cmd/compile: missing BCE case

[choleraehyq]

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

$ go version
go version go1.15.2 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/cholerae/Library/Caches/go-build"
GOENV="/Users/cholerae/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/cholerae/Documents/gopath/pkg/mod"
GONOPROXY="code.byted.org,git.byted.org"
GONOSUMDB="code.byted.org,git.byted.org"
GOOS="darwin"
GOPATH="/Users/cholerae/Documents/gopath"
GOPRIVATE="code.byted.org,git.byted.org"
GOPROXY="https://goproxy.cn,direct"
GOROOT="/Users/cholerae/Documents/gopath/go"
GOSUMDB="sum.golang.google.cn"
GOTMPDIR=""
GOTOOLDIR="/Users/cholerae/Documents/gopath/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

https://godbolt.org/z/31M9no

package main

//go:noinline
func test(val uint, dst []byte) byte {
    if val+3 > val {
        _ = dst[val+3]
        dst[val] = 1
        dst[val+1] = 1
        dst[val+2] = 1
        dst[val+3] = 1
    }
    return 1
}

func main() {
    println(test(100, make([]byte, 10)))
}

Bound checks in function test after _ = dst[val+3] could be eliminated, but not.

What did you expect to see?

Only one bound check exists in function test.

What did you see instead?

There are three bound checks in function test.

[mdempsky]

FYI, you can write that if body as:

t := dst[val : val+4]
t[0] = 1
t[1] = 1
t[2] = 1
t[3] = 1

and then there will be no bounds checks for the individual assignments.