Re: arm ldxr/stxr vs cas

On 9/4/2024 5:59 PM, MitchAlsup1 wrote:

On Wed, 4 Sep 2024 23:48:48 +0000, Chris M. Thomasson wrote:
 

On 9/4/2024 2:27 PM, MitchAlsup1 wrote:

On Mon, 2 Sep 2024 17:27:57 +0000, jseigh wrote:
>

I read that arm added the cas instruction because they didn't think
ldxr/stxr would scale well.  It wasn't clear to me as to why that
would be the case.  I would think the memory lock mechanism would
have really low overhead vs cas having to do an interlocked load
and store.  Unless maybe the memory lock size might be large
enough to cause false sharing issues.  Any ideas?

>
A pipeline lock between the LD part of a CAS and the ST part of a
CAS is essentially FREE. But the same is true for LL followed by
a later SC.

>
100% sure on that? No way to break the reservation from an unrelated
aspect wrt LL/SC?

 I have been building pipelines like that since 1983.
 Now, it is completely possible to build a pipeline that gives
one grief (lesser or greater) in doing these things--but it is
definitely possible to build a grief free pipeline for LL- SC,
and by extension CAS.

Well, the software must play along as well? Padded and aligned on certain boundaries helps things out. Big time.