MIR-opt not const-folding

[simonvandel]

I expected the following:

fn f() -> usize  {       
    1 + if true { 1 } else { 2 }  
}

to const-fold into _0 = 2.

Godbolt: https://godbolt.org/z/e89dYj

Using -Zdump_mir=all shows that this may be a pass ordering issue.

Before SimplifyCfg-final

fn f() -> usize {
    let mut _0: usize;                   // return place in scope 0 at test.rs:2:11: 2:16
    let mut _1: usize;                   // in scope 0 at test.rs:3:9: 3:33
    let mut _2: bool;                    // in scope 0 at test.rs:3:12: 3:16

    bb0: {
        StorageLive(_1);                 // scope 0 at test.rs:3:9: 3:33
        StorageLive(_2);                 // scope 0 at test.rs:3:12: 3:16
        _2 = const true;                 // scope 0 at test.rs:3:12: 3:16
        goto -> bb1;                     // scope 0 at test.rs:3:9: 3:33
    }

    bb1: {
        _1 = const 1_usize;              // scope 0 at test.rs:3:19: 3:20
        goto -> bb3;                     // scope 0 at test.rs:3:9: 3:33
    }

    bb2: {
        _1 = const 2_usize;              // scope 0 at test.rs:3:30: 3:31
        goto -> bb3;                     // scope 0 at test.rs:3:9: 3:33
    }

    bb3: {
        StorageDead(_2);                 // scope 0 at test.rs:3:32: 3:33
        _0 = Add(const 1_usize, move _1); // scope 0 at test.rs:3:5: 3:33
        StorageDead(_1);                 // scope 0 at test.rs:3:32: 3:33
        return;                          // scope 0 at test.rs:4:2: 4:2
    }

    bb4 (cleanup): {
        resume;                          // scope 0 at test.rs:2:1: 4:2
    }
}

After SimplifyCfg-final

fn f() -> usize {
    let mut _0: usize;                   // return place in scope 0 at test.rs:2:11: 2:16
    let mut _1: usize;                   // in scope 0 at test.rs:3:9: 3:33
    let mut _2: bool;                    // in scope 0 at test.rs:3:12: 3:16

    bb0: {
        StorageLive(_1);                 // scope 0 at test.rs:3:9: 3:33
        StorageLive(_2);                 // scope 0 at test.rs:3:12: 3:16
        _2 = const true;                 // scope 0 at test.rs:3:12: 3:16
        _1 = const 1_usize;              // scope 0 at test.rs:3:19: 3:20
        StorageDead(_2);                 // scope 0 at test.rs:3:32: 3:33
        _0 = Add(const 1_usize, move _1); // scope 0 at test.rs:3:5: 3:33
        StorageDead(_1);                 // scope 0 at test.rs:3:32: 3:33
        return;                          // scope 0 at test.rs:4:2: 4:2
    }
}

So after SimplifyCfg-final it is clear that _0 = Add(const 1_usize, move _1) can be folded into _0 = 2. However, this is not done as ConstProp is only running early in the pipeline.

[klensy]

Godbolt currently shows this on nightly, 1.49 with -O --emit=mir

fn f() -> usize {
    let mut _0: usize;                   // return place in scope 0 at ./example.rs:2:15: 2:20
    let mut _1: usize;                   // in scope 0 at ./example.rs:3:9: 3:33

    bb0: {
        StorageLive(_1);                 // scope 0 at ./example.rs:3:9: 3:33
        _1 = const 1_usize;              // scope 0 at ./example.rs:3:19: 3:20
        _0 = Add(const 1_usize, move _1); // scope 0 at ./example.rs:3:5: 3:33
        StorageDead(_1);                 // scope 0 at ./example.rs:3:32: 3:33
        return;                          // scope 0 at ./example.rs:4:2: 4:2
    }
}

[nagisa]

This is a constant/value propagation which I'm sure has an issue for it already.

[michaelmaitland]

Is this issue something I could pick up and work on?