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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 expanded "><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 expanded "><a href="../queries/incremental-compilation.html" class="active"><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 "><a href="../mir/dataflow.html"><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 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| <h1 id="incremental-compilation"><a class="header" href="#incremental-compilation">Incremental compilation</a></h1> |
| <ul> |
| <li><a href="#the-basic-algorithm">The basic algorithm</a> |
| <ul> |
| <li><a href="#the-try-mark-green-algorithm">The try-mark-green algorithm</a></li> |
| <li><a href="#the-query-dag">The query DAG</a></li> |
| </ul> |
| </li> |
| <li><a href="#improvements-to-the-basic-algorithm">Improvements to the basic algorithm</a></li> |
| <li><a href="#resources">Resources</a></li> |
| <li><a href="#footnotes">Footnotes</a></li> |
| </ul> |
| <p>The incremental compilation scheme is, in essence, a surprisingly |
| simple extension to the overall query system. We'll start by describing |
| a slightly simplified variant of the real thing – the "basic algorithm" – |
| and then describe some possible improvements.</p> |
| <h2 id="the-basic-algorithm"><a class="header" href="#the-basic-algorithm">The basic algorithm</a></h2> |
| <p>The basic algorithm is |
| called the <strong>red-green</strong> algorithm<sup class="footnote-reference"><a href="#salsa">1</a></sup>. The high-level idea is |
| that, after each run of the compiler, we will save the results of all |
| the queries that we do, as well as the <strong>query DAG</strong>. The |
| <strong>query DAG</strong> is a <a href="https://en.wikipedia.org/wiki/Directed_acyclic_graph">DAG</a> that indexes which queries executed which |
| other queries. So, for example, there would be an <a href="https://en.wikipedia.org/wiki/Glossary_of_graph_theory_terms#edge">edge</a> from a query Q1 |
| to another query Q2 if computing Q1 required computing Q2 (note that |
| because queries cannot depend on themselves, this results in a DAG and |
| not a general graph).</p> |
| <p>On the next run of the compiler, then, we can sometimes reuse these |
| query results to avoid re-executing a query. We do this by assigning |
| every query a <strong>color</strong>:</p> |
| <ul> |
| <li>If a query is colored <strong>red</strong>, that means that its result during |
| this compilation has <strong>changed</strong> from the previous compilation.</li> |
| <li>If a query is colored <strong>green</strong>, that means that its result is |
| the <strong>same</strong> as the previous compilation.</li> |
| </ul> |
| <p>There are two key insights here:</p> |
| <ul> |
| <li>First, if all the inputs to query Q are colored green, then the |
| query Q <strong>must</strong> result in the same value as last time and hence |
| need not be re-executed (or else the compiler is not deterministic).</li> |
| <li>Second, even if some inputs to a query changes, it may be that it |
| <strong>still</strong> produces the same result as the previous compilation. In |
| particular, the query may only use part of its input. |
| <ul> |
| <li>Therefore, after executing a query, we always check whether it |
| produced the same result as the previous time. <strong>If it did,</strong> we |
| can still mark the query as green, and hence avoid re-executing |
| dependent queries.</li> |
| </ul> |
| </li> |
| </ul> |
| <h3 id="the-try-mark-green-algorithm"><a class="header" href="#the-try-mark-green-algorithm">The try-mark-green algorithm</a></h3> |
| <p>At the core of incremental compilation is an algorithm called |
| "try-mark-green". It has the job of determining the color of a given |
| query Q (which must not have yet been executed). In cases where Q has |
| red inputs, determining Q's color may involve re-executing Q so that |
| we can compare its output, but if all of Q's inputs are green, then we |
| can conclude that Q must be green without re-executing it or inspecting |
| its value at all. In the compiler, this allows us to avoid |
| deserializing the result from disk when we don't need it, and in fact |
| enables us to sometimes skip <em>serializing</em> the result as well |
| (see the refinements section below).</p> |
| <p>Try-mark-green works as follows:</p> |
| <ul> |
| <li>First check if the query Q was executed during the previous compilation. |
| <ul> |
| <li>If not, we can just re-execute the query as normal, and assign it the |
| color of red.</li> |
| </ul> |
| </li> |
| <li>If yes, then load the 'dependent queries' of Q.</li> |
| <li>If there is a saved result, then we load the <code>reads(Q)</code> vector from the |
| query DAG. The "reads" is the set of queries that Q executed during |
| its execution. |
| <ul> |
| <li>For each query R in <code>reads(Q)</code>, we recursively demand the color |
| of R using try-mark-green. |
| <ul> |
| <li>Note: it is important that we visit each node in <code>reads(Q)</code> in same order |
| as they occurred in the original compilation. See <a href="#dag">the section on the |
| query DAG below</a>.</li> |
| <li>If <strong>any</strong> of the nodes in <code>reads(Q)</code> wind up colored <strong>red</strong>, then Q is |
| dirty. |
| <ul> |
| <li>We re-execute Q and compare the hash of its result to the hash of the |
| result from the previous compilation.</li> |
| <li>If the hash has not changed, we can mark Q as <strong>green</strong> and return.</li> |
| </ul> |
| </li> |
| <li>Otherwise, <strong>all</strong> of the nodes in <code>reads(Q)</code> must be <strong>green</strong>. In that |
| case, we can color Q as <strong>green</strong> and return.</li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| <p><a name="dag"></a></p> |
| <h3 id="the-query-dag"><a class="header" href="#the-query-dag">The query DAG</a></h3> |
| <p>The query DAG code is stored in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/dep_graph/index.html"><code>compiler/rustc_middle/src/dep_graph</code></a>. Construction of the DAG is done |
| by instrumenting the query execution.</p> |
| <p>One key point is that the query DAG also tracks ordering; that is, for |
| each query Q, we not only track the queries that Q reads, we track the |
| <strong>order</strong> in which they were read. This allows try-mark-green to walk |
| those queries back in the same order. This is important because once a |
| subquery comes back as red, we can no longer be sure that Q will continue |
| along the same path as before. That is, imagine a query like this:</p> |
| <pre><code class="language-rust ignore">fn main_query(tcx) { |
| if tcx.subquery1() { |
| tcx.subquery2() |
| } else { |
| tcx.subquery3() |
| } |
| } |
| </code></pre> |
| <p>Now imagine that in the first compilation, <code>main_query</code> starts by |
| executing <code>subquery1</code>, and this returns true. In that case, the next |
| query <code>main_query</code> executes will be <code>subquery2</code>, and <code>subquery3</code> will |
| not be executed at all.</p> |
| <p>But now imagine that in the <strong>next</strong> compilation, the input has |
| changed such that <code>subquery1</code> returns <strong>false</strong>. In this case, <code>subquery2</code> |
| would never execute. If try-mark-green were to visit <code>reads(main_query)</code> out |
| of order, however, it might visit <code>subquery2</code> before <code>subquery1</code>, and hence |
| execute it. |
| This can lead to ICEs and other problems in the compiler.</p> |
| <h2 id="improvements-to-the-basic-algorithm"><a class="header" href="#improvements-to-the-basic-algorithm">Improvements to the basic algorithm</a></h2> |
| <p>In the description of the basic algorithm, we said that at the end of |
| compilation we would save the results of all the queries that were |
| performed. In practice, this can be quite wasteful – many of those |
| results are very cheap to recompute, and serializing and deserializing |
| them is not a particular win. In practice, what we would do is to save |
| <strong>the hashes</strong> of all the subqueries that we performed. Then, in select cases, |
| we <strong>also</strong> save the results.</p> |
| <p>This is why the incremental algorithm separates computing the |
| <strong>color</strong> of a node, which often does not require its value, from |
| computing the <strong>result</strong> of a node. Computing the result is done via a simple |
| algorithm like so:</p> |
| <ul> |
| <li>Check if a saved result for Q is available. If so, compute the color of Q. |
| If Q is green, deserialize and return the saved result.</li> |
| <li>Otherwise, execute Q. |
| <ul> |
| <li>We can then compare the hash of the result and color Q as green if |
| it did not change.</li> |
| </ul> |
| </li> |
| </ul> |
| <h2 id="resources"><a class="header" href="#resources">Resources</a></h2> |
| <p>The initial design document can be found <a href="https://github.com/nikomatsakis/rustc-on-demand-incremental-design-doc/blob/master/0000-rustc-on-demand-and-incremental.md">here</a>, which expands |
| on the memoization details, provides more high-level overview and motivation |
| for this system.</p> |
| <h1 id="footnotes"><a class="header" href="#footnotes">Footnotes</a></h1> |
| <div class="footnote-definition" id="salsa"><sup class="footnote-definition-label">1</sup> |
| <p>I have long wanted to rename it to the Salsa algorithm, but it never caught on. -@nikomatsakis</p> |
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