I have been investigating a 15% performance regression in my C++ primesum program (https://github.com/kimwalisch/primesum/tree/256-bit) over the last 2 days. By lots of benchmarking I was able to identify the boost multiprecision library together with -std=c++11 (or -std=gnu++11) as the culprit for the performance regression because I have also a version of the primesum program which does not use the boost multiprecision library and in this version there is no performance regression when compiling using -std=c++11. I have tested using multiple versions of the boost multiprecision library including the latest 1.65.1. The slowdown happens on both GCC (versions: 5.4, 6.4, 7.2) and Clang (version 3.8) on x86_64 Linux. I am only using the int256_t and uint256_t types (hence cpp_int backend) and I am doing only simple integer arithmetic: +, - and *. Is this a known issue and is there a known workaround e.g. special compiler flag? I could revert to C++98 but I really don't want to do that... Thanks, Kim Walisch