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4.8 Getting the Best Efficiency Out of MPFR

Here are a few hints to get the best efficiency out of MPFR:

• you should avoid allocating and clearing variables. Reuse variables whenever possible, allocate or clear outside of loops, pass temporary variables to subroutines instead of allocating them inside the subroutines;
• use mpfr_swap instead of mpfr_set whenever possible. This will avoid copying the significands;
• avoid using MPFR from C++, or make sure your C++ interface does not perform unnecessary allocations or copies. Slowdowns of up to a factor 15 have been observed on some applications with a C++ interface;
• MPFR functions work in-place: to compute a = a + b you don’t need an auxiliary variable, you can directly write mpfr_add (a, a, b, ...).