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cmd/compile: prove/BCE don't seem affected by hints that signed integers are not negative #28956

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mvdan opened this Issue Nov 26, 2018 · 0 comments

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mvdan commented Nov 26, 2018

$ cat f.go
package p

func f(i int, data []byte) {
        if i >= 0 {
                for i < len(data) {
                        _ = data[i] // bounds check!
                        i++
                }
        }
}
$ go build -gcflags=-d=ssa/check_bce/debug=1 f.go
# command-line-arguments
./f.go:6:33: Found IsInBounds
$ go version
go version devel +048c9164a0 Sat Nov 24 23:55:07 2018 +0000 linux/amd64

If we make the index an unsigned integer instead of using the if i >= 0 hint, the bounds check goes away.

I haven't found this in real code, but I tried this hint it while trying to remove a bounds check from real code. I was surprised to see that BCE would still not do its thing with the hint. Seems to me like this should be a simple fix somewhere in the prove or BCE passes.

/cc @aclements @rasky

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