cmd/compile: compiler hangs on tiny program

[ALTree]

go version devel +371c83b594 Thu Mar 30 02:45:45 2017 +0000 linux/amd64

The following program:

package p

func Spin() {
l1:
	for true {
		goto l1
	l2:
		if true {
			goto l2
		}
	}
}

hangs the tip compiler (spinning at 100% cpu).

It compiles in no time in go1.8

[ALTree]

The issue was introduced in CL 37760 (cmd/compile: avoid generating some dead blocks).

cc @josharian

[josharian]

Thanks! I have a simple fix but it touches some very hot code, so benchmarking now.

[gopherbot]

CL https://golang.org/cl/38913 mentions this issue.