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

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.

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])
}

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

Upstream fix: llvm/llvm-project@a13c0f6

Fixed on the latest nightly by #81451

I confirm the fix.

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Missed optimization: computing smaller index than length does not elliminate bound check #80963

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Missed optimization: computing smaller index than length does not elliminate bound check #80963

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the8472 commented on Jan 13, 2021

nikic commented on Jan 13, 2021

nikic commented on Jan 18, 2021

MSxDOS commented on Mar 5, 2021

newpavlov commented on Mar 5, 2021

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