Detailed Steps for Implementing MIR Optimizations

optimize. This should be kept simple, so avoid `println!` or other formatting code if it's not necessary for the optimization. The reason for this is that `println!`, `format!`, etc. generate a lot of MIR that can make it harder to understand what the optimization does to the test. 2. Run `./x test --bless tests/mir-opt/<your-test>.rs` to generate a MIR dump. Read [this README][mir-opt-test-readme] for instructions on how to dump things. 3. Commit the current working directory state. The reason you should commit the test output before you implement the optimization is so that you (and your reviewers) can see a before/after diff of what the optimization changed. 4. Implement a new optimization in [`compiler/rustc_mir_transform/src`]. The fastest and easiest way to do this is to 1. pick a small optimization (such as [`remove_storage_markers`]) and copy it to a new file, 2. add your optimization to one of the lists in the [`run_optimization_passes()`] function, 3. and then start modifying the copied optimization. 5. Rerun `./x test --bless tests/mir-opt/<your-test>.rs` to regenerate the MIR dumps. Look at the diffs to see if they are what you expect. 6. Run `./x test tests/ui` to see if your optimization broke anything. 7. If there are issues with your optimization, experiment with it a bit and repeat steps 5 and 6. 8. Commit and open a PR. You can do this at any point, even if things aren't working yet, so that you can ask for feedback on the PR. Open a "WIP" PR (just prefix your PR title with `[WIP]` or otherwise note that it is a work in progress) in that case. Make sure to commit the blessed test output as well! It's necessary for CI to pass and it's very helpful to reviewers. If you have any questions along the way, feel free to ask in `#t-compiler/wg-mir-opt` on Zulip. ## Defining optimization passes The list of passes run and the order in which they are run is defined by the [`run_optimization_passes`][rop] function. It contains an array of passes to run. Each pass in the array is a struct that implements the [`MirPass`] trait. The array is an array of `&dyn MirPass` trait objects. Typically, a pass is implemented in its own module of the [`rustc_mir_transform`][trans] crate. Some examples of passes are: - `CleanupPostBorrowck`: Remove some of the info that is only needed for analyses, rather than codegen. - `ConstProp`: Does [constant propagation][constprop]. You can see the ["Implementors" section of the `MirPass` rustdocs][impl] for more examples. ## MIR optimization levels MIR optimizations can come in various levels of readiness. Experimental optimizations may cause miscompilations, or slow down compile times. These passes are still included in nightly builds to gather feedback and make it easier to modify the pass. To enable working with slow or otherwise experimental optimization passes, you can specify the `-Z mir-opt-level` debug flag. You can find the definitions of the levels in the [compiler MCP]. If you are developing a MIR pass and want to query whether your optimization pass should run, you can check the current level using [`tcx.sess.opts.unstable_opts.mir_opt_level`][mir_opt_level].

This section provides detailed steps for implementing a new MIR optimization, including creating simple test cases, generating MIR dumps, implementing the optimization by copying and modifying an existing one, and then regenerating the MIR dumps to observe the changes. It also covers running UI tests to check for regressions, experimenting with the optimization, and submitting a pull request with the blessed test output. Furthermore, it explains how optimization passes are defined using the `run_optimization_passes` function and the `MirPass` trait, with examples such as `CleanupPostBorrowck` and `ConstProp`. The section also explains the different MIR optimization levels and how to enable experimental optimizations using the `-Z mir-opt-level` flag.