An optimisation that could change the result (even by the smallest possible amount) is not a safe optimisation. As a general rule, unless the build script tells it otherwise, a compile won't perform an optimisation that could make the optimised version give a different result from the basic one.
Of course you may decide it is safe enough, in which case you might find your compiler has an option to tell it that you would like such optimisations to be considered. Stepping away from arithmetic for a moment, variable aliasing is another example of this. If your code could potentially update a value several ways through pointer jiggery-pokery a compile may decide certain register based optimisations are not safe, and there are often directives you can give to tell the compile that it can consider these optimisations as you are sure the code does nothing that would make them a problem (the compile can't decide this for itself as that would potentially take for ever due to the "halting" problem).
An optimisation that could change the result (even by the smallest possible amount) is not a safe optimisation. As a general rule, unless the build script tells it otherwise, a compile won't perform an optimisation that could make the optimised version give a different result from the basic one.
Of course you may decide it is safe enough, in which case you might find your compiler has an option to tell it that you would like such optimisations to be considered. Stepping away from arithmetic for a moment, variable aliasing is another example of this. If your code could potentially update a value several ways through pointer jiggery-pokery a compile may decide certain register based optimisations are not safe, and there are often directives you can give to tell the compile that it can consider these optimisations as you are sure the code does nothing that would make them a problem (the compile can't decide this for itself as that would potentially take for ever due to the "halting" problem).