cmd/compile: missed bounds check elimination when paired slices are resliced

[FiloSottile]

go version devel go1.24-140308837f Mon Oct 21 15:30:47 2024 +0200 darwin/arm64

When the lengths of two slices are "paired" (dst = dst[:len(src)]), the compiler is smart enough to apply ifs to both, but if you reslice them the pairing is lost.

func foo(src, dst []byte) {
    dst = dst[:len(src)]
    for len(src) >= 50 {
        _ = (*[50]byte)(src) // no bounds check
        _ = (*[50]byte)(dst) // no bounds check
    }
}

func bar(src, dst []byte) {
    dst = dst[:len(src)]
    for len(src) >= 50 {
        src = src[50:] // no bounds check
        dst = dst[50:] // bounds check!
    }
}

[gabyhelp]

Related Issues and Documentation

• cmd/compile: bce: if slice[n] is in bounds, then slice[:n+1] should have bounds check eliminated #32431 (closed)
• cmd/compile: unnecessary bounds check for position len/2 #27249 (closed)
• cmd/compile: No BCE for len(x)\&y in slice expression #52563 (closed)
• cmd/compile: teach prove about slice expressions #28941
• cmd/compile: improve BCE to detect safe unbounded slicing #35483 (closed)
• cmd/compile: don't generate call to growslice if the slice has capacity #33853 (closed)
• cmd/compile: unnecessary bounds check with slice offset #66691
• cmd/compile: missing BCE optimization cases #64272
• cmd/compile: Prove doesn't understand division of slice length #57077 (closed)
• cmd/compile: No BCE for str[len(str):] #53409 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}