00437ebe73
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/12444
Reviewed-by: Keith Randall <khr@golang.org>
|
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| CONTRIBUTORS | ||
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| LICENSE | ||
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| robots.txt | ||
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For example, if you extracted the tar file into $HOME/go, you might put the following in your .profile:
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