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> 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

An assert() also does the trick: https://godbolt.org/z/MecvMGPdW

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

9*x/64 still reduces to 2 instructions