Here is a concrete case in which this goes wrong.

func f_ssa() int {
	var n int
Next:
	for j := 0; j < 3; j++ {
		for i := 0; i < 10; i++ {
			if i == 6 {
				continue Next
			}
			n = i
		}
		n += j + j + j + j + j + j + j + j + j + j // j * 10
	}
	return n
}

What follows is the function printout before and after CSE.

Note blocks b8 and b10 in the before case.

b8 is the inner loop's condition: i < 10.
b10 is the inner loop's increment: i++.
v82 is i. On entry to b8, it is either 0 (v19) the first time,
or the result of incrementing v82, by way of v29.

The CSE pass considered v82 and v49 to be common subexpressions,
and eliminated v82 in favor of v49.

In the after case, v82 is now dead and will shortly be eliminated.
As a result, v29 is also dead, and we have lost the increment.
The loop runs forever.

BEFORE CSE

f_ssa <nil>
  b1:
    v1 = Arg <mem>
    v2 = SP <uint64>
    v4 = Addr <*int> {~r0} v2
    v13 = Zero <mem> [8] v4 v1
    v14 = Const <int>
    v15 = Const <int>
    v17 = Const <int> [3]
    v19 = Const <int>
    v21 = Const <int> [10]
    v24 = Const <int> [6]
    v28 = Const <int> [1]
    v43 = Const <int> [1]
    Plain -> b3
  b2: <- b7
    Exit v47
  b3: <- b1
    Plain -> b4
  b4: <- b3 b6
    v49 = Phi <int> v15 v44
    v68 = Phi <int> v14 v67
    v81 = Phi <mem> v13 v81
    v18 = Less <bool> v49 v17
    If v18 -> b5 b7
  b5: <- b4
    Plain -> b8
  b6: <- b12 b11
    v67 = Phi <int> v66 v41
    v44 = Add <int> v49 v43
    Plain -> b4
  b7: <- b4
    v47 = Store <mem> v4 v68 v81
    Plain -> b2
  b8: <- b5 b10
    v66 = Phi <int> v68 v82
    v82 = Phi <int> v19 v29
    v22 = Less <bool> v82 v21
    If v22 -> b9 b11
  b9: <- b8
    v25 = Eq <bool> v82 v24
    If v25 -> b12 b13
  b10: <- b13
    v29 = Add <int> v82 v28
    Plain -> b8
  b11: <- b8
    v32 = Add <int> v49 v49
    v33 = Add <int> v32 v49
    v34 = Add <int> v33 v49
    v35 = Add <int> v34 v49
    v36 = Add <int> v35 v49
    v37 = Add <int> v36 v49
    v38 = Add <int> v37 v49
    v39 = Add <int> v38 v49
    v40 = Add <int> v39 v49
    v41 = Add <int> v66 v40
    Plain -> b6
  b12: <- b9
    Plain -> b6
  b13: <- b9
    Plain -> b10

AFTER CSE

f_ssa <nil>
  b1:
    v1 = Arg <mem>
    v2 = SP <uint64>
    v4 = Addr <*int> {~r0} v2
    v13 = Zero <mem> [8] v4 v1
    v14 = Const <int>
    v15 = Const <int>
    v17 = Const <int> [3]
    v19 = Const <int>
    v21 = Const <int> [10]
    v24 = Const <int> [6]
    v28 = Const <int> [1]
    v43 = Const <int> [1]
    Plain -> b3
  b2: <- b7
    Exit v47
  b3: <- b1
    Plain -> b4
  b4: <- b3 b6
    v49 = Phi <int> v19 v44
    v68 = Phi <int> v19 v67
    v81 = Phi <mem> v13 v81
    v18 = Less <bool> v49 v17
    If v18 -> b5 b7
  b5: <- b4
    Plain -> b8
  b6: <- b12 b11
    v67 = Phi <int> v66 v41
    v44 = Add <int> v49 v43
    Plain -> b4
  b7: <- b4
    v47 = Store <mem> v4 v68 v81
    Plain -> b2
  b8: <- b5 b10
    v66 = Phi <int> v68 v49
    v82 = Phi <int> v19 v29
    v22 = Less <bool> v49 v21
    If v22 -> b9 b11
  b9: <- b8
    v25 = Eq <bool> v49 v24
    If v25 -> b12 b13
  b10: <- b13
    v29 = Add <int> v49 v43
    Plain -> b8
  b11: <- b8
    v32 = Add <int> v49 v49
    v33 = Add <int> v32 v49
    v34 = Add <int> v33 v49
    v35 = Add <int> v34 v49
    v36 = Add <int> v35 v49
    v37 = Add <int> v36 v49
    v38 = Add <int> v37 v49
    v39 = Add <int> v38 v49
    v40 = Add <int> v39 v49
    v41 = Add <int> v66 v40
    Plain -> b6
  b12: <- b9
    Plain -> b6
  b13: <- b9
    Plain -> b10

Change-Id: I16fc4ec527ec63f24f7d0d79d1a4a59bf37269de
Reviewed-on: https://go-review.googlesource.com/12444Reviewed-by: Keith Randall <khr@golang.org>