Hi,
David Laight wrote:
>Better to code as:
> status = wmm->dwStatus;
> if (...)
Incidentally, that's what I did tonight in patch 20/25 try 2.
>but even then I think the compiler is allowed to perform extra reads.
>It might, for example, do so if the condition was complicayted and
>there was a lot of pressure on registers - so instead of spilling 'status'
>to stack, it would re-read it.
Interesting. Do you think/believe or are you sure?
>With gcc you can force it to only to one read by adding:
> asm volatile("":::"memory");
>The only portable way is with volatile.
"only portable way" from the C perspective. However, given a specific target
environment, we could use its API functions.
Either
MemoryBarrier(); /* which MSDN documents but Wine does not provide in
include/*.h
Or
InterlockedExchange(&status, wmm->dwStatus);
So if AJ is still not satisfied with try 2, I'll change all reads of
wmm->dwStatus
within the player into InterlockedExchange.
Yet I think that would be a superfluous extra memory barrier within the player.
Thank you,
Jörg Höhle
