How to deal with compiler optimization problems

Question

I'm having an annoying problem with my iPhone app. Whenever I set the optimization level to something other than "None", I get computation errors. This only happens in when building for the iPhone SDK (the iPhone Simulator is always fine).

I wouldn't mind disabling optimizations in release mode, but the application is a tiny bit too slow when I do that.

The application is complex, so it is hard to locate the part that is too aggressively optimized.

I think that the problem is on the GCC side since it seems to have problem optimizing the code for the ARM architecture.

Is there a way to only disable optimizations only for certain part of the code? How would you deal with that kind of issue?

Answer 1

Yes, that's entirely possible. GCC has an attribute for that:

/* disable optimization for this function */
void my_function(void) __attribute__((optimize(0)));

void my_function(void) {
    /* ... */
}

Sets the optimization level for that function to -O0. You can enable/disable specific optimizations:

/* disable optimization for this function */
void my_function(void) __attribute__((optimize("no-inline-functions")));

void my_function(void) {
    /* ... */
}

Answer 2

If optimization changes your program's behavior, you might unwittingly be relying on undefined or implementation-defined behavior. It could be worth taking a closer look at your code with an eye toward assumptions about variables' values and orders of evaluation.