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

[go101]

One more:

func ComparePackagePaths(pa, pb string) {
	var n = len(pa)
	if n > len(pb) {
		n = len(pb)
		pa, pb = pb, pa
	}
	if n <= len(pb) { // len(pa) <= len(pb) will get BCE
		for i := 0; i < len(pa); i++ {
			_, _ = pa[i], 
			pb[i] // bound check here
		}
	}
}

[go101]

One more, all bounds checks could be removed in this test:

package allocate

import "testing"
import "reflect"

func init() {
	s3 := Filter3(buildOrginalData())
	s4 := Filter4(buildOrginalData())
	if !reflect.DeepEqual(s3, s4) {
		panic("s3 != s4")
	}
}

func buildOrginalData() []int {
	s := make([]int, 1024)
	for i := range s {
		s[i] = i
	}
	return s
}

func check(v int) bool {
	return v%2 == 0
}

// Lose elements.
func Filter3(data []int) []int {
	var k = 0
	for _, v := range data {
		if check(v) {
			data[k] = v // Found IsInBounds
			k++
		}
	}
	return data[:k] // Found IsSliceInBounds
}

// Keep all elements.
func Filter4(data []int) []int {
	var k = 0
	for i, v := range data {
		if check(v) {
			data[i] = data[k] // Found IsInBounds
			data[k] = v
			k++
		}
	}
	return data[:k] // Found IsSliceInBounds
}

var r3 []int
func BenchmarkFilter3(b *testing.B) {
	data := buildOrginalData()
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		r3 = Filter3(data)
	}
}

var r4 []int
func BenchmarkFilter4(b *testing.B) {
	data := buildOrginalData()
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		r4 = Filter4(data)
	}
}

[go101]

Another:

func foo(s [10]uint, n uint) {
	i := n % uint(len(s))
	s[i] = i // Found IsInBounds
}

func bar(s []uint, n uint) {
	i := n % uint(len(s))
	s[i] = i // BCEed
}

[go101]

package main

func f3(s []int) {
	for i := len(s) - 1; i >= 0; i-- {
		//_ = s[i]
		_ = s[i:len(s)]
		_ = s[:i+1] // Found IsSliceInBounds
	}
}


func f5(s []int) {
	for i := 0; i < len(s) - 1; i += 2 {
		//_ = s[i]
		_ = s[i:len(s)]
		_ = s[:i+1] // Found IsSliceInBounds
		_ = s[i+1]  // Found IsInBounds
	}
}

func main() {}

By turning on the _ = s[i] lines, the two Found IsSliceInBounds messages will disappear, but the Found IsInBounds message is still there.