cmd/compile: unnecessary bounds check with slice offset

[dsnet]

Go version

go1.22

Output of go env in your module/workspace:

GOARCH=amd64
GOOS=linux

What did you do?

Compile the following:

package main

import "encoding/binary"

var src []byte
var dst uint64

func main() {
	const offset = 1
	if len(src) < offset+8 {
		return
	}
	dst = binary.BigEndian.Uint64(src[offset:])
}

What did you see happen?

I see the following in the assembly:

	0x003f 00063 (/usr/local/go1.22.0/src/encoding/binary/binary.go:183)	MOVL	$7, AX
	0x0044 00068 (/usr/local/go1.22.0/src/encoding/binary/binary.go:183)	PCDATA	$1, $0
	0x0044 00068 (/usr/local/go1.22.0/src/encoding/binary/binary.go:183)	CALL	runtime.panicIndex(SB)
	0x0049 00073 (/usr/local/go1.22.0/src/encoding/binary/binary.go:183)	XCHGL	AX, AX

What did you expect to see?

No bounds check.

The bounds check is avoided if offset is 0, but fails once it is non-zero.

However, this provably safe so long as offset+8 does not overflow an int.

[dsnet]

Interesting. If I rewrite the conditional as len(src)-offset < 8, the compiler is happy again.

[dmitshur]

CC @golang/compiler.