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bug#70474: [PATCH] Move the spin loop target to the LDAXR instruction.
From: |
Tony Garnock-Jones |
Subject: |
bug#70474: [PATCH] Move the spin loop target to the LDAXR instruction. |
Date: |
Mon, 22 Apr 2024 10:18:46 +0200 |
Oh man. This little patch all by itself makes the problem behaviour go
away. No switching to SWPAL/CASAL, just tightening the spinloop. (And no
changes at all to the CAS code, so nothing to do with the fibers bug I
guess.)
With the patch, the spinloop goes LDAXR-STLXR-CBNZ (which is what GCC
does when SWPAL isn't there) instead of potentially MOV-LDAXR-STLXR-CBNZ
(which isn't).
Could the machine really be so sensitive to the target of the CBNZ?
---
libguile/lightening/lightening/aarch64-cpu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/libguile/lightening/lightening/aarch64-cpu.c
b/libguile/lightening/lightening/aarch64-cpu.c
index 13aa351e9..4df712a0e 100644
--- a/libguile/lightening/lightening/aarch64-cpu.c
+++ b/libguile/lightening/lightening/aarch64-cpu.c
@@ -2532,10 +2532,10 @@ str_atomic(jit_state_t *_jit, int32_t loc,
int32_t val)
static void
swap_atomic(jit_state_t *_jit, int32_t dst, int32_t loc, int32_t val)
{
- void *retry = jit_address(_jit);
int32_t result = jit_gpr_regno(get_temp_gpr(_jit));
int32_t val_or_tmp = dst == val ? jit_gpr_regno(get_temp_gpr(_jit))
: val;
movr(_jit, val_or_tmp, val);
+ void *retry = jit_address(_jit);
LDAXR(_jit, dst, loc);
STLXR(_jit, val_or_tmp, loc, result);
jit_patch_there(_jit, bnei(_jit, result, 0), retry);
--
2.44.0
- bug#70474: Possible bug with `atomic-box-swap!` on OSX/M3 (?!?!), Tony Garnock-Jones, 2024/04/19
- bug#70474: Possible bug with `atomic-box-swap!` on OSX/M3 (?!?!), Christopher Baines, 2024/04/19
- bug#70474: Also manifests on an M1 running 14.1.1 and with newer Guile versions, Tony Garnock-Jones, 2024/04/19
- bug#70474: [PATCH 1/2] Including the cast makes Apple clang 15.0.0 happy; without it, clang is sad, Tony Garnock-Jones, 2024/04/19
- bug#70474: [PATCH 2/2] Replace aarch64 CAS and atomic-swap generated JIT code with CASAL and SWPAL instructions, Tony Garnock-Jones, 2024/04/19
- bug#70474: Just adding DMB doesn't help, Tony Garnock-Jones, 2024/04/22
- bug#70474: [PATCH] Move the spin loop target to the LDAXR instruction.,
Tony Garnock-Jones <=