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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 13:23:13 +0200

Andy Wingo in IRC pointed out that the reason the patch appears to work is that the `movr` isn't idempotent! By the time it comes round again, `val` has already been overwritten by LDAXR in the case that `dst == val`.

On 22/04/2024 10:18, Tony Garnock-Jones wrote:
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);

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