-O3 shouldn't enable flushing denormals to zero on gcc, the standard optimisation flags shouldn't affect the results - if they do it's likely a bug. To enable

-O3 shouldn't enable flushing denormals to zero on gcc, the standard optimisation flags shouldn't affect the results - if they do it's likely a bug. To enable that, you need to use -Ofast or -ffast-math, there's a reason they're not default enabled after all.