Comment by jmcomets

Obvious caveat: pushing this a bit further it can quickly fallback to the default case. The optimizer is a superpower but you still need to try to write efficient code.

    unsigned add_v5(unsigned x, unsigned y) {
      if (x == y) return 2 * x;
      return x + y;
    }

Results in:

    add_v5(unsigned int, unsigned int):
      lsl w8, w0, #1
      add w9, w1, w0
      cmp w0, w1
      csel w0, w8, w9, eq
      ret

(armv8-a clang 21.1.0 with O3)

If compiler folks can chime in, I'm curious why incrementing in a loop can be unrolled and inspected to optimize to an addition, but doubling the number when both operands are equal can't?