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cmd/compile: make poset use sufficient conditions for OrderedOrEqual
When assessing whether A <= B, the poset's OrderedOrEqual has a passing condition which permits A <= B, but is not sufficient to infer that A <= B. This CL removes that incorrect passing condition. Having identified that A and B are in the poset, the method will report that A <= B if any of these three conditions are true: (1) A and B are the same node in the poset. - This means we know that A == B. (2) There is a directed path, strict or not, from A -> B - This means we know that, at least, A <= B, but A < B is possible. (3) There is a directed path from B -> A, AND that path has no strict edges. - This means we know that B <= A, but do not know that B < A. In condition (3), we do not have enough information to say that A <= B, rather we only know that B == A (which satisfies A <= B) is possible. The way I understand it, a strict edge shows a known, strictly-ordered relation (<) but the lack of a strict edge does not show the lack of a strictly-ordered relation. The difference is highlighted by the example in #34802, where a bounds check is incorrectly removed by prove, such that negative indexes into a slice succeed: n := make([]int, 1) for i := -1; i <= 0; i++ { fmt.Printf("i is %d\n", i) n[i] = 1 // No Bounds check, program runs, assignment to n[-1] succeeds!! } When prove is checking the negative/failed branch from the bounds check at n[i], in the signed domain we learn (0 > i || i >= len(n)). Because prove can't learn the OR condition, we check whether we know that i is non-negative so we can learn something, namely that i >= len(n). Prove uses the poset to check whether we know that i is non-negative. At this point the poset holds the following relations as a directed graph: -1 <= i <= 0 -1 < 0 In poset.OrderedOrEqual, we are testing for 0 <= i. In this case, condition (3) above is true because there is a non-strict path from i -> 0, and that path does NOT have any strict edges. Because this condition is true, the poset reports to prove that i is known to be >= 0. Knowing, incorrectly, that i >= 0, prove learns from the failed bounds check that i >= len(n) in the signed domain. When the slice, n, was created, prove learned that len(n) == 1. Because i is also the induction variable for the loop, upon entering the loop, prove previously learned that i is in [-1,0]. So when prove attempts to learn from the failed bounds check, it finds the new fact, i > len(n), unsatisfiable given that it previously learned that i <= 0 and len(n) = 1. Fixes #34802 Change-Id: I235f4224bef97700c3aa5c01edcc595eb9f13afc Reviewed-on: https://go-review.googlesource.com/c/go/+/200759 Run-TryBot: Zach Jones <zachj1@gmail.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Giovanni Bajo <rasky@develer.com> Reviewed-by: Keith Randall <khr@golang.org>
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@ -813,8 +813,7 @@ func (po *poset) OrderedOrEqual(n1, n2 *Value) bool {
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return false
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}
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return i1 == i2 || po.reaches(i1, i2, false) ||
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(po.reaches(i2, i1, false) && !po.reaches(i2, i1, true))
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return i1 == i2 || po.reaches(i1, i2, false)
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}
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// Equal reports whether n1==n2. It returns false either when it is
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@ -184,7 +184,7 @@ func TestPoset(t *testing.T) {
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{OrderedOrEqual, 4, 12},
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{OrderedOrEqual_Fail, 12, 4},
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{OrderedOrEqual, 4, 7},
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{OrderedOrEqual, 7, 4},
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{OrderedOrEqual_Fail, 7, 4},
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// Dag #1: 1<4<=7<12
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{Checkpoint, 0, 0},
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@ -448,7 +448,7 @@ func TestSetEqual(t *testing.T) {
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{SetOrderOrEqual, 20, 100},
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{SetOrder, 100, 110},
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{OrderedOrEqual, 10, 30},
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{OrderedOrEqual, 30, 10},
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{OrderedOrEqual_Fail, 30, 10},
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{Ordered_Fail, 10, 30},
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{Ordered_Fail, 30, 10},
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{Ordered, 10, 40},
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@ -934,6 +934,28 @@ func zeroExt32to64Fence(x []int, j uint32) int {
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return 0
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}
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// Ensure that bounds checks with negative indexes are not incorrectly removed.
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func negIndex() {
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n := make([]int, 1)
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for i := -1; i <= 0; i++ { // ERROR "Induction variable: limits \[-1,0\], increment 1$"
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n[i] = 1
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}
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}
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func negIndex2(n int) {
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a := make([]int, 5)
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b := make([]int, 5)
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c := make([]int, 5)
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for i := -1; i <= 0; i-- {
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b[i] = i
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n++
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if n > 10 {
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break
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}
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}
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useSlice(a)
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useSlice(c)
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}
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//go:noinline
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func useInt(a int) {
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}
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