First iteration of simplify match branches

[JulianKnodt]

This is a simple MIR pass that attempts to convert

   bb0: {
        StorageLive(_2);
        _3 = discriminant(_1);
        switchInt(move _3) -> [0isize: bb2, otherwise: bb1];
    }

    bb1: {
        _2 = const false;
        goto -> bb3;
    }

    bb2: {
        _2 = const true;
        goto -> bb3;
    }

into

    bb0: {
        StorageLive(_2);
        _3 = discriminant(_1);
        _2 = _3 == 0;
        goto -> bb3;
    }

There are still missing components(like checking if the assignments are bools).
Was hoping that this could get some review though.

Handles #75141

r? @oli-obk

[oli-obk]

cc @rust-lang/wg-mir-opt

[JulianKnodt]

Uh, would you happen to know why this is breaking? I can't figure out why this won't build.

[oli-obk]

Uh, would you happen to know why this is breaking? I can't figure out why this won't build.

your optimization breaks things. You can test with ./x.py test --stage 1 --bless src/test/mir-opt to not build rustc with your changes so you can see what's going on. Basically your optimization is too liberal in what code it is applied to, so likely we're misoptimizing some code that was never intended to get optimized

[JulianKnodt]

Ah, I just assumed that since it was breaking in dependencies it was not the fault of the changes. It's good to know this may yet optimize something somewhere

[oli-obk]

Yea, I always work with --stage 1 because it's faster, but the additional benefit of actually getting to tests is really helpful when doing these kinds of shenanigans

[JulianKnodt]

The diff for the simple MIR test looks ok, so I'm wondering where this would break things otherwise...

[oli-obk]

So one thing that we overlooked is that there may be more branches pointing to the first block that we're modifying. We can work around that by not modifying it and instead cloning all statements from that block and placing them at the end of the current block

[oli-obk]

awesome!

@bors r+

[bors]

📌 Commit 46e5699 has been approved by oli-obk

[wesleywiser]

MIR opts (hopefully 🤞) improve performance so they shouldn't be rolled up.

@bors rollup=never

[bors]

⌛ Testing commit 46e5699 with merge 5e3f1b1...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: oli-obk
Pushing 5e3f1b1 to master...