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-O3 -march=native. Your CPU supports thepopcntinstruction, and compilers can sometimes recognize some pure C implementations of it and compile to the single instruction. (Or better, use__builtin_popcountllon GNU C, like Dennis's answer). \$\endgroup\$-march=nativeon your Haswell CPU will also enable BMI2 for more efficient variable-count shift instructions. (SHLX instead of the legacy SHL that needs count incl.) The OP's AMD Piledriver CPU doesn't have BMI2, but it does have popcnt. But AMD CPUs run variable-count SHL faster than Intel CPUs, so compiling with BMI2 while tuning might still be appropriate. Piledriver is pretty different from Haswell as far as micro-optimizations go, but asking for-march=nativeis good \$\endgroup\$