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* [Bug target/111404] New: [AArch64] 128-bit __sync_val_compare_and_swap is not atomic
@ 2023-09-13 14:45 wilco at gcc dot gnu.org
2023-09-14 17:13 ` [Bug target/111404] " wilco at gcc dot gnu.org
2023-11-30 16:45 ` cvs-commit at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: wilco at gcc dot gnu.org @ 2023-09-13 14:45 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111404
Bug ID: 111404
Summary: [AArch64] 128-bit __sync_val_compare_and_swap is not
atomic
Product: gcc
Version: 8.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: wilco at gcc dot gnu.org
Target Milestone: ---
This compiles
__int128 f(__int128 *p, __int128 *q, __int128 x)
{
return __sync_val_compare_and_swap (p, *q, x);
}
into:
f:
ldp x6, x7, [x1]
mov x4, x0
.L3:
ldxp x0, x1, [x4]
cmp x0, x6
ccmp x1, x7, 0, eq
bne .L4
stlxp w5, x2, x3, [x4]
cbnz w5, .L3
.L4:
dmb ish
ret
This means if the compare fails, we return the value loaded via LDXP. However
unless the STXP succeeds, this returned value is not single-copy atomic.
So on failure we still need to execute STLXP.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug target/111404] [AArch64] 128-bit __sync_val_compare_and_swap is not atomic
2023-09-13 14:45 [Bug target/111404] New: [AArch64] 128-bit __sync_val_compare_and_swap is not atomic wilco at gcc dot gnu.org
@ 2023-09-14 17:13 ` wilco at gcc dot gnu.org
2023-11-30 16:45 ` cvs-commit at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: wilco at gcc dot gnu.org @ 2023-09-14 17:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111404
Wilco <wilco at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2023-09-14
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Target|arm64-*-* |aarch64
--- Comment #1 from Wilco <wilco at gcc dot gnu.org> ---
Patch: https://gcc.gnu.org/pipermail/gcc-patches/2023-September/630198.html
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug target/111404] [AArch64] 128-bit __sync_val_compare_and_swap is not atomic
2023-09-13 14:45 [Bug target/111404] New: [AArch64] 128-bit __sync_val_compare_and_swap is not atomic wilco at gcc dot gnu.org
2023-09-14 17:13 ` [Bug target/111404] " wilco at gcc dot gnu.org
@ 2023-11-30 16:45 ` cvs-commit at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2023-11-30 16:45 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111404
--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Wilco Dijkstra <wilco@gcc.gnu.org>:
https://gcc.gnu.org/g:df8958e6bc5d050dab8bdc5954c1632fb0e98d18
commit r14-6021-gdf8958e6bc5d050dab8bdc5954c1632fb0e98d18
Author: Wilco Dijkstra <wilco.dijkstra@arm.com>
Date: Thu Nov 30 16:14:35 2023 +0000
AArch64: Fix __sync_val_compare_and_swap [PR111404]
__sync_val_compare_and_swap may be used on 128-bit types and either calls
the
outline atomic code or uses an inline loop. On AArch64 LDXP is only atomic
if
the value is stored successfully using STXP, but the current
implementations
do not perform the store if the comparison fails. In this case the value
returned is not read atomically.
gcc/ChangeLog:
PR target/111404
* config/aarch64/aarch64.cc (aarch64_split_compare_and_swap):
For 128-bit store the loaded value and loop if needed.
libgcc/ChangeLog:
PR target/111404
* config/aarch64/lse.S (__aarch64_cas16_acq_rel): Execute STLXP
using
either new value or loaded value.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2023-09-13 14:45 [Bug target/111404] New: [AArch64] 128-bit __sync_val_compare_and_swap is not atomic wilco at gcc dot gnu.org
2023-09-14 17:13 ` [Bug target/111404] " wilco at gcc dot gnu.org
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