Statement reordering with locks

Here some C++ code that is accessed from multiple threads in parallel. It has a critical section:

lock.Acquire();
current_id = shared_id;
// small amounts of other code
shared_id = (shared_id + 1) % max_id;
lock.Release();
// do something with current_id

The class of the lock variable is wrapper around the POSIX mutex implement开发者_JAVA技巧ation. Because of the module operations, it is not possible to use atomic operations.

Is it possible that a gcc compiler with a O3 flag optimizes the code so that the assignment of current_id is moved before the lock?

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It is possible to compile with O3!

The compiler will never optimize across a function call unless the function is marked as pure using function-attributes.

The mutex functions aren't pure, so it's absolutely safe to use them with O3.

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Normally the compiler should not make such harmful optimizations. If you're still unsure, you could use the volatile keyword to prevent optimizations on that id variables.