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| <ol class="chapter"><li class="chapter-item affix "><a href="../about-this-guide.html">About this guide</a></li><li class="chapter-item affix "><a href="../getting-started.html">Getting Started</a></li><li class="spacer"></li><li class="chapter-item affix "><li class="part-title">Building and debugging rustc</li><li class="chapter-item "><a href="../building/how-to-build-and-run.html"><strong aria-hidden="true">1.</strong> How to Build and Run the Compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../building/prerequisites.html"><strong aria-hidden="true">1.1.</strong> Prerequisites</a></li><li class="chapter-item "><a href="../building/suggested.html"><strong aria-hidden="true">1.2.</strong> Suggested Workflows</a></li><li class="chapter-item "><a href="../building/build-install-distribution-artifacts.html"><strong aria-hidden="true">1.3.</strong> Distribution artifacts</a></li><li class="chapter-item "><a href="../building/compiler-documenting.html"><strong aria-hidden="true">1.4.</strong> Documenting Compiler</a></li><li class="chapter-item "><a href="../rustdoc.html"><strong aria-hidden="true">1.5.</strong> Rustdoc overview</a></li><li class="chapter-item "><a href="../building/new-target.html"><strong aria-hidden="true">1.6.</strong> Adding a new target</a></li></ol></li><li class="chapter-item "><a href="../tests/intro.html"><strong aria-hidden="true">2.</strong> The compiler testing framework</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../tests/running.html"><strong aria-hidden="true">2.1.</strong> Running tests</a></li><li class="chapter-item "><a href="../tests/adding.html"><strong aria-hidden="true">2.2.</strong> Adding new tests</a></li><li class="chapter-item "><a href="../compiletest.html"><strong aria-hidden="true">2.3.</strong> Using compiletest commands to control test execution</a></li></ol></li><li class="chapter-item "><a href="../compiler-debugging.html"><strong aria-hidden="true">3.</strong> Debugging the Compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../tracing.html"><strong aria-hidden="true">3.1.</strong> Using the tracing/logging instrumentation</a></li></ol></li><li class="chapter-item "><a href="../profiling.html"><strong aria-hidden="true">4.</strong> Profiling the compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../profiling/with_perf.html"><strong aria-hidden="true">4.1.</strong> with the linux perf tool</a></li><li class="chapter-item "><a href="../profiling/wpa_profiling.html"><strong aria-hidden="true">4.2.</strong> with Windows Performance Analyzer</a></li></ol></li><li class="chapter-item "><a href="../crates-io.html"><strong aria-hidden="true">5.</strong> crates.io Dependencies</a></li><li class="chapter-item affix "><li class="part-title">Contributing to Rust</li><li class="chapter-item "><a href="../contributing.html"><strong aria-hidden="true">6.</strong> Introduction</a></li><li class="chapter-item "><a href="../compiler-team.html"><strong aria-hidden="true">7.</strong> About the compiler team</a></li><li class="chapter-item "><a href="../git.html"><strong aria-hidden="true">8.</strong> Using Git</a></li><li class="chapter-item "><a href="../rustbot.html"><strong aria-hidden="true">9.</strong> Mastering @rustbot</a></li><li class="chapter-item "><a href="../walkthrough.html"><strong aria-hidden="true">10.</strong> Walkthrough: a typical contribution</a></li><li class="chapter-item "><a href="../bug-fix-procedure.html"><strong aria-hidden="true">11.</strong> Bug Fix Procedure</a></li><li class="chapter-item "><a href="../implementing_new_features.html"><strong aria-hidden="true">12.</strong> Implementing new features</a></li><li class="chapter-item "><a href="../stability.html"><strong aria-hidden="true">13.</strong> Stability attributes</a></li><li class="chapter-item "><a href="../stabilization_guide.html"><strong aria-hidden="true">14.</strong> Stabilizing Features</a></li><li class="chapter-item "><a href="../feature-gates.html"><strong aria-hidden="true">15.</strong> Feature Gates</a></li><li class="chapter-item "><a href="../conventions.html"><strong aria-hidden="true">16.</strong> Coding conventions</a></li><li class="chapter-item "><a href="../notification-groups/about.html"><strong aria-hidden="true">17.</strong> Notification groups</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../notification-groups/arm.html"><strong aria-hidden="true">17.1.</strong> ARM</a></li><li class="chapter-item "><a href="../notification-groups/cleanup-crew.html"><strong aria-hidden="true">17.2.</strong> Cleanup Crew</a></li><li class="chapter-item "><a href="../notification-groups/llvm.html"><strong aria-hidden="true">17.3.</strong> LLVM</a></li><li class="chapter-item "><a href="../notification-groups/risc-v.html"><strong aria-hidden="true">17.4.</strong> RISC-V</a></li><li class="chapter-item "><a href="../notification-groups/windows.html"><strong aria-hidden="true">17.5.</strong> Windows</a></li></ol></li><li class="chapter-item "><a href="../licenses.html"><strong aria-hidden="true">18.</strong> Licenses</a></li><li class="chapter-item affix "><li class="part-title">High-level Compiler Architecture</li><li class="chapter-item "><a href="../part-2-intro.html"><strong aria-hidden="true">19.</strong> Prologue</a></li><li class="chapter-item "><a href="../overview.html"><strong aria-hidden="true">20.</strong> Overview of the Compiler</a></li><li class="chapter-item "><a href="../compiler-src.html"><strong aria-hidden="true">21.</strong> The compiler source code</a></li><li class="chapter-item "><a href="../building/bootstrapping.html"><strong aria-hidden="true">22.</strong> Bootstrapping</a></li><li class="chapter-item "><a href="../query.html"><strong aria-hidden="true">23.</strong> Queries: demand-driven compilation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../queries/query-evaluation-model-in-detail.html"><strong aria-hidden="true">23.1.</strong> The Query Evaluation Model in Detail</a></li><li class="chapter-item "><a href="../queries/incremental-compilation.html"><strong aria-hidden="true">23.2.</strong> Incremental compilation</a></li><li class="chapter-item "><a href="../queries/incremental-compilation-in-detail.html"><strong aria-hidden="true">23.3.</strong> Incremental compilation In Detail</a></li><li class="chapter-item "><a href="../incrcomp-debugging.html"><strong aria-hidden="true">23.4.</strong> Debugging and Testing</a></li><li class="chapter-item "><a href="../salsa.html"><strong aria-hidden="true">23.5.</strong> Salsa</a></li></ol></li><li class="chapter-item "><a href="../memory.html"><strong aria-hidden="true">24.</strong> Memory Management in Rustc</a></li><li class="chapter-item "><a href="../serialization.html"><strong aria-hidden="true">25.</strong> Serialization in Rustc</a></li><li class="chapter-item "><a href="../parallel-rustc.html"><strong aria-hidden="true">26.</strong> Parallel Compilation</a></li><li class="chapter-item "><a href="../rustdoc-internals.html"><strong aria-hidden="true">27.</strong> Rustdoc internals</a></li><li class="chapter-item affix "><li class="part-title">Source Code Representation</li><li class="chapter-item "><a href="../part-3-intro.html"><strong aria-hidden="true">28.</strong> Prologue</a></li><li class="chapter-item "><a href="../cli.html"><strong aria-hidden="true">29.</strong> Command-line arguments</a></li><li class="chapter-item "><a href="../rustc-driver.html"><strong aria-hidden="true">30.</strong> The Rustc Driver and Interface</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../rustc-driver-interacting-with-the-ast.html"><strong aria-hidden="true">30.1.</strong> Ex: Type checking through rustc_interface</a></li><li class="chapter-item "><a href="../rustc-driver-getting-diagnostics.html"><strong aria-hidden="true">30.2.</strong> Ex: Getting diagnostics through rustc_interface</a></li></ol></li><li class="chapter-item "><a href="../syntax-intro.html"><strong aria-hidden="true">31.</strong> Syntax and the AST</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../the-parser.html"><strong aria-hidden="true">31.1.</strong> Lexing and Parsing</a></li><li class="chapter-item "><a href="../macro-expansion.html"><strong aria-hidden="true">31.2.</strong> Macro expansion</a></li><li class="chapter-item "><a href="../name-resolution.html"><strong aria-hidden="true">31.3.</strong> Name resolution</a></li><li class="chapter-item "><a href="../test-implementation.html"><strong aria-hidden="true">31.4.</strong> #[test] Implementation</a></li><li class="chapter-item "><a href="../panic-implementation.html"><strong aria-hidden="true">31.5.</strong> Panic Implementation</a></li><li class="chapter-item "><a href="../ast-validation.html"><strong aria-hidden="true">31.6.</strong> AST Validation</a></li><li class="chapter-item "><a href="../feature-gate-ck.html"><strong aria-hidden="true">31.7.</strong> Feature Gate Checking</a></li><li class="chapter-item "><a href="../lang-items.html"><strong aria-hidden="true">31.8.</strong> Lang Items</a></li></ol></li><li class="chapter-item "><a href="../hir.html"><strong aria-hidden="true">32.</strong> The HIR (High-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../lowering.html"><strong aria-hidden="true">32.1.</strong> Lowering AST to HIR</a></li><li class="chapter-item "><a href="../hir-debugging.html"><strong aria-hidden="true">32.2.</strong> Debugging</a></li></ol></li><li class="chapter-item "><a href="../thir.html"><strong aria-hidden="true">33.</strong> The THIR (Typed High-level IR)</a></li><li class="chapter-item "><a href="../mir/index.html"><strong aria-hidden="true">34.</strong> The MIR (Mid-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../mir/construction.html"><strong aria-hidden="true">34.1.</strong> MIR construction</a></li><li class="chapter-item "><a href="../mir/visitor.html"><strong aria-hidden="true">34.2.</strong> MIR visitor and traversal</a></li><li class="chapter-item "><a href="../mir/passes.html"><strong aria-hidden="true">34.3.</strong> MIR passes: getting the MIR for a function</a></li></ol></li><li class="chapter-item "><a href="../identifiers.html"><strong aria-hidden="true">35.</strong> Identifiers in the Compiler</a></li><li class="chapter-item "><a href="../closure.html"><strong aria-hidden="true">36.</strong> Closure expansion</a></li><li class="chapter-item affix "><li class="part-title">Analysis</li><li class="chapter-item "><a href="../part-4-intro.html"><strong aria-hidden="true">37.</strong> Prologue</a></li><li class="chapter-item "><a href="../ty.html"><strong aria-hidden="true">38.</strong> The ty module: representing types</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../generics.html"><strong aria-hidden="true">38.1.</strong> Generics and substitutions</a></li><li class="chapter-item "><a href="../ty-fold.html"><strong aria-hidden="true">38.2.</strong> TypeFolder and TypeFoldable</a></li><li class="chapter-item "><a href="../generic_arguments.html"><strong aria-hidden="true">38.3.</strong> Generic arguments</a></li><li class="chapter-item "><a href="../constants.html"><strong aria-hidden="true">38.4.</strong> Constants in the type system</a></li></ol></li><li class="chapter-item "><a href="../type-inference.html"><strong aria-hidden="true">39.</strong> Type inference</a></li><li class="chapter-item "><a href="../traits/resolution.html"><strong aria-hidden="true">40.</strong> Trait solving</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../early-late-bound.html"><strong aria-hidden="true">40.1.</strong> Early and Late Bound Parameters</a></li><li class="chapter-item "><a href="../traits/hrtb.html"><strong aria-hidden="true">40.2.</strong> Higher-ranked trait bounds</a></li><li class="chapter-item "><a href="../traits/caching.html"><strong aria-hidden="true">40.3.</strong> Caching subtleties</a></li><li class="chapter-item "><a href="../traits/specialization.html"><strong aria-hidden="true">40.4.</strong> Specialization</a></li><li class="chapter-item "><a href="../traits/chalk.html"><strong aria-hidden="true">40.5.</strong> Chalk-based trait solving</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../traits/lowering-to-logic.html"><strong aria-hidden="true">40.5.1.</strong> Lowering to logic</a></li><li class="chapter-item "><a href="../traits/goals-and-clauses.html"><strong aria-hidden="true">40.5.2.</strong> Goals and clauses</a></li><li class="chapter-item "><a href="../traits/canonical-queries.html"><strong aria-hidden="true">40.5.3.</strong> Canonical queries</a></li></ol></li></ol></li><li class="chapter-item "><a href="../type-checking.html"><strong aria-hidden="true">41.</strong> Type checking</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../method-lookup.html"><strong aria-hidden="true">41.1.</strong> Method Lookup</a></li><li class="chapter-item "><a href="../variance.html"><strong aria-hidden="true">41.2.</strong> Variance</a></li><li class="chapter-item "><a href="../opaque-types-type-alias-impl-trait.html"><strong aria-hidden="true">41.3.</strong> Opaque Types</a></li></ol></li><li class="chapter-item "><a href="../pat-exhaustive-checking.html"><strong aria-hidden="true">42.</strong> Pattern and Exhaustiveness Checking</a></li><li class="chapter-item expanded "><a href="../mir/dataflow.html" class="active"><strong aria-hidden="true">43.</strong> MIR dataflow</a></li><li class="chapter-item "><a href="../borrow_check.html"><strong aria-hidden="true">44.</strong> The borrow checker</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../borrow_check/moves_and_initialization.html"><strong aria-hidden="true">44.1.</strong> Tracking moves and initialization</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../borrow_check/moves_and_initialization/move_paths.html"><strong aria-hidden="true">44.1.1.</strong> Move paths</a></li></ol></li><li class="chapter-item "><a href="../borrow_check/type_check.html"><strong aria-hidden="true">44.2.</strong> MIR type checker</a></li><li class="chapter-item "><a href="../borrow_check/region_inference.html"><strong aria-hidden="true">44.3.</strong> Region inference</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../borrow_check/region_inference/constraint_propagation.html"><strong aria-hidden="true">44.3.1.</strong> Constraint propagation</a></li><li class="chapter-item "><a href="../borrow_check/region_inference/lifetime_parameters.html"><strong aria-hidden="true">44.3.2.</strong> Lifetime parameters</a></li><li class="chapter-item "><a href="../borrow_check/region_inference/member_constraints.html"><strong aria-hidden="true">44.3.3.</strong> Member constraints</a></li><li class="chapter-item "><a href="../borrow_check/region_inference/placeholders_and_universes.html"><strong aria-hidden="true">44.3.4.</strong> Placeholders and universes</a></li><li class="chapter-item "><a href="../borrow_check/region_inference/closure_constraints.html"><strong aria-hidden="true">44.3.5.</strong> Closure constraints</a></li><li class="chapter-item "><a href="../borrow_check/region_inference/error_reporting.html"><strong aria-hidden="true">44.3.6.</strong> Error reporting</a></li></ol></li><li class="chapter-item "><a href="../borrow_check/two_phase_borrows.html"><strong aria-hidden="true">44.4.</strong> Two-phase-borrows</a></li></ol></li><li class="chapter-item "><a href="../param_env.html"><strong aria-hidden="true">45.</strong> Parameter Environments</a></li><li class="chapter-item "><a href="../diagnostics.html"><strong aria-hidden="true">46.</strong> Errors and Lints</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../diagnostics/sessiondiagnostic.html"><strong aria-hidden="true">46.1.</strong> Creating Errors With SessionDiagnostic</a></li><li class="chapter-item "><a href="../diagnostics/lintstore.html"><strong aria-hidden="true">46.2.</strong> LintStore</a></li><li class="chapter-item "><a href="../diagnostics/diagnostic-codes.html"><strong aria-hidden="true">46.3.</strong> Diagnostic Codes</a></li><li class="chapter-item "><a href="../diagnostics/diagnostic-items.html"><strong aria-hidden="true">46.4.</strong> Diagnostic Items</a></li></ol></li><li class="chapter-item "><li class="part-title">MIR to Binaries</li><li class="chapter-item "><a href="../part-5-intro.html"><strong aria-hidden="true">47.</strong> Prologue</a></li><li class="chapter-item "><a href="../mir/optimizations.html"><strong aria-hidden="true">48.</strong> MIR optimizations</a></li><li class="chapter-item "><a href="../mir/debugging.html"><strong aria-hidden="true">49.</strong> Debugging</a></li><li class="chapter-item "><a href="../const-eval.html"><strong aria-hidden="true">50.</strong> Constant evaluation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../miri.html"><strong aria-hidden="true">50.1.</strong> miri const evaluator</a></li></ol></li><li class="chapter-item "><a href="../backend/monomorph.html"><strong aria-hidden="true">51.</strong> Monomorphization</a></li><li class="chapter-item "><a href="../backend/lowering-mir.html"><strong aria-hidden="true">52.</strong> Lowering MIR</a></li><li class="chapter-item "><a href="../backend/codegen.html"><strong aria-hidden="true">53.</strong> Code Generation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../backend/updating-llvm.html"><strong aria-hidden="true">53.1.</strong> Updating LLVM</a></li><li class="chapter-item "><a href="../backend/debugging.html"><strong aria-hidden="true">53.2.</strong> Debugging LLVM</a></li><li class="chapter-item "><a href="../backend/backend-agnostic.html"><strong aria-hidden="true">53.3.</strong> Backend Agnostic Codegen</a></li><li class="chapter-item "><a href="../backend/implicit-caller-location.html"><strong aria-hidden="true">53.4.</strong> Implicit Caller Location</a></li></ol></li><li class="chapter-item "><a href="../backend/libs-and-metadata.html"><strong aria-hidden="true">54.</strong> Libraries and Metadata</a></li><li class="chapter-item "><a href="../profile-guided-optimization.html"><strong aria-hidden="true">55.</strong> Profile-guided Optimization</a></li><li class="chapter-item "><a href="../llvm-coverage-instrumentation.html"><strong aria-hidden="true">56.</strong> LLVM Source-Based Code Coverage</a></li><li class="chapter-item "><a href="../sanitizers.html"><strong aria-hidden="true">57.</strong> Sanitizers Support</a></li><li class="chapter-item "><a href="../debugging-support-in-rustc.html"><strong aria-hidden="true">58.</strong> Debugging Support in the Rust Compiler</a></li><li class="spacer"></li><li class="chapter-item affix "><a href="../appendix/background.html">Appendix A: Background topics</a></li><li class="chapter-item affix "><a href="../appendix/glossary.html">Appendix B: Glossary</a></li><li class="chapter-item affix "><a href="../appendix/code-index.html">Appendix C: Code Index</a></li><li class="chapter-item affix "><a href="../appendix/compiler-lecture.html">Appendix D: Compiler Lecture Series</a></li><li class="chapter-item affix "><a href="../appendix/bibliography.html">Appendix E: Bibliography</a></li><li class="chapter-item affix "><a href="../appendix/humorust.html">Appendix Z: HumorRust</a></li><li class="spacer"></li></ol> |
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| <h1 id="dataflow-analysis"><a class="header" href="#dataflow-analysis">Dataflow Analysis</a></h1> |
| <ul> |
| <li><a href="#defining-a-dataflow-analysis">Defining a Dataflow Analysis</a> |
| <ul> |
| <li><a href="#transfer-functions-and-effects">Transfer Functions and Effects</a></li> |
| <li><a href="#before-effects">"Before" Effects</a></li> |
| <li><a href="#convergence">Convergence</a></li> |
| </ul> |
| </li> |
| <li><a href="#a-brief-example">A Brief Example</a></li> |
| <li><a href="#inspecting-the-results-of-a-dataflow-analysis">Inspecting the Results of a Dataflow Analysis</a> |
| <ul> |
| <li><a href="#graphviz-diagrams">Graphviz Diagrams</a></li> |
| </ul> |
| </li> |
| </ul> |
| <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/%7Eamoeller/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, the Goethe University Frankfurt has published a series of short |
| <a href="https://www.youtube.com/watch?v=NVBQSR_HdL0&list=PL_sGR8T76Y58l3Gck3ZwIIHLWEmXrOLV_&index=2">lectures on YouTube</a> in English that are very approachable.</p> |
| <h2 id="defining-a-dataflow-analysis"><a class="header" href="#defining-a-dataflow-analysis">Defining a Dataflow Analysis</a></h2> |
| <p>The interface for dataflow analyses is split into three traits. The first is |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.AnalysisDomain.html"><code>AnalysisDomain</code></a>, which must be implemented by <em>all</em> analyses. 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> |
| <p>You must then provide <em>either</em> a direct implementation of the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.Analysis.html"><code>Analysis</code></a> trait |
| <em>or</em> an implementation of the proxy trait <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.GenKillAnalysis.html"><code>GenKillAnalysis</code></a>. The latter is for |
| so-called <a href="https://en.wikipedia.org/wiki/Data-flow_analysis#Bit_vector_problems">"gen-kill" problems</a>, which have a simple class of transfer function |
| that can be applied very efficiently. Analyses whose domain is not a <code>BitSet</code> |
| of some index type, or whose transfer functions cannot be expressed through |
| "gen" and "kill" operations, must implement <code>Analysis</code> directly, and will run |
| slower as a result. All implementers of <code>GenKillAnalysis</code> also implement |
| <code>Analysis</code> automatically via a default <code>impl</code>.</p> |
| <pre><code class="language-text"> AnalysisDomain |
| ^ |
| | | = has as a supertrait |
| | . = provides a default impl for |
| | |
| Analysis |
| ^ ^ |
| | . |
| | . |
| | . |
| GenKillAnalysis |
| |
| </code></pre> |
| <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 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> |
| <p>The only meaningful difference (besides the "apply" prefix) between the methods |
| of the <code>GenKillAnalysis</code> trait and the <code>Analysis</code> trait is that an <code>Analysis</code> |
| has direct, mutable access to the dataflow state, whereas a <code>GenKillAnalysis</code> |
| only sees an implementer of the <code>GenKill</code> trait, which only allows the <code>gen</code> |
| and <code>kill</code> operations for mutation.</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 |
| 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"><a href="#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> |
| <div class="footnote-definition" id="bottom-purpose"><sup class="footnote-definition-label">1</sup> |
| <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.</p> |
| </div> |
| <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><code>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); |
| </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 pass it to an <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/struct.Engine.html"><code>Engine</code></a>, which |
| is responsible for finding the steady-state solution to your dataflow problem. |
| You should use the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/trait.Analysis.html#method.into_engine"><code>into_engine</code></a> method defined on the <code>Analysis</code> trait for |
| this, since it will use the more efficient <code>Engine::new_gen_kill</code> constructor |
| when possible.</p> |
| <p>Calling <code>iterate_to_fixpoint</code> on your <code>Engine</code> 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 |
| | |
| | into_engine(…) |
| | |
| Engine |
| | |
| | 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() |
| .into_engine(tcx, body, def_id) |
| .iterate_to_fixpoint() |
| .visit_in_rpo_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() |
| .into_engine(tcx, body, def_id) |
| .iterate_to_fixpoint() |
| .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.AnalysisDomain.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> |
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