bug#70474: [PATCH] Move the spin loop target to the LDAXR instruction.

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);