Optimizing compiler removes bug

Question

So a lot of people have posted on stack overflow about how compiler optimizations introduce bugs into their program. However, I'm having the opposite problem. I'm writing a C++ program and I have a bug that seems like undefined behavior that only shows up when I set XCode's compiler to off (None[-O0]). If I set it to "fastest, aggressive optimizations [-Ofast]", then the bug disappears.

So I was wondering if anyone has any experience with optimization getting rid of a bug (instead of the more common case of optimization introducing bugs). What are some common causes of this? I'm kind of at a loss of what to do or where to check.

Answer 1

Optimization can change evaluation order of expressions, so if your bug is the result, for example, of a specified evaluation order of foo() and bar() in:

 some_function(foo(), bar());

then different optimization settings will expose the bug while other ones will hide it.

There are lots of other possibilities.

Answer 2

Enabling optimizations can cause freed stack/heap memory to contain valid data for a longer period of time than in a debug build. Some compilers even scrub freed stack/heap memory in debug builds, meaning that freed data is immediately invalid.