cmd/compile: incorrect loop variable detection

[randall77]

func main() {
	for i := math.MinInt64 + 2; i >= math.MinInt64; i-- {
		if i == math.MaxInt64 {
			break
		}
		println(i)
	}
}

This program will loop forever when it shouldn't. You can "fix" it by passing -gcflags=-N.

Similar to #53600, this is a bug in induction variable detection. Just reversing the direction when step is negative in cmd/compile/internal/ssa/loopbce.go (added in CL 104041 ) isn't quite right. The "distance until overflow" isn't preserved under mirroring.

@dr2chase @rasky

[gopherbot]

Change https://go.dev/cl/415854 mentions this issue: cmd/compile: fix prove pass when lower condition is >= minint

[gopherbot]

Change https://go.dev/cl/415874 mentions this issue: cmd/compile: rework induction variable detector