My favorite short bit of code is from 1987:
It's a sign function (1 if d0 > 0, -1 if d0 < -1, and 0 if d0 == 0) and uses no conditionals thanks to a trick discovered by an automated optimization tool! It's explained in the paper "Superoptimizer" by Henry Massalin.
assembly
add.l d0,d0 subx.l d1,d1 negx.l d0 addx.l d1,d1
commit 5aa1d48