cmd/compile: some missing BCE cases

[go101]

What version of Go are you using (go version)?

$ go version
go version go1.15 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

package foo

func f(a []int) {
	//a = a[:256] // works
	_ = a[255] // doesn't work
	for i := 0; i < 50000; i++ {
	    _ = a[byte(i)] // BCEed?
	}
}

func g(a []int) {
	//a = a[:4096] // works
	_ = a[4095] // doesn't work
	for i := 0; i < 50000; i++ {
	    _ = a[i&4095] // BCEed?
	}
}

What did you expect to see?

Both tricks work.

What did you see instead?

One of them doesn't work.

[go101]

And this:

package foo

func f1(a []int) (r int) {
	n := len(a)
	for i := 0; i < n - 3; i += 4 {
		r += a[i] // BCEed 
	}
	return r
}

func f2(a []int) (r int) {
	n := len(a)
	for i := 0; i <= n - 4; i += 4 {
		r += a[i] // not BCEed
	}
	return r
}

I rememberd that before it behaved the inverse.

[randall77]

@rasky @brtzsnr @dr2chase

[mvdan]

And @zdjones