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It is bad in general, but it ends up being worse in Julia because C and C++ generally aren't compiled with whole program optimization. global fastmath is more aggressive the more you inline, and in C/C++ the math library is usually a statically linked library which creates an inlining barrier. Julia has all the code at runtime, and therefore is often able to run faster by inlining more code. The downside of this is that a global fastmath flags will optimize more than you think they should and give even more wrong answers than usual.