...one of the most highly
regarded and expertly designed C++ library projects in the
world. — Herb Sutter and Andrei
In the beginning of the project and throughout, many performance analyses, counts of multiprecision-operations-per-second and the like have been performed. Some of these are already listed in the ensuing sections.
We will now provide some general notes on performance, valid for all of the multiprecision backends, before the detailed benchmarks of the following sections.
The header-only, library-independent Boost-licenses integer and floating-point backends including cpp_int for multiprecision integers, cpp_bin_float and cpp_dec_float for multiprecision floating-point types are significantly slower than the world's fastest implementations generally agreed to be found in GMP/MPIR, MPFR and MPC (which are based on GMP). Complex types cpp_complex that are synthesized from these types share similar relative performances.
The backends which effectively wrap GMP/MPIR and MPFR retain the superior performance of the low-level big-number engines. When these are used (in association with at least some level of optmization) they achieve and retain the expected low-level performances.
At low digit counts, however, it is noted that the performances of cpp_int, cpp_bin_float and cpp_dec_float can actually meet or exceed those encountered for GMP/MPIR, MPFR, etc. The reason for this is because stack allocation and/or the use of fast container storage can actually out-perform the allocation mechanisms in GMP/MPIR, which dominate run-time costs at low digit counts.
As digit counts rise above about 50 or so, however, GMP/MPIR performance steadily increases, and simultaneously increases beyond (in relation to) the performances of the Boost-licensed, self-written backends. At around a few hundred to several thousands of digits, factors of about two through five are observed, whereby GMP/MPIR-based calculations are (performance-wise) supreior ones.
At a few thousand decimal digits, the upper end of the Boost backends is reached. At the moment, advanced big-number multiplication schemes in the Boost-licensed, self-written backends is limited to school multiplication and Karatsuba multiplication. Higher-orders of Toom-Cook and FFT-based multiplication are not (yet) implemented. So it is not yet feasible to perform mega-digit calculations with the Boost-licensed, self-written backends, whereas these are readily possible with the GMP/MPIR and MPRF based backends.