Hi,
Is it possible that the gcc optimiser could re-order the following
code:
sem_wait(&mutex);
bool different = val != buf_level;
sem_post(&mutex);
if (different) {
...
e.g. to:
sem_wait(&mutex);
sem_post(&mutex);
if (val != buf_level) {
...
???
I've asked this question generally on comp.lang.c++.moderated
The answer I have received is that although the standard does not
specifically forbifd it, it won't be happening because it knows that
code between sem_wait() and sem_post() need to stay within the
"locked block". I've also been told that I can always look at the
produced assembly, but that does not really help me, because there are
thousands of far more complicated bits of code I've written using
semaphores. I don't know what actually causes the optimiser
re-ordering to kick in.
The following re-oredring would also be "bad" for me, because
conditional block should be able to be run in a number of threads at
once, but the test only, should be synchronised:
sem_wait(&mutex);
bool different = val != buf_level;
if (different) {
...
}
sem_post(&mutex);
(This is because otherwise deadlocks can happen).
I will appreciate your reply, thanks.
Brian
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