cmd/compile: BCE not clever enough or intended?

[yaxinlx]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   go1.7
2. What operating system and processor architecture are you using (go env)?
   linux/amd64
3. What did you do?

// example4.go
package main

import "math/rand"

func fa2() {
    s := []int{0, 1, 2, 3, 4, 5, 6}
    index := rand.Intn(7)
    _ = s[index:] // line 9: bounds check 
    _ = s[:index] // line 10: bounds check eliminatd!
}

func fb2(s []int, index int) {
    _ = s[index:] // line 14: bounds check
    _ = s[:index] // line 15: bounds check // not clever enough?
}

func fc2() {
    s := []int{0, 1, 2, 3, 4, 5, 6}
    s = s[:4]
    index := rand.Intn(7)
    _ = s[index:] // line 22: bounds check 
    _ = s[:index] // line 23: bounds check eliminatd!
}

func main() {}

go build -gcflags="-d=ssa/check_bce/debug=1" example4.go
# command-line-arguments
./11.go:9: Found IsSliceInBounds
./11.go:14: Found IsSliceInBounds
./11.go:15: Found IsSliceInBounds
./11.go:22: Found IsSliceInBounds

1. What did you expect to see?

BCE should also applied for line 15.

1. What did you see instead?

BCE is not applied for line 15.

[josharian]

cc @brtzsnr

[brtzsnr]

Thanks for reporting. Definitely not smart enough:

v10 = SliceLen <int> v6
v11 = SliceCap <int> v6
v12 = IsSliceInBounds <bool> v7 v10
v32 = IsSliceInBounds <bool> v7 v11

Implication v12 -> v32 is not seen by the prove pass. This is a shortcoming of the current algorithm.

[gopherbot]

CL https://golang.org/cl/33633 mentions this issue.