When can optimizations done by the compiler cause my C++ code to exhibit wrong behaviour which would not be present had those optimizations not been performed? For example, not using volatile in certain circumstances can cause the program to behave incorrectly (e.g. not re-reading the value of a variable from memory and instead only reads it once and stores it in register). But are there other pitfalls which one should know about before turning on the most aggressive optimization flag and afterwards wondering why the program doesn’t work anymore?
When can optimizations done by the compiler cause my C++ code to exhibit wrong
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Compiler optimizations should not affect the observable behavior of your program, so in theory, you don’t need to worry. In practice, if your program strays in to undefined behavior, anything could already happen, so if your program breaks when you enable optimizations, you’ve merely exposed existing bugs – it wasn’t optimization that broke it.
One common optimization point is the return value optimisation (RVO) and named return value optimization (NRVO) which basically means objects returned by value from functions get constructed directly in the object which is receiving them, rather than making a copy. This adjusts the order and number of constructor, copy constructor and destructor calls – but usually with those functions correctly written, there’s still no observable difference in the behavior.