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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 "><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 expanded "><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 expanded "><a href="../backend/debugging.html" class="active"><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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| <h2 id="debugging-llvm"><a class="header" href="#debugging-llvm">Debugging LLVM</a></h2> |
| <blockquote> |
| <p>NOTE: If you are looking for info about code generation, please see <a href="./codegen.html">this |
| chapter</a> instead.</p> |
| </blockquote> |
| <p>This section is about debugging compiler bugs in code generation (e.g. why the |
| compiler generated some piece of code or crashed in LLVM). LLVM is a big |
| project on its own that probably needs to have its own debugging document (not |
| that I could find one). But here are some tips that are important in a rustc |
| context:</p> |
| <p>As a general rule, compilers generate lots of information from analyzing code. |
| Thus, a useful first step is usually to find a minimal example. One way to do |
| this is to</p> |
| <ol> |
| <li> |
| <p>create a new crate that reproduces the issue (e.g. adding whatever crate is |
| at fault as a dependency, and using it from there)</p> |
| </li> |
| <li> |
| <p>minimize the crate by removing external dependencies; that is, moving |
| everything relevant to the new crate</p> |
| </li> |
| <li> |
| <p>further minimize the issue by making the code shorter (there are tools that |
| help with this like <code>creduce</code>)</p> |
| </li> |
| </ol> |
| <p>The official compilers (including nightlies) have LLVM assertions disabled, |
| which means that LLVM assertion failures can show up as compiler crashes (not |
| ICEs but "real" crashes) and other sorts of weird behavior. If you are |
| encountering these, it is a good idea to try using a compiler with LLVM |
| assertions enabled - either an "alt" nightly or a compiler you build yourself |
| by setting <code>[llvm] assertions=true</code> in your config.toml - and see whether |
| anything turns up.</p> |
| <p>The rustc build process builds the LLVM tools into |
| <code>./build/<host-triple>/llvm/bin</code>. They can be called directly.</p> |
| <p>The default rustc compilation pipeline has multiple codegen units, which is |
| hard to replicate manually and means that LLVM is called multiple times in |
| parallel. If you can get away with it (i.e. if it doesn't make your bug |
| disappear), passing <code>-C codegen-units=1</code> to rustc will make debugging easier.</p> |
| <p>For rustc to generate LLVM IR, you need to pass the <code>--emit=llvm-ir</code> flag. If |
| you are building via cargo, use the <code>RUSTFLAGS</code> environment variable (e.g. |
| <code>RUSTFLAGS='--emit=llvm-ir'</code>). This causes rustc to spit out LLVM IR into the |
| target directory.</p> |
| <p><code>cargo llvm-ir [options] path</code> spits out the LLVM IR for a particular function |
| at <code>path</code>. (<code>cargo install cargo-asm</code> installs <code>cargo asm</code> and <code>cargo llvm-ir</code>). <code>--build-type=debug</code> emits code for debug builds. There are also |
| other useful options. Also, debug info in LLVM IR can clutter the output a lot: |
| <code>RUSTFLAGS="-C debuginfo=0"</code> is really useful.</p> |
| <p><code>RUSTFLAGS="-C save-temps"</code> outputs LLVM bitcode (not the same as IR) at |
| different stages during compilation, which is sometimes useful. One just needs |
| to convert the bitcode files to <code>.ll</code> files using <code>llvm-dis</code> which should be in |
| the target local compilation of rustc.</p> |
| <p>If you are seeing incorrect behavior due to an optimization pass, a very handy |
| LLVM option is <code>-opt-bisect-limit</code>, which takes an integer denoting the index |
| value of the highest pass to run. Index values for taken passes are stable |
| from run to run; by coupling this with software that automates bisecting the |
| search space based on the resulting program, an errant pass can be quickly |
| determined. When an <code>-opt-bisect-limit</code> is specified, all runs are displayed |
| to standard error, along with their index and output indicating if the |
| pass was run or skipped. Setting the limit to an index of -1 (e.g., |
| <code>RUSTFLAGS="-C llvm-args=-opt-bisect-limit=-1"</code>) will show all passes and |
| their corresponding index values.</p> |
| <p>If you want to play with the optimization pipeline, you can use the <code>opt</code> tool |
| from <code>./build/<host-triple>/llvm/bin/</code> with the LLVM IR emitted by rustc. Note |
| that rustc emits different IR depending on whether <code>-O</code> is enabled, even |
| without LLVM's optimizations, so if you want to play with the IR rustc emits, |
| you should:</p> |
| <pre><code class="language-bash">$ rustc +local my-file.rs --emit=llvm-ir -O -C no-prepopulate-passes \ |
| -C codegen-units=1 |
| $ OPT=./build/$TRIPLE/llvm/bin/opt |
| $ $OPT -S -O2 < my-file.ll > my |
| </code></pre> |
| <p>If you just want to get the LLVM IR during the LLVM pipeline, to e.g. see which |
| IR causes an optimization-time assertion to fail, or to see when LLVM performs |
| a particular optimization, you can pass the rustc flag <code>-C llvm-args=-print-after-all</code>, and possibly add <code>-C llvm-args='-filter-print-funcs=EXACT_FUNCTION_NAME</code> (e.g. <code>-C llvm-args='-filter-print-funcs=_ZN11collections3str21_$LT$impl$u20$str$GT$\ 7replace17hbe10ea2e7c809b0bE'</code>).</p> |
| <p>That produces a lot of output into standard error, so you'll want to pipe that |
| to some file. Also, if you are using neither <code>-filter-print-funcs</code> nor <code>-C codegen-units=1</code>, then, because the multiple codegen units run in parallel, the |
| printouts will mix together and you won't be able to read anything.</p> |
| <p>If you want just the IR for a specific function (say, you want to see why it |
| causes an assertion or doesn't optimize correctly), you can use <code>llvm-extract</code>, |
| e.g.</p> |
| <pre><code class="language-bash">$ ./build/$TRIPLE/llvm/bin/llvm-extract \ |
| -func='_ZN11collections3str21_$LT$impl$u20$str$GT$7replace17hbe10ea2e7c809b0bE' \ |
| -S \ |
| < unextracted.ll \ |
| > extracted.ll |
| </code></pre> |
| <h3 id="getting-help-and-asking-questions"><a class="header" href="#getting-help-and-asking-questions">Getting help and asking questions</a></h3> |
| <p>If you have some questions, head over to the <a href="https://rust-lang.zulipchat.com/">rust-lang Zulip</a> and |
| specifically the <code>#t-compiler/wg-llvm</code> stream.</p> |
| <h3 id="compiler-options-to-know-and-love"><a class="header" href="#compiler-options-to-know-and-love">Compiler options to know and love</a></h3> |
| <p>The <code>-C help</code> and <code>-Z help</code> compiler switches will list out a variety |
| of interesting options you may find useful. Here are a few of the most |
| common that pertain to LLVM development (some of them are employed in the |
| tutorial above):</p> |
| <ul> |
| <li>The <code>--emit llvm-ir</code> option emits a <code><filename>.ll</code> file with LLVM IR in textual format |
| <ul> |
| <li>The <code>--emit llvm-bc</code> option emits in bytecode format (<code><filename>.bc</code>)</li> |
| </ul> |
| </li> |
| <li>Passing <code>-C llvm-args=<foo></code> allows passing pretty much all the |
| options that tools like llc and opt would accept; |
| e.g. <code>-C llvm-args=-print-before-all</code> to print IR before every LLVM |
| pass.</li> |
| <li>The <code>-C no-prepopulate-passes</code> will avoid pre-populate the LLVM pass |
| manager with a list of passes. This will allow you to view the LLVM |
| IR that rustc generates, not the LLVM IR after optimizations.</li> |
| <li>The <code>-C passes=val</code> option allows you to supply a space separated list of extra LLVM passes to run</li> |
| <li>The <code>-C save-temps</code> option saves all temporary output files during compilation</li> |
| <li>The <code>-Z print-llvm-passes</code> option will print out LLVM optimization passes being run</li> |
| <li>The <code>-Z time-llvm-passes</code> option measures the time of each LLVM pass</li> |
| <li>The <code>-Z verify-llvm-ir</code> option will verify the LLVM IR for correctness</li> |
| <li>The <code>-Z no-parallel-llvm</code> will disable parallel compilation of distinct compilation units</li> |
| <li>The <code>-Z llvm-time-trace</code> option will output a Chrome profiler compatible JSON file |
| which contains details and timings for LLVM passes.</li> |
| <li>The <code>-C llvm-args=-opt-bisect-limit=<index></code> option allows for bisecting LLVM |
| optimizations.</li> |
| </ul> |
| <h3 id="filing-llvm-bug-reports"><a class="header" href="#filing-llvm-bug-reports">Filing LLVM bug reports</a></h3> |
| <p>When filing an LLVM bug report, you will probably want some sort of minimal |
| working example that demonstrates the problem. The Godbolt compiler explorer is |
| really helpful for this.</p> |
| <ol> |
| <li> |
| <p>Once you have some LLVM IR for the problematic code (see above), you can |
| create a minimal working example with Godbolt. Go to |
| <a href="https://gcc.godbolt.org">gcc.godbolt.org</a>.</p> |
| </li> |
| <li> |
| <p>Choose <code>LLVM-IR</code> as programming language.</p> |
| </li> |
| <li> |
| <p>Use <code>llc</code> to compile the IR to a particular target as is:</p> |
| <ul> |
| <li>There are some useful flags: <code>-mattr</code> enables target features, <code>-march=</code> |
| selects the target, <code>-mcpu=</code> selects the CPU, etc.</li> |
| <li>Commands like <code>llc -march=help</code> output all architectures available, which |
| is useful because sometimes the Rust arch names and the LLVM names do not |
| match.</li> |
| <li>If you have compiled rustc yourself somewhere, in the target directory |
| you have binaries for <code>llc</code>, <code>opt</code>, etc.</li> |
| </ul> |
| </li> |
| <li> |
| <p>If you want to optimize the LLVM-IR, you can use <code>opt</code> to see how the LLVM |
| optimizations transform it.</p> |
| </li> |
| <li> |
| <p>Once you have a godbolt link demonstrating the issue, it is pretty easy to |
| fill in an LLVM bug. Just visit <a href="https://bugs.llvm.org/">bugs.llvm.org</a>.</p> |
| </li> |
| </ol> |
| <h3 id="porting-bug-fixes-from-llvm"><a class="header" href="#porting-bug-fixes-from-llvm">Porting bug fixes from LLVM</a></h3> |
| <p>Once you've identified the bug as an LLVM bug, you will sometimes |
| find that it has already been reported and fixed in LLVM, but we haven't |
| gotten the fix yet (or perhaps you are familiar enough with LLVM to fix it yourself).</p> |
| <p>In that case, we can sometimes opt to port the fix for the bug |
| directly to our own LLVM fork, so that rustc can use it more easily. |
| Our fork of LLVM is maintained in <a href="https://github.com/rust-lang/llvm-project/">rust-lang/llvm-project</a>. Once |
| you've landed the fix there, you'll also need to land a PR modifying |
| our submodule commits -- ask around on Zulip for help.</p> |
| |
| </main> |
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