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Animats • today at 3:02 AM • 1 reply • view on HN

There's a discussion of "delayed bounds checking", but not "hoisted bounds checking", where bounds checking is done early. Consider

    let mut tab: [usize;100] = [0;100];
    ...
    for i in 0..101 {
        tab[i] = i;
    }
This must panic at i=100. Panic becomes inevitable at entry to the loop. Is the compiler entitled to generate a check that will panic at loop entry? The slides suggest that Rust does not hoist such checks, and, so, with nested loops, it has trouble getting checks out of the loop, which prevents vectorization.

Replies

afdbcreid • today at 3:27 AM

Currently LLVM cannot do that because the panic message includes the erroneous index. You can do it manually though if you add `_ = tab[100]`.

Even if the panic message would not include the index, LLVM was unable to do that if the previous iterations had side effects (for example if `tab` is not a local variable).