Nice! I tried it in Nim and it appears to trigger it with:

    {.overflowChecks:off.}
    proc run_switches*(input: cstring): int {.exportc.} =
      result = 0
      for c in input:
        result.inc int('s' == c)
        result.dec int('p' == c)

That gives a ~5x speedup on an Apple M1. Keeping overflow checks on only gets it up to ~2x the default C version. Always nice to know good ways to trigger SIMD opts.