Missed optimization: computing smaller index than length does not elliminate bound check

[newpavlov]

Code:

pub fn foo(buf: &[u8]) -> Option<&[u8]> {
    // guard against potential overflow
    if buf.len() > usize::MAX/3 {
        return None;
    }
    let n = (3*buf.len())/4;
    Some(&buf[..n])
}

godbolt

Even replacing usize::MAX/3 with any non-zero small number (even 1) does not result in removal of the bound check.

This behavior can be reproduced on current Nightly and previous stable compiler versions.

[the8472]

Workaround:

pub fn foo(buf: &[u8]) -> Option<&[u8]> {
    if buf.len() > usize::MAX/3 {
        return None;
    }
    let n = (3*buf.len())/4;
    let n = std::cmp::min(n, buf.len());
    Some(&buf[..n])
}

[nikic]

https://bugs.llvm.org/show_bug.cgi?id=48744

[nikic]

Upstream fix: llvm/llvm-project@a13c0f6

[MSxDOS]

Fixed on the latest nightly by #81451

[newpavlov]

I confirm the fix.