cmd/compile: improve BCE to detect safe unbounded slicing

[nhooyr]

See #31586 (comment)

Core problem can be reduced to:

package scratch

import "encoding/binary"

func A1(b []byte) {
	key64 := uint64(1024)
	for len(b) >= 32 {
		v := binary.LittleEndian.Uint64(b)
		binary.LittleEndian.PutUint64(b, v^key64)
		v = binary.LittleEndian.Uint64(b[8:])
		binary.LittleEndian.PutUint64(b[8:], v^key64)
		v = binary.LittleEndian.Uint64(b[16:])
		binary.LittleEndian.PutUint64(b[16:], v^key64)
		v = binary.LittleEndian.Uint64(b[24:])
		binary.LittleEndian.PutUint64(b[24:], v^key64)
		b = b[32:]
	}
}

func A2(b []byte) {
	key64 := uint64(1024)
	for len(b) >= 32 {
		v := binary.LittleEndian.Uint64(b)
		binary.LittleEndian.PutUint64(b, v^key64)
		v = binary.LittleEndian.Uint64(b[8:16])
		binary.LittleEndian.PutUint64(b[8:16], v^key64)
		v = binary.LittleEndian.Uint64(b[16:24])
		binary.LittleEndian.PutUint64(b[16:24], v^key64)
		v = binary.LittleEndian.Uint64(b[24:32])
		binary.LittleEndian.PutUint64(b[24:32], v^key64)
		b = b[32:]
	}
}

BCE:

$ go build -gcflags="-d=ssa/check_bce/debug=1" scratch.go
# command-line-arguments
./scratch.go:10:34: Found IsInBounds
./scratch.go:12:34: Found IsInBounds
./scratch.go:14:34: Found IsInBounds

Bound checks remain in A1 even though they are not needed but are eliminated in A2 due to the explicit and not unbounded slices.

@renthraysk stated:

It was the _ = b[7] line in PutUint64() that was cause the bounds checks. The compiler failed to prove it's not needed with PutUint64(b[8:], x) however it does eliminate the first check in PutUint64(b, x).

I'm on 1.13.4 on macOS.

[josharian]

cc @zdjones

[zdjones]

This is a duplicate of #24876. Also see #19126

The issue is that the prove pass does not currently do any arithimetic when assesing b[16:] and therefore has no information about the length of the resulting slice. see this comment: #19126 (comment).

I have a chain of CLs that fix this, but they are not complete this cycle.

[zdjones]

Here is the CL chain that fixes this issue: https://go-review.googlesource.com/c/go/+/190657

[agnivade]

Closing as a duplicate.