Those algorithms become more important when you're dividing by a value known at runtime but which remains the same during parts of the program. That's where libdivide comes in.
> libdivide allows you to replace expensive integer divides with comparatively cheap multiplication and bitshifts. Compilers usually do this, but only when the divisor is known at compile time. libdivide allows you to take advantage of it at runtime. The result is that integer division can become faster - a lot faster. [...] divide SIMD vectors by runtime constants, [1]
> libdivide.h is a header-only C/C++ library for optimizing integer division. Integer division is one of the slowest instructions on most CPUs e.g. on current x64 CPUs a 64-bit integer division has a latency of up to 90 clock cycles whereas a multiplication has a latency of only 3 clock cycles. libdivide allows you to replace expensive integer divsion instructions by a sequence of shift, add and multiply instructions that will calculate the integer division much faster.
> On current CPUs you can get a speedup of up to 10x for 64-bit integer division and a speedup of up to to 5x for 32-bit integer division when using libdivide. libdivide also supports SSE2, AVX2 and AVX512 vector division which provides an even larger speedup. You can test how much speedup you can achieve on your CPU using the benchmark program.[2]
[1] https://libdivide.com/ [2] https://github.com/ridiculousfish/libdivide
if the divisor is in a variable or otherwise 'hidden', the compiler can't deduce enought to get to it, is my guess.
loop for i from 0 to n
f(a[i]/y)
Keep in mind that this doesn't use the fact that we know that the input is between 0 to 63.
You can use __builtin_assume for this: https://godbolt.org/z/K4jKhxnTq
edit uh but when asserts are disabled it won't work: https://godbolt.org/z/4TMs1Wc5z
unless you roll your own assert: https://godbolt.org/z/4v35rrTvn