On 10/10/2012 10:42 AM, Joerg-Cyril.Hoehle@t-systems.com wrote:
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 at -O0 'status' will get a stack slot http://gcc.gnu.org/ml/gcc/2010-05/msg00116.html With optimization turned on probably not as it is just a reference to an other memory location which can be easily optimized away.
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.
bye michael