On Fri, 21 Mar 2025 15:50:05 GMT, Maurizio Cimadamore <[email protected]>
wrote:
> In all these cases there is a fast path: e.g. when we know we have already
> warned for enable native access, or for Unsafe. In the SV API, the fast path
> is when we know that the SV is set already. In my experience, the volatile
> access in this fast path costs nothing: whenever I looked at the generated C2
> code for hot paths of FFM code using enable-native-access, it seems that,
> once the stable field is set, the fact that it is volatile no longer matters.
> There's no barrier generated by C2 -- access is as fast as plain access.
An acquire load is allowed to be reordered with a preceding volatile store and
I believe this is the only case where it makes a difference. E.g::
x = load_acquire(p);
store_volatile(p, v);
y = load_acquire(p);
can be transformed into:
x = load_acquire(p);
y = x;
store_volatile(p, v);
Furthermore, on Aarch64, volatile load is implemented with `ldar` while acquire
load can be implemented with `ldr`.
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PR Comment: https://git.openjdk.org/jdk/pull/23972#issuecomment-2744005070
