in modification order. We correctly ignore the lower-half's
overflow bit in this case. If we read from S4, then we will see the
value of S3 in the next load (or a later value), which does not have
the overflow bit set anymore.
*/
h = atomic_load_acquire (&c->__value32.__high);
/* This will read from the release sequence of S3 (i.e, either the S3
store or the read-modify-writes at S1 following S3 in modification
order). Thus, the read synchronizes with S3, and the following load
of the higher-order half will read from the matching S2 (or a later
value).
Thus, if we read a lower-half value here that already overflowed and
belongs to an increased higher-order half value, we will see the
latter and h and h2 will not be equal. */
l = atomic_load_acquire (&c->__value32.__low);
/* See above. */
h2 = atomic_load_relaxed (&c->__value32.__high);
}
while (h != h2);
if (((l >> 31) > 0) && ((h >> 31) > 0))
l ^= (unsigned int) 1 << 31;
return ((uint64_t) (h & ~((unsigned int) 1 << 31)) << 31) + l;
}
#endif /* !__HAVE_64B_ATOMICS */
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