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| <h1 id="dataflow-analysis"><a class="header" href="#dataflow-analysis">Dataflow Analysis</a></h1> |
| <p>If you work on the MIR, you will frequently come across various flavors of |
| <a href="https://en.wikipedia.org/wiki/Data-flow_analysis#Basic_principles">dataflow analysis</a>. <code>rustc</code> uses dataflow to find uninitialized |
| variables, determine what variables are live across a generator <code>yield</code> |
| statement, and compute which <code>Place</code>s are borrowed at a given point in the |
| control-flow graph. Dataflow analysis is a fundamental concept in modern |
| compilers, and knowledge of the subject will be helpful to prospective |
| contributors.</p> |
| <p>However, this documentation is not a general introduction to dataflow analysis. |
| It is merely a description of the framework used to define these analyses in |
| <code>rustc</code>. It assumes that the reader is familiar with the core ideas as well as |
| some basic terminology, such as “transfer function”, “fixpoint” and “lattice”. |
| If you’re unfamiliar with these terms, or if you want a quick refresher, |
| <a href="https://cs.au.dk/~amoeller/spa/"><em>Static Program Analysis</em></a> by Anders Møller and Michael I. Schwartzbach is an |
| excellent, freely available textbook. For those who prefer audiovisual |
| learning, we previously recommended a series of short lectures |
| by the Goethe University Frankfurt on YouTube, but it has since been deleted. |
| See <a href="https://github.com/rust-lang/rustc-dev-guide/pull/1295">this PR</a> for the context and <a href="https://github.com/rust-lang/rustc-dev-guide/pull/1295#issuecomment-1118131294">this comment</a> |
| for the alternative lectures.</p> |
| <h2 id="defining-a-dataflow-analysis"><a class="header" href="#defining-a-dataflow-analysis">Defining a Dataflow Analysis</a></h2> |
| <p>A dataflow analysis is defined by the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.Analysis.html"><code>Analysis</code></a> trait. In addition to the |
| type of the dataflow state, this trait defines the initial value of that state |
| at entry to each block, as well as the direction of the analysis, either |
| forward or backward. The domain of your dataflow analysis must be a <a href="https://en.wikipedia.org/wiki/Lattice_(order)">lattice</a> |
| (strictly speaking a join-semilattice) with a well-behaved <code>join</code> operator. See |
| documentation for the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/lattice/index.html"><code>lattice</code></a> module, as well as the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/lattice/trait.JoinSemiLattice.html"><code>JoinSemiLattice</code></a> |
| trait, for more information.</p> |
| <h3 id="transfer-functions-and-effects"><a class="header" href="#transfer-functions-and-effects">Transfer Functions and Effects</a></h3> |
| <p>The dataflow framework in <code>rustc</code> allows each statement (and terminator) inside |
| a basic block to define its own transfer function. For brevity, these |
| individual transfer functions are known as “effects”. Each effect is applied |
| successively in dataflow order, and together they define the transfer function |
| for the entire basic block. It’s also possible to define an effect for |
| particular outgoing edges of some terminators (e.g. |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.Analysis.html#tymethod.apply_call_return_effect"><code>apply_call_return_effect</code></a> for the <code>success</code> edge of a <code>Call</code> |
| terminator). Collectively, these are referred to as “per-edge effects”.</p> |
| <h3 id="before-effects"><a class="header" href="#before-effects">“Before” Effects</a></h3> |
| <p>Observant readers of the documentation may notice that there are actually <em>two</em> |
| possible effects for each statement and terminator, the “before” effect and the |
| unprefixed (or “primary”) effect. The “before” effects are applied immediately |
| before the unprefixed effect <strong>regardless of the direction of the analysis</strong>. |
| In other words, a backward analysis will apply the “before” effect and then the |
| “primary” effect when computing the transfer function for a basic block, just |
| like a forward analysis.</p> |
| <p>The vast majority of analyses should use only the unprefixed effects: Having |
| multiple effects for each statement makes it difficult for consumers to know |
| where they should be looking. However, the “before” variants can be useful in |
| some scenarios, such as when the effect of the right-hand side of an assignment |
| statement must be considered separately from the left-hand side.</p> |
| <h3 id="convergence"><a class="header" href="#convergence">Convergence</a></h3> |
| <p>Your analysis must converge to “fixpoint”, otherwise it will run forever. |
| Converging to fixpoint is just another way of saying “reaching equilibrium”. |
| In order to reach equilibrium, your analysis must obey some laws. One of the |
| laws it must obey is that the bottom value<sup class="footnote-reference" id="fr-bottom-purpose-1"><a href="#footnote-bottom-purpose">1</a></sup> joined with some |
| other value equals the second value. Or, as an equation:</p> |
| <blockquote> |
| <p><em>bottom</em> join <em>x</em> = <em>x</em></p> |
| </blockquote> |
| <p>Another law is that your analysis must have a “top value” such that</p> |
| <blockquote> |
| <p><em>top</em> join <em>x</em> = <em>top</em></p> |
| </blockquote> |
| <p>Having a top value ensures that your semilattice has a finite height, and the |
| law state above ensures that once the dataflow state reaches top, it will no |
| longer change (the fixpoint will be top).</p> |
| <h2 id="a-brief-example"><a class="header" href="#a-brief-example">A Brief Example</a></h2> |
| <p>This section provides a brief example of a simple data-flow analysis at a high |
| level. It doesn’t explain everything you need to know, but hopefully it will |
| make the rest of this page clearer.</p> |
| <p>Let’s say we want to do a simple analysis to find if <code>mem::transmute</code> may have |
| been called by a certain point in the program. Our analysis domain will just |
| be a <code>bool</code> that records whether <code>transmute</code> has been called so far. The bottom |
| value will be <code>false</code>, since by default <code>transmute</code> has not been called. The top |
| value will be <code>true</code>, since our analysis is done as soon as we determine that |
| <code>transmute</code> has been called. Our join operator will just be the boolean OR (<code>||</code>) |
| operator. We use OR and not AND because of this case:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span><span class="boring">unsafe fn example(some_cond: bool) { |
| </span>let x = if some_cond { |
| std::mem::transmute::<i32, u32>(0_i32) // transmute was called! |
| } else { |
| 1_u32 // transmute was not called |
| }; |
| |
| // Has transmute been called by this point? We conservatively approximate that |
| // as yes, and that is why we use the OR operator. |
| println!("x: {}", x); |
| <span class="boring">} |
| </span><span class="boring">}</span></code></pre> |
| <h2 id="inspecting-the-results-of-a-dataflow-analysis"><a class="header" href="#inspecting-the-results-of-a-dataflow-analysis">Inspecting the Results of a Dataflow Analysis</a></h2> |
| <p>Once you have constructed an analysis, you must call <code>iterate_to_fixpoint</code> |
| which will return a <code>Results</code>, which contains the dataflow state at fixpoint |
| upon entry of each block. Once you have a <code>Results</code>, you can inspect the |
| dataflow state at fixpoint at any point in the CFG. If you only need the state |
| at a few locations (e.g., each <code>Drop</code> terminator) use a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/struct.ResultsCursor.html"><code>ResultsCursor</code></a>. If |
| you need the state at <em>every</em> location, a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.ResultsVisitor.html"><code>ResultsVisitor</code></a> will be more |
| efficient.</p> |
| <pre><code class="language-text"> Analysis |
| | |
| | iterate_to_fixpoint() |
| | |
| Results |
| / \ |
| into_results_cursor(…) / \ visit_with(…) |
| / \ |
| ResultsCursor ResultsVisitor |
| </code></pre> |
| <p>For example, the following code uses a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.ResultsVisitor.html"><code>ResultsVisitor</code></a>…</p> |
| <pre><code class="language-rust ignore">// Assuming `MyVisitor` implements `ResultsVisitor<FlowState = MyAnalysis::Domain>`... |
| let mut my_visitor = MyVisitor::new(); |
| |
| // inspect the fixpoint state for every location within every block in RPO. |
| let results = MyAnalysis::new() |
| .iterate_to_fixpoint(tcx, body, None); |
| results.visit_with(body, &mut my_visitor);`</code></pre> |
| <p>whereas this code uses <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/struct.ResultsCursor.html"><code>ResultsCursor</code></a>:</p> |
| <pre><code class="language-rust ignore">let mut results = MyAnalysis::new() |
| .iterate_to_fixpoint(tcx, body, None); |
| .into_results_cursor(body); |
| |
| // Inspect the fixpoint state immediately before each `Drop` terminator. |
| for (bb, block) in body.basic_blocks().iter_enumerated() { |
| if let TerminatorKind::Drop { .. } = block.terminator().kind { |
| results.seek_before_primary_effect(body.terminator_loc(bb)); |
| let state = results.get(); |
| println!("state before drop: {:#?}", state); |
| } |
| }</code></pre> |
| <h3 id="graphviz-diagrams"><a class="header" href="#graphviz-diagrams">Graphviz Diagrams</a></h3> |
| <p>When the results of a dataflow analysis are not what you expect, it often helps |
| to visualize them. This can be done with the <code>-Z dump-mir</code> flags described in |
| <a href="./debugging.html">Debugging MIR</a>. Start with <code>-Z dump-mir=F -Z dump-mir-dataflow</code>, where <code>F</code> is |
| either “all” or the name of the MIR body you are interested in.</p> |
| <p>These <code>.dot</code> files will be saved in your <code>mir_dump</code> directory and will have the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.Analysis.html#associatedconstant.NAME"><code>NAME</code></a> of the analysis (e.g. <code>maybe_inits</code>) as part of their filename. Each |
| visualization will display the full dataflow state at entry and exit of each |
| block, as well as any changes that occur in each statement and terminator. See |
| the example below:</p> |
| <p><img src="../img/dataflow-graphviz-example.png" alt="A graphviz diagram for a dataflow analysis"></p> |
| <hr> |
| <ol class="footnote-definition"> |
| <li id="footnote-bottom-purpose"> |
| <p>The bottom value’s primary purpose is as the initial dataflow |
| state. Each basic block’s entry state is initialized to bottom before the |
| analysis starts. <a href="#fr-bottom-purpose-1">↩</a></p> |
| </li> |
| </ol> |
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