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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 "><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 expanded "><a href="debugging-support-in-rustc.html" class="active"><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="debugging-support-in-the-rust-compiler"><a class="header" href="#debugging-support-in-the-rust-compiler">Debugging support in the Rust compiler</a></h1> |
| <ul> |
| <li><a href="#preliminaries">Preliminaries</a> |
| <ul> |
| <li><a href="#debuggers">Debuggers</a></li> |
| <li><a href="#dwarf">DWARF</a></li> |
| </ul> |
| </li> |
| <li><a href="#supported-debuggers">Supported debuggers</a> |
| <ul> |
| <li><a href="#gdb">GDB</a> |
| <ul> |
| <li><a href="#rust-expression-parser">Rust expression parser</a></li> |
| <li><a href="#parser-extensions">Parser extensions</a></li> |
| <li><a href="#developer-notes">Developer notes</a></li> |
| </ul> |
| </li> |
| <li><a href="#lldb">LLDB</a> |
| <ul> |
| <li><a href="#rust-expression-parser-1">Rust expression parser</a></li> |
| <li><a href="#parser-extensions-1">Parser extensions</a></li> |
| <li><a href="#developer-notes-1">Developer notes</a></li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| <li><a href="#dwarf-and-rustc">DWARF and Rustc</a> |
| <ul> |
| <li><a href="#current-limitations-of-dwarf">Current limitations of DWARF</a></li> |
| </ul> |
| </li> |
| <li><a href="#developer-notes-2">Developer notes</a></li> |
| <li><a href="#what-is-missing">What is missing</a> |
| <ul> |
| <li><a href="#shipping-gdb-in-rustup">Shipping GDB in Rustup</a></li> |
| <li><a href="#code-signing-for-lldb-debug-server-on-macos">Code signing for LLDB debug server on macOS</a></li> |
| <li><a href="#dwarf-and-traits">DWARF and Traits</a></li> |
| </ul> |
| </li> |
| <li><a href="#typical-process-for-a-debug-info-change-llvm">Typical process for a Debug Info change (LLVM)</a> |
| <ul> |
| <li><a href="#procedural-macro-stepping">Procedural macro stepping</a></li> |
| </ul> |
| </li> |
| <li><a href="#source-file-checksums-in-debug-info">Source file checksums in debug info</a> |
| <ul> |
| <li><a href="#dwarf-5">DWARF 5</a></li> |
| <li><a href="#llvm">LLVM</a></li> |
| <li><a href="#microsoft-visual-c-compiler-zh-option">Microsoft Visual C++ Compiler /ZH option</a></li> |
| <li><a href="#clang">Clang</a></li> |
| </ul> |
| </li> |
| <li><a href="#future-work">Future work</a> |
| <ul> |
| <li><a href="#name-mangling-changes">Name mangling changes</a></li> |
| <li><a href="#reuse-rust-compiler-for-expressions">Reuse Rust compiler for expressions</a></li> |
| </ul> |
| </li> |
| </ul> |
| <p>This document explains the state of debugging tools support in the Rust compiler (rustc). |
| The document gives an overview of debugging tools like GDB, LLDB etc. and infrastructure |
| around Rust compiler to debug Rust code. If you want to learn how to debug the Rust compiler |
| itself, then you must see <a href="compiler-debugging.html">Debugging the Compiler</a> page.</p> |
| <p>The material is gathered from YouTube video <a href="https://www.youtube.com/watch?v=elBxMRSNYr4">Tom Tromey discusses debugging support in rustc</a>.</p> |
| <h2 id="preliminaries"><a class="header" href="#preliminaries">Preliminaries</a></h2> |
| <h3 id="debuggers"><a class="header" href="#debuggers">Debuggers</a></h3> |
| <p>According to Wikipedia</p> |
| <blockquote> |
| <p>A <a href="https://en.wikipedia.org/wiki/Debugger">debugger or debugging tool</a> is a computer program that is used to test and debug |
| other programs (the "target" program).</p> |
| </blockquote> |
| <p>Writing a debugger from scratch for a language requires a lot of work, especially if |
| debuggers have to be supported on various platforms. GDB and LLDB, however, can be |
| extended to support debugging a language. This is the path that Rust has chosen. |
| This document's main goal is to document the said debuggers support in Rust compiler.</p> |
| <h3 id="dwarf"><a class="header" href="#dwarf">DWARF</a></h3> |
| <p>According to the <a href="http://dwarfstd.org">DWARF</a> standard website</p> |
| <blockquote> |
| <p>DWARF is a debugging file format used by many compilers and debuggers to support source level |
| debugging. It addresses the requirements of a number of procedural languages, |
| such as C, C++, and Fortran, and is designed to be extensible to other languages. |
| DWARF is architecture independent and applicable to any processor or operating system. |
| It is widely used on Unix, Linux and other operating systems, |
| as well as in stand-alone environments.</p> |
| </blockquote> |
| <p>DWARF reader is a program that consumes the DWARF format and creates debugger compatible output. |
| This program may live in the compiler itself. DWARF uses a data structure called |
| Debugging Information Entry (DIE) which stores the information as "tags" to denote functions, |
| variables etc., e.g., <code>DW_TAG_variable</code>, <code>DW_TAG_pointer_type</code>, <code>DW_TAG_subprogram</code> etc. |
| You can also invent your own tags and attributes.</p> |
| <h2 id="supported-debuggers"><a class="header" href="#supported-debuggers">Supported debuggers</a></h2> |
| <h3 id="gdb"><a class="header" href="#gdb">GDB</a></h3> |
| <p>We have our own fork of GDB - <a href="https://github.com/rust-dev-tools/gdb">https://github.com/rust-dev-tools/gdb</a></p> |
| <h4 id="rust-expression-parser"><a class="header" href="#rust-expression-parser">Rust expression parser</a></h4> |
| <p>To be able to show debug output we need an expression parser. |
| This (GDB) expression parser is written in <a href="https://www.gnu.org/software/bison/">Bison</a> and is only a subset of Rust expressions. |
| This means that this parser can parse only a subset of Rust expressions. |
| GDB parser was written from scratch and has no relation to any other parser. |
| For example, this parser is not related to Rustc's parser.</p> |
| <p>GDB has Rust like value and type output. It can print values and types in a way |
| that look like Rust syntax in the output. Or when you print a type as <a href="https://ftp.gnu.org/old-gnu/Manuals/gdb/html_node/gdb_109.html">ptype</a> in GDB, |
| it also looks like Rust source code. Checkout the documentation in the <a href="https://sourceware.org/gdb/onlinedocs/gdb/Rust.html">manual for GDB/Rust</a>.</p> |
| <h4 id="parser-extensions"><a class="header" href="#parser-extensions">Parser extensions</a></h4> |
| <p>Expression parser has a couple of extensions in it to facilitate features that you cannot do |
| with Rust. Some limitations are listed in the <a href="https://sourceware.org/gdb/onlinedocs/gdb/Rust.html">manual for GDB/Rust</a>. There is some special |
| code in the DWARF reader in GDB to support the extensions.</p> |
| <p>A couple of examples of DWARF reader support needed are as follows -</p> |
| <ol> |
| <li> |
| <p>Enum: Needed for support for enum types. The Rustc writes the information about enum into |
| DWARF and GDB reads the DWARF to understand where is the tag field or is there a tag |
| field or is the tag slot shared with non-zero optimization etc.</p> |
| </li> |
| <li> |
| <p>Dissect trait objects: DWARF extension where the trait object's description in the DWARF |
| also points to a stub description of the corresponding vtable which in turn points to the |
| concrete type for which this trait object exists. This means that you can do a <code>print *object</code> |
| for that trait object, and GDB will understand how to find the correct type of the payload in |
| the trait object.</p> |
| </li> |
| </ol> |
| <p><strong>TODO</strong>: Figure out if the following should be mentioned in the GDB-Rust document rather than |
| this guide page so there is no duplication. This is regarding the following comments:</p> |
| <p><a href="https://github.com/rust-lang/rustc-dev-guide/pull/316#discussion_r284027340">This comment by Tom</a></p> |
| <blockquote> |
| <p>gdb's Rust extensions and limitations are documented in the gdb manual: |
| https://sourceware.org/gdb/onlinedocs/gdb/Rust.html -- however, this neglects to mention that |
| gdb convenience variables and registers follow the gdb $ convention, and that the Rust parser |
| implements the gdb @ extension.</p> |
| </blockquote> |
| <p><a href="https://github.com/rust-lang/rustc-dev-guide/pull/316#discussion_r285401353">This question by Aman</a></p> |
| <blockquote> |
| <p>@tromey do you think we should mention this part in the GDB-Rust document rather than this |
| document so there is no duplication etc.?</p> |
| </blockquote> |
| <h4 id="developer-notes"><a class="header" href="#developer-notes">Developer notes</a></h4> |
| <ul> |
| <li>This work is now upstream. Bugs can be reported in <a href="https://sourceware.org/bugzilla/">GDB Bugzilla</a>.</li> |
| </ul> |
| <h3 id="lldb"><a class="header" href="#lldb">LLDB</a></h3> |
| <p>Fork of LLVM project - <a href="https://github.com/rust-lang/llvm-project">https://github.com/rust-lang/llvm-project</a></p> |
| <p>LLDB currently only works on macOS because of a dependency issue. This issue was easier to |
| solve for macOS as compared to Linux. However, Tom has a possible solution which can enable |
| us to ship LLDB everywhere.</p> |
| <h4 id="rust-expression-parser-1"><a class="header" href="#rust-expression-parser-1">Rust expression parser</a></h4> |
| <p>This expression parser is written in C++. It is a type of <a href="https://en.wikipedia.org/wiki/Recursive_descent_parser">Recursive Descent parser</a>. |
| Implements slightly less of the Rust language than GDB. LLDB has Rust like value and type output.</p> |
| <h4 id="parser-extensions-1"><a class="header" href="#parser-extensions-1">Parser extensions</a></h4> |
| <p>There is some special code in the DWARF reader in LLDB to support the extensions. |
| A couple of examples of DWARF reader support needed are as follows -</p> |
| <ol> |
| <li>Enum: Needed for support for enum types. The Rustc writes the information about |
| enum into DWARF and LLDB reads the DWARF to understand where is the tag field or |
| is there a tag field or is the tag slot shared with non-zero optimization etc. |
| In other words, it has enum support as well.</li> |
| </ol> |
| <h4 id="developer-notes-1"><a class="header" href="#developer-notes-1">Developer notes</a></h4> |
| <ul> |
| <li>None of the LLDB work is upstream. This <a href="https://github.com/rust-lang/lldb/wiki">rust-lang/lldb wiki page</a> explains a few details.</li> |
| <li>The reason for forking LLDB is that LLDB recently removed all the other language plugins |
| due to lack of maintenance.</li> |
| <li>LLDB has a plugin architecture but that does not work for language support.</li> |
| <li>LLDB is available via Rust build (<code>rustup</code>).</li> |
| <li>GDB generally works better on Linux.</li> |
| </ul> |
| <h2 id="dwarf-and-rustc"><a class="header" href="#dwarf-and-rustc">DWARF and Rustc</a></h2> |
| <p><a href="http://dwarfstd.org">DWARF</a> is the standard way compilers generate debugging information that debuggers read. |
| It is <em>the</em> debugging format on macOS and Linux. It is a multi-language, extensible format |
| and is mostly good enough for Rust's purposes. Hence, the current implementation reuses DWARF's |
| concepts. This is true even if some of the concepts in DWARF do not align with Rust |
| semantically because generally there can be some kind of mapping between the two.</p> |
| <p>We have some DWARF extensions that the Rust compiler emits and the debuggers understand that |
| are <em>not</em> in the DWARF standard.</p> |
| <ul> |
| <li> |
| <p>Rust compiler will emit DWARF for a virtual table, and this <code>vtable</code> object will have a |
| <code>DW_AT_containing_type</code> that points to the real type. This lets debuggers dissect a trait object |
| pointer to correctly find the payload. E.g., here's such a DIE, from a test case in the gdb |
| repository:</p> |
| <pre><code class="language-asm"><1><1a9>: Abbrev Number: 3 (DW_TAG_structure_type) |
| <1aa> DW_AT_containing_type: <0x1b4> |
| <1ae> DW_AT_name : (indirect string, offset: 0x23d): vtable |
| <1b2> DW_AT_byte_size : 0 |
| <1b3> DW_AT_alignment : 8 |
| </code></pre> |
| </li> |
| <li> |
| <p>The other extension is that the Rust compiler can emit a tagless discriminated union. |
| See <a href="http://dwarfstd.org/ShowIssue.php?issue=180517.2">DWARF feature request</a> for this item.</p> |
| </li> |
| </ul> |
| <h3 id="current-limitations-of-dwarf"><a class="header" href="#current-limitations-of-dwarf">Current limitations of DWARF</a></h3> |
| <ul> |
| <li>Traits - require a bigger change than normal to DWARF, on how to represent Traits in DWARF.</li> |
| <li>DWARF provides no way to differentiate between Structs and Tuples. Rust compiler emits |
| fields with <code>__0</code> and debuggers look for a sequence of such names to overcome this limitation. |
| For example, in this case the debugger would look at a field via <code>x.__0</code> instead of <code>x.0</code>. |
| This is resolved via the Rust parser in the debugger so now you can do <code>x.0</code>.</li> |
| </ul> |
| <p>DWARF relies on debuggers to know some information about platform ABI. |
| Rust does not do that all the time.</p> |
| <h2 id="developer-notes-2"><a class="header" href="#developer-notes-2">Developer notes</a></h2> |
| <p>This section is from the talk about certain aspects of development.</p> |
| <h2 id="what-is-missing"><a class="header" href="#what-is-missing">What is missing</a></h2> |
| <h3 id="shipping-gdb-in-rustup"><a class="header" href="#shipping-gdb-in-rustup">Shipping GDB in Rustup</a></h3> |
| <p>Tracking issue: <a href="https://github.com/rust-lang/rust/issues/34457">https://github.com/rust-lang/rust/issues/34457</a></p> |
| <p>Shipping GDB requires change to Rustup delivery system. To manage Rustup build size and |
| times we need to build GDB separately, on its own and somehow provide the artifacts produced |
| to be included in the final build. However, if we can ship GDB with rustup, it will simplify |
| the development process by having compiler emit new debug info which can be readily consumed.</p> |
| <p>Main issue in achieving this is setting up dependencies. One such dependency is Python. That |
| is why we have our own fork of GDB because one of the drivers is patched on Rust's side to |
| check the correct version of Python (Python 2.7 in this case. <em>Note: Python3 is not chosen |
| for this purpose because Python's stable ABI is limited and is not sufficient for GDB's needs. |
| See <a href="https://docs.python.org/3/c-api/stable.html">https://docs.python.org/3/c-api/stable.html</a></em>).</p> |
| <p>This is to keep updates to debugger as fast as possible as we make changes to the debugging symbols. |
| In essence, to ship the debugger as soon as new debugging info is added. GDB only releases |
| every six months or so. However, the changes that are |
| not related to Rust itself should ideally be first merged to upstream eventually.</p> |
| <h3 id="code-signing-for-lldb-debug-server-on-macos"><a class="header" href="#code-signing-for-lldb-debug-server-on-macos">Code signing for LLDB debug server on macOS</a></h3> |
| <p>According to Wikipedia, <a href="https://en.wikipedia.org/wiki/System_Integrity_Protection">System Integrity Protection</a> is</p> |
| <blockquote> |
| <p>System Integrity Protection (SIP, sometimes referred to as rootless) is a security feature |
| of Apple's macOS operating system introduced in OS X El Capitan. It comprises a number of |
| mechanisms that are enforced by the kernel. A centerpiece is the protection of system-owned |
| files and directories against modifications by processes without a specific "entitlement", |
| even when executed by the root user or a user with root privileges (sudo).</p> |
| </blockquote> |
| <p>It prevents processes using <code>ptrace</code> syscall. If a process wants to use <code>ptrace</code> it has to be |
| code signed. The certificate that signs it has to be trusted on your machine.</p> |
| <p>See <a href="https://developer.apple.com/library/archive/releasenotes/MacOSX/WhatsNewInOSX/Articles/MacOSX10_11.html#//apple_ref/doc/uid/TP40016227-SW11">Apple developer documentation for System Integrity Protection</a>.</p> |
| <p>We may need to sign up with Apple and get the keys to do this signing. Tom has looked into if |
| Mozilla cannot do this because it is at the maximum number of |
| keys it is allowed to sign. Tom does not know if Mozilla could get more keys.</p> |
| <p>Alternatively, Tom suggests that maybe a Rust legal entity is needed to get the keys via Apple. |
| This problem is not technical in nature. If we had such a key we could sign GDB as well and |
| ship that.</p> |
| <h3 id="dwarf-and-traits"><a class="header" href="#dwarf-and-traits">DWARF and Traits</a></h3> |
| <p>Rust traits are not emitted into DWARF at all. The impact of this is calling a method <code>x.method()</code> |
| does not work as is. The reason being that method is implemented by a trait, as opposed |
| to a type. That information is not present so finding trait methods is missing.</p> |
| <p>DWARF has a notion of interface types (possibly added for Java). Tom's idea was to use this |
| interface type as traits.</p> |
| <p>DWARF only deals with concrete names, not the reference types. So, a given implementation of a |
| trait for a type would be one of these interfaces (<code>DW_tag_interface</code> type). Also, the type for |
| which it is implemented would describe all the interfaces this type implements. This requires a |
| DWARF extension.</p> |
| <p>Issue on Github: <a href="https://github.com/rust-lang/rust/issues/33014">https://github.com/rust-lang/rust/issues/33014</a></p> |
| <h2 id="typical-process-for-a-debug-info-change-llvm"><a class="header" href="#typical-process-for-a-debug-info-change-llvm">Typical process for a Debug Info change (LLVM)</a></h2> |
| <p>LLVM has Debug Info (DI) builders. This is the primary thing that Rust calls into. |
| This is why we need to change LLVM first because that is emitted first and not DWARF directly. |
| This is a kind of metadata that you construct and hand-off to LLVM. For the Rustc/LLVM hand-off |
| some LLVM DI builder methods are called to construct representation of a type.</p> |
| <p>The steps of this process are as follows -</p> |
| <ol> |
| <li> |
| <p>LLVM needs changing.</p> |
| <p>LLVM does not emit Interface types at all, so this needs to be implemented in the LLVM first.</p> |
| <p>Get sign off on LLVM maintainers that this is a good idea.</p> |
| </li> |
| <li> |
| <p>Change the DWARF extension.</p> |
| </li> |
| <li> |
| <p>Update the debuggers.</p> |
| <p>Update DWARF readers, expression evaluators.</p> |
| </li> |
| <li> |
| <p>Update Rust compiler.</p> |
| <p>Change it to emit this new information.</p> |
| </li> |
| </ol> |
| <h3 id="procedural-macro-stepping"><a class="header" href="#procedural-macro-stepping">Procedural macro stepping</a></h3> |
| <p>A deeply profound question is that how do you actually debug a procedural macro? |
| What is the location you emit for a macro expansion? Consider some of the following cases -</p> |
| <ul> |
| <li>You can emit location of the invocation of the macro.</li> |
| <li>You can emit the location of the definition of the macro.</li> |
| <li>You can emit locations of the content of the macro.</li> |
| </ul> |
| <p>RFC: <a href="https://github.com/rust-lang/rfcs/pull/2117">https://github.com/rust-lang/rfcs/pull/2117</a></p> |
| <p>Focus is to let macros decide what to do. This can be achieved by having some kind of attribute |
| that lets the macro tell the compiler where the line marker should be. This affects where you |
| set the breakpoints and what happens when you step it.</p> |
| <h2 id="source-file-checksums-in-debug-info"><a class="header" href="#source-file-checksums-in-debug-info">Source file checksums in debug info</a></h2> |
| <p>Both DWARF and CodeView (PDB) support embedding a cryptographic hash of each source file that |
| contributed to the associated binary.</p> |
| <p>The cryptographic hash can be used by a debugger to verify that the source file matches the |
| executable. If the source file does not match, the debugger can provide a warning to the user.</p> |
| <p>The hash can also be used to prove that a given source file has not been modified since it was |
| used to compile an executable. Because MD5 and SHA1 both have demonstrated vulnerabilities, |
| using SHA256 is recommended for this application.</p> |
| <p>The Rust compiler stores the hash for each source file in the corresponding <code>SourceFile</code> in |
| the <code>SourceMap</code>. The hashes of input files to external crates are stored in <code>rlib</code> metadata.</p> |
| <p>A default hashing algorithm is set in the target specification. This allows the target to |
| specify the best hash available, since not all targets support all hash algorithms.</p> |
| <p>The hashing algorithm for a target can also be overridden with the <code>-Z source-file-checksum=</code> |
| command-line option.</p> |
| <h4 id="dwarf-5"><a class="header" href="#dwarf-5">DWARF 5</a></h4> |
| <p>DWARF version 5 supports embedding an MD5 hash to validate the source file version in use. |
| DWARF 5 - Section 6.2.4.1 opcode DW_LNCT_MD5</p> |
| <h4 id="llvm"><a class="header" href="#llvm">LLVM</a></h4> |
| <p>LLVM IR supports MD5 and SHA1 (and SHA256 in LLVM 11+) source file checksums in the DIFile node.</p> |
| <p><a href="https://llvm.org/docs/LangRef.html#difile">LLVM DIFile documentation</a></p> |
| <h4 id="microsoft-visual-c-compiler-zh-option"><a class="header" href="#microsoft-visual-c-compiler-zh-option">Microsoft Visual C++ Compiler /ZH option</a></h4> |
| <p>The MSVC compiler supports embedding MD5, SHA1, or SHA256 hashes in the PDB using the <code>/ZH</code> |
| compiler option.</p> |
| <p><a href="https://docs.microsoft.com/en-us/cpp/build/reference/zh">MSVC /ZH documentation</a></p> |
| <h4 id="clang"><a class="header" href="#clang">Clang</a></h4> |
| <p>Clang always embeds an MD5 checksum, though this does not appear in documentation.</p> |
| <h2 id="future-work"><a class="header" href="#future-work">Future work</a></h2> |
| <h4 id="name-mangling-changes"><a class="header" href="#name-mangling-changes">Name mangling changes</a></h4> |
| <ul> |
| <li>New demangler in <code>libiberty</code> (gcc source tree).</li> |
| <li>New demangler in LLVM or LLDB.</li> |
| </ul> |
| <p><strong>TODO</strong>: Check the location of the demangler source. <a href="https://github.com/rust-lang/rustc-dev-guide/issues/1157">#1157</a></p> |
| <h4 id="reuse-rust-compiler-for-expressions"><a class="header" href="#reuse-rust-compiler-for-expressions">Reuse Rust compiler for expressions</a></h4> |
| <p>This is an important idea because debuggers by and large do not try to implement type |
| inference. You need to be much more explicit when you type into the debugger than your |
| actual source code. So, you cannot just copy and paste an expression from your source |
| code to debugger and expect the same answer but this would be nice. This can be helped |
| by using compiler.</p> |
| <p>It is certainly doable but it is a large project. You certainly need a bridge to the |
| debugger because the debugger alone has access to the memory. Both GDB (gcc) and LLDB (clang) |
| have this feature. LLDB uses Clang to compile code to JIT and GDB can do the same with GCC.</p> |
| <p>Both debuggers expression evaluation implement both a superset and a subset of Rust. |
| They implement just the expression language but they also add some extensions like GDB has |
| convenience variables. Therefore, if you are taking this route then you not only need |
| to do this bridge but may have to add some mode to let the compiler understand some extensions.</p> |
| |
| </main> |
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