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cmd/compile: teach prove that x ∈ [0, c) ⇒ c - x ∈ [0, c) #29477

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josharian opened this issue Dec 31, 2018 · 1 comment
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cmd/compile: teach prove that x ∈ [0, c) ⇒ c - x ∈ [0, c) #29477

josharian opened this issue Dec 31, 2018 · 1 comment

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@josharian
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@josharian josharian commented Dec 31, 2018

Consider:

package p

func f(x int64, s uint) int64 {
	s1 := s & 63
	s2 := 64 - s1
	return x >> s2
}

(Extracted from code that does multi-word shifts.)

The compiler should be able to prove that s2 ∈ [0, 64), and thus emit a plain shift, rather than a shift + mask. It does not.

cc @rasky @aclements

@josharian josharian added this to the Unplanned milestone Dec 31, 2018
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@josharian josharian commented Dec 31, 2018

Except that that is not sound. Time for bed.

@josharian josharian closed this Dec 31, 2018
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