gcc reordering

[Brian Modra <brian at zwartberg dot com>]

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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