Beating an optimising compiler at compiling division calculations: https://lemire.me/blog/2019/02/08/faster-remainders-when-the-divisor-is-a-constant-beating-compilers-and-libdivide/
(TIL that gcc, clang and libdivide use different techniques!)
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Interpreters versus compilers as a spectrum, and a fun worked example of optimising Brainfuck: