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| <h1 id="mir-optimizations"><a class="header" href="#mir-optimizations">MIR optimizations</a></h1> |
| <p>MIR optimizations are optimizations run on the <a href="../mir/index.html">MIR</a> to produce better MIR |
| before codegen. This is important for two reasons: first, it makes the final |
| generated executable code better, and second, it means that LLVM has less work |
| to do, so compilation is faster. Note that since MIR is generic (not |
| <a href="../appendix/glossary.html#mono">monomorphized</a> yet), these optimizations are particularly |
| effective; we can optimize the generic version, so all of the monomorphizations |
| are cheaper!</p> |
| <p>MIR optimizations run after borrow checking. We run a series of optimization |
| passes over the MIR to improve it. Some passes are required to run on all code, |
| some passes don't actually do optimizations but only check stuff, and some |
| passes are only turned on in <code>release</code> mode.</p> |
| <p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/fn.optimized_mir.html"><code>optimized_mir</code></a> <a href="../query.html">query</a> is called to produce the optimized MIR |
| for a given <a href="../appendix/glossary.html#def-id"><code>DefId</code></a>. This query makes sure that the borrow checker has |
| run and that some validation has occurred. Then, it <a href="../mir/passes.html#stealing">steals</a> the MIR, |
| optimizes it, and returns the improved MIR.</p> |
| <h2 id="quickstart-for-adding-a-new-optimization"><a class="header" href="#quickstart-for-adding-a-new-optimization">Quickstart for adding a new optimization</a></h2> |
| <ol> |
| <li> |
| <p>Make a Rust source file in <code>tests/mir-opt</code> that shows the code you want to |
| optimize. This should be kept simple, so avoid <code>println!</code> or other formatting |
| code if it's not necessary for the optimization. The reason for this is that |
| <code>println!</code>, <code>format!</code>, etc. generate a lot of MIR that can make it harder to |
| understand what the optimization does to the test.</p> |
| </li> |
| <li> |
| <p>Run <code>./x test --bless tests/mir-opt/<your-test>.rs</code> to generate a MIR |
| dump. Read <a href="https://github.com/rust-lang/rust/blob/HEAD/tests/mir-opt/README.md">this README</a> for instructions on how to dump |
| things.</p> |
| </li> |
| <li> |
| <p>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.</p> |
| </li> |
| <li> |
| <p>Implement a new optimization in <a href="https://github.com/rust-lang/rust/tree/HEAD/compiler/rustc_mir_transform/src"><code>compiler/rustc_mir_transform/src</code></a>. |
| The fastest and easiest way to do this is to</p> |
| <ol> |
| <li>pick a small optimization (such as <a href="https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_mir_transform/src/remove_storage_markers.rs"><code>remove_storage_markers</code></a>) and copy it |
| to a new file,</li> |
| <li>add your optimization to one of the lists in the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/fn.run_optimization_passes.html"><code>run_optimization_passes()</code></a> function,</li> |
| <li>and then start modifying the copied optimization.</li> |
| </ol> |
| </li> |
| <li> |
| <p>Rerun <code>./x test --bless tests/mir-opt/<your-test>.rs</code> to regenerate the |
| MIR dumps. Look at the diffs to see if they are what you expect.</p> |
| </li> |
| <li> |
| <p>Run <code>./x test tests/ui</code> to see if your optimization broke anything.</p> |
| </li> |
| <li> |
| <p>If there are issues with your optimization, experiment with it a bit and |
| repeat steps 5 and 6.</p> |
| </li> |
| <li> |
| <p>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 <code>[WIP]</code> or otherwise note that it is a |
| work in progress) in that case.</p> |
| <p>Make sure to commit the blessed test output as well! It's necessary for CI to |
| pass and it's very helpful to reviewers.</p> |
| </li> |
| </ol> |
| <p>If you have any questions along the way, feel free to ask in |
| <code>#t-compiler/wg-mir-opt</code> on Zulip.</p> |
| <h2 id="defining-optimization-passes"><a class="header" href="#defining-optimization-passes">Defining optimization passes</a></h2> |
| <p>The list of passes run and the order in which they are run is defined by the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/fn.run_optimization_passes.html"><code>run_optimization_passes</code></a> function. It contains an array of passes to |
| run. Each pass in the array is a struct that implements the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/pass_manager/trait.MirPass.html"><code>MirPass</code></a> trait. |
| The array is an array of <code>&dyn MirPass</code> trait objects. Typically, a pass is |
| implemented in its own module of the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/index.html"><code>rustc_mir_transform</code></a> crate.</p> |
| <p>Some examples of passes are:</p> |
| <ul> |
| <li><code>CleanupPostBorrowck</code>: Remove some of the info that is only needed for |
| analyses, rather than codegen.</li> |
| <li><code>ConstProp</code>: Does <a href="https://en.wikipedia.org/wiki/Constant_folding#Constant_propagation">constant propagation</a>.</li> |
| </ul> |
| <p>You can see the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/pass_manager/trait.MirPass.html#implementors">"Implementors" section of the <code>MirPass</code> rustdocs</a> for more examples.</p> |
| <h2 id="mir-optimization-levels"><a class="header" href="#mir-optimization-levels">MIR optimization levels</a></h2> |
| <p>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 <code>-Z mir-opt-level</code> debug flag. You can find the |
| definitions of the levels in the <a href="https://github.com/rust-lang/compiler-team/issues/319">compiler MCP</a>. If you are developing a MIR pass and |
| want to query whether your optimization pass should run, you can check the |
| current level using <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/config/struct.UnstableOptions.html#structfield.mir_opt_level"><code>tcx.sess.opts.unstable_opts.mir_opt_level</code></a>.</p> |
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