Re: gcc reordering

From:

BeeOnRope <travis.downs@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 30 Jul 2009 05:36:03 CST

Message-ID:

<60f7f109-d5b2-4acb-96ca-eed52935fc29@b14g2000yqd.googlegroups.com>

{ Please avoid top-posting. -mod }

Generally speaking, the compiler can re-order things if it the re-
ordering is invisible to the correct operation of the code - it must
preserve "as-if" semantics in the visible behavior of the problem.

Now if val and buf_level were declared on the stack earlier in the
function, and their address has not escaped the function, the compiler
could perform the re-ordering you mention - since there is no way that
either the sem_wait or sem_post functions could modify these values as
a side effect.

Assuming these values are somehow shared between threads, that pretty
much implies that they are global, class static or have otherwise had
their address "escape" whatever local context they were declared in,
and then the re-ordering wouldn't be possible. That general behavior
is why mutexes tend to work correct on the "interesting" data they are
intended to protect, even though the C++ standard is silent on the
subject of threads.

Another way the reordering is possible is if the compiler can see the
full definitions of sem_wait and sem_post - it could then determine
they have no side effects, and the re-ordering is legal. Of course
sem_wait and sem_post in practice will involve functions hidden away
in libraries, inline assembly, or other compiler-specific commands to
surpress this, so that wouldn't occur for those two functions in real
life.

On Jul 29, 4:46 pm, "br...@zwartberg.com" <epai...@googlemail.com>
wrote:

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) {
           ...

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