import "math"

func f(x []float64) int {
    max := 0.0
    idx := 0
    for i, v := range x {
        absV := math.Abs(v)
        if absV > max {
            max = absV
            idx = i
        }
    }
    return idx
}

It looks like SSA is spilling i during the loop. Kind of an unfortunate consequence of how registers are picked for phi ops. It registerizes i correctly at the top of the loop, but then it decides to also allocate (Phi idx i) to the same register. At the Phi it basically flips a coin as to whether to use the register of idx or i for the phi, and picks the wrong one. Not sure what the right fix would be yet. Some sort of lookahead might help.

Keith, where's that coin flip? I am pretty sure I can improve it with the loop-related information. I'll go look, if not right away, then after lunch.

It goes back to the layout pass. We place the idx generating block after the phi, so the i-providing block is the primary predecessor.

Didn't happen for 1.7, punting to 1.8.