[mir-opt] avoid `*&` when reading primitive from slice

[scottmcm]

Today, this code https://play.rust-lang.org/?version=nightly&mode=release&edition=2024&gist=09a5eaf64b3be527fef5a0589d6fe377

use std::cmp::Ordering;
pub fn foo(x: [i32; 1], y: [i32; 1]) -> Ordering {
    Ord::cmp(&x[0], &y[0])
}

gives this optimized MIR:

fn foo(_1: [i32; 1], _2: [i32; 1]) -> std::cmp::Ordering {
    debug x => _1;
    debug y => _2;
    let mut _0: std::cmp::Ordering;
    let _3: &i32;
    let _4: &i32;
    scope 1 (inlined std::cmp::impls::<impl Ord for i32>::cmp) {
        let mut _5: i32;
        let mut _6: i32;
    }

    bb0: {
        _3 = &_1[0 of 1];
        _4 = &_2[0 of 1];
        StorageLive(_5);
        _5 = copy (*_3);
        StorageLive(_6);
        _6 = copy (*_4);
        _0 = Cmp(move _5, move _6);
        StorageDead(_6);
        StorageDead(_5);
        return;
    }
}

It's a shame that we need to take a reference there in the MIR instead of just copying out of the place directly, like

    bb0: {
        StorageLive(_5);
        _5 = copy _1[0 of 1];
        StorageLive(_6);
        _6 = copy _2[0 of 1];
        _0 = Cmp(move _5, move _6);
        StorageDead(_6);
        StorageDead(_5);
        return;
    }

@cjgillot, is this something you think GVN could plausibly do?

[dianqk]

I think GVN only tracks the value that is a local, then reuse the value (local). Maybe we can do a pre-processing before GVN to put these values into locals: https://play.rust-lang.org/?version=nightly&mode=release&edition=2024&gist=38d1e888c464c4bc190f04d7aa27ecd3.

[cjgillot]

I think it can be done with GVN. The hard part is checking that the place still exists and has the same value. I'm surprised ReferencePropagation doesn't do it already. #132527 may be enough to unblock GVN.

[cjgillot]

In this case, ReferencePropagation cannot do it, as the index is not known when it runs. GVN could do it. Most of the required code already exists in simplify_aggregate_to_copy and needs to be generalized.