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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 expanded "><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 expanded "><a href="miri.html" class="active"><strong aria-hidden="true">50.1.</strong> miri const evaluator</a></li></ol></li><li class="chapter-item "><a href="backend/monomorph.html"><strong aria-hidden="true">51.</strong> Monomorphization</a></li><li class="chapter-item "><a href="backend/lowering-mir.html"><strong aria-hidden="true">52.</strong> Lowering MIR</a></li><li class="chapter-item "><a href="backend/codegen.html"><strong aria-hidden="true">53.</strong> Code Generation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="backend/updating-llvm.html"><strong aria-hidden="true">53.1.</strong> Updating LLVM</a></li><li class="chapter-item "><a href="backend/debugging.html"><strong aria-hidden="true">53.2.</strong> Debugging LLVM</a></li><li class="chapter-item "><a href="backend/backend-agnostic.html"><strong aria-hidden="true">53.3.</strong> Backend Agnostic Codegen</a></li><li class="chapter-item "><a href="backend/implicit-caller-location.html"><strong aria-hidden="true">53.4.</strong> Implicit Caller Location</a></li></ol></li><li class="chapter-item "><a href="backend/libs-and-metadata.html"><strong aria-hidden="true">54.</strong> Libraries and Metadata</a></li><li class="chapter-item "><a href="profile-guided-optimization.html"><strong aria-hidden="true">55.</strong> Profile-guided Optimization</a></li><li class="chapter-item "><a href="llvm-coverage-instrumentation.html"><strong aria-hidden="true">56.</strong> LLVM Source-Based Code Coverage</a></li><li class="chapter-item "><a href="sanitizers.html"><strong aria-hidden="true">57.</strong> Sanitizers Support</a></li><li class="chapter-item "><a href="debugging-support-in-rustc.html"><strong aria-hidden="true">58.</strong> Debugging Support in the Rust Compiler</a></li><li class="spacer"></li><li class="chapter-item affix "><a href="appendix/background.html">Appendix A: Background topics</a></li><li class="chapter-item affix "><a href="appendix/glossary.html">Appendix B: Glossary</a></li><li class="chapter-item affix "><a href="appendix/code-index.html">Appendix C: Code Index</a></li><li class="chapter-item affix "><a href="appendix/compiler-lecture.html">Appendix D: Compiler Lecture Series</a></li><li class="chapter-item affix "><a href="appendix/bibliography.html">Appendix E: Bibliography</a></li><li class="chapter-item affix "><a href="appendix/humorust.html">Appendix Z: HumorRust</a></li><li class="spacer"></li></ol> |
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| <main> |
| <h1 id="miri"><a class="header" href="#miri">Miri</a></h1> |
| <ul> |
| <li><a href="#datastructures">Datastructures</a></li> |
| <li><a href="#memory">Memory</a> |
| <ul> |
| <li><a href="#global-memory-and-exotic-allocations">Global memory and exotic allocations</a></li> |
| <li><a href="#pointer-values-vs-pointer-types">Pointer values vs Pointer types</a></li> |
| </ul> |
| </li> |
| <li><a href="#interpretation">Interpretation</a></li> |
| </ul> |
| <p>The Miri (<strong>MIR</strong> <strong>I</strong>nterpreter) engine is a virtual machine for executing MIR without |
| compiling to machine code. It is usually invoked via <code>tcx.const_eval_*</code> functions. |
| In the following, we will refer to the Miri engine as just "Miri", but note that |
| there also is a stand-alone |
| <a href="https://github.com/rust-lang/miri/">tool called "Miri"</a> that is based on the |
| engine (sometimes referred to as Miri-the-tool to disambiguate it from the |
| engine).</p> |
| <p>If you start out with a constant:</p> |
| <pre><pre class="playground"><code class="language-rust"> |
| <span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>const FOO: usize = 1 << 12; |
| <span class="boring">} |
| </span></code></pre></pre> |
| <p>rustc doesn't actually invoke anything until the constant is either used or |
| placed into metadata.</p> |
| <p>Once you have a use-site like:</p> |
| <pre><code class="language-rust ignore">type Foo = [u8; FOO - 42]; |
| </code></pre> |
| <p>The compiler needs to figure out the length of the array before being able to |
| create items that use the type (locals, constants, function arguments, ...).</p> |
| <p>To obtain the (in this case empty) parameter environment, one can call |
| <code>let param_env = tcx.param_env(length_def_id);</code>. The <code>GlobalId</code> needed is</p> |
| <pre><code class="language-rust ignore">let gid = GlobalId { |
| promoted: None, |
| instance: Instance::mono(length_def_id), |
| }; |
| </code></pre> |
| <p>Invoking <code>tcx.const_eval(param_env.and(gid))</code> will now trigger the creation of |
| the MIR of the array length expression. The MIR will look something like this:</p> |
| <pre><code class="language-mir">Foo::{{constant}}#0: usize = { |
| let mut _0: usize; |
| let mut _1: (usize, bool); |
| |
| bb0: { |
| _1 = CheckedSub(const FOO, const 42usize); |
| assert(!move (_1.1: bool), "attempt to subtract with overflow") -> bb1; |
| } |
| |
| bb1: { |
| _0 = move (_1.0: usize); |
| return; |
| } |
| } |
| </code></pre> |
| <p>Before the evaluation, a virtual memory location (in this case essentially a |
| <code>vec![u8; 4]</code> or <code>vec![u8; 8]</code>) is created for storing the evaluation result.</p> |
| <p>At the start of the evaluation, <code>_0</code> and <code>_1</code> are |
| <code>Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Undef))</code>. This is quite |
| a mouthful: <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/enum.Operand.html"><code>Operand</code></a> can represent either data stored somewhere in the |
| <a href="#memory">interpreter memory</a> (<code>Operand::Indirect</code>), or (as an optimization) |
| immediate data stored in-line. And <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/enum.Immediate.html"><code>Immediate</code></a> can either be a single |
| (potentially uninitialized) <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/enum.Scalar.html">scalar value</a> (integer or thin pointer), |
| or a pair of two of them. In our case, the single scalar value is <em>not</em> (yet) |
| initialized.</p> |
| <p>When the initialization of <code>_1</code> is invoked, the value of the <code>FOO</code> constant is |
| required, and triggers another call to <code>tcx.const_eval_*</code>, which will not be shown |
| here. If the evaluation of FOO is successful, <code>42</code> will be subtracted from its |
| value <code>4096</code> and the result stored in <code>_1</code> as |
| <code>Operand::Immediate(Immediate::ScalarPair(Scalar::Raw { data: 4054, .. }, Scalar::Raw { data: 0, .. })</code>. The first part of the pair is the computed value, |
| the second part is a bool that's true if an overflow happened. A <code>Scalar::Raw</code> |
| also stores the size (in bytes) of this scalar value; we are eliding that here.</p> |
| <p>The next statement asserts that said boolean is <code>0</code>. In case the assertion |
| fails, its error message is used for reporting a compile-time error.</p> |
| <p>Since it does not fail, <code>Operand::Immediate(Immediate::Scalar(Scalar::Raw { data: 4054, .. }))</code> is stored in the virtual memory was allocated before the |
| evaluation. <code>_0</code> always refers to that location directly.</p> |
| <p>After the evaluation is done, the return value is converted from <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/enum.Operand.html"><code>Operand</code></a> to |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/enum.ConstValue.html"><code>ConstValue</code></a> by <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/const_eval/eval_queries/fn.op_to_const.html"><code>op_to_const</code></a>: the former representation is geared towards |
| what is needed <em>during</em> cost evaluation, while <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/enum.ConstValue.html"><code>ConstValue</code></a> is shaped by the |
| needs of the remaining parts of the compiler that consume the results of const |
| evaluation. As part of this conversion, for types with scalar values, even if |
| the resulting <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/enum.Operand.html"><code>Operand</code></a> is <code>Indirect</code>, it will return an immediate |
| <code>ConstValue::Scalar(computed_value)</code> (instead of the usual <code>ConstValue::ByRef</code>). |
| This makes using the result much more efficient and also more convenient, as no |
| further queries need to be executed in order to get at something as simple as a |
| <code>usize</code>.</p> |
| <p>Future evaluations of the same constants will not actually invoke |
| Miri, but just use the cached result.</p> |
| <h2 id="datastructures"><a class="header" href="#datastructures">Datastructures</a></h2> |
| <p>Miri's outside-facing datastructures can be found in |
| <a href="https://github.com/rust-lang/rust/blob/master/compiler/rustc_middle/src/mir/interpret">rustc_middle/src/mir/interpret</a>. |
| This is mainly the error enum and the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/enum.ConstValue.html"><code>ConstValue</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/enum.Scalar.html"><code>Scalar</code></a> types. A |
| <code>ConstValue</code> can be either <code>Scalar</code> (a single <code>Scalar</code>, i.e., integer or thin |
| pointer), <code>Slice</code> (to represent byte slices and strings, as needed for pattern |
| matching) or <code>ByRef</code>, which is used for anything else and refers to a virtual |
| allocation. These allocations can be accessed via the methods on |
| <code>tcx.interpret_interner</code>. A <code>Scalar</code> is either some <code>Raw</code> integer or a pointer; |
| see <a href="#memory">the next section</a> for more on that.</p> |
| <p>If you are expecting a numeric result, you can use <code>eval_usize</code> (panics on |
| anything that can't be represented as a <code>u64</code>) or <code>try_eval_usize</code> which results |
| in an <code>Option<u64></code> yielding the <code>Scalar</code> if possible.</p> |
| <h2 id="memory"><a class="header" href="#memory">Memory</a></h2> |
| <p>To support any kind of pointers, Miri needs to have a "virtual memory" that the |
| pointers can point to. This is implemented in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.Memory.html"><code>Memory</code></a> type. In the |
| simplest model, every global variable, stack variable and every dynamic |
| allocation corresponds to an <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/struct.Allocation.html"><code>Allocation</code></a> in that memory. (Actually using an |
| allocation for every MIR stack variable would be very inefficient; that's why we |
| have <code>Operand::Immediate</code> for stack variables that are both small and never have |
| their address taken. But that is purely an optimization.)</p> |
| <p>Such an <code>Allocation</code> is basically just a sequence of <code>u8</code> storing the value of |
| each byte in this allocation. (Plus some extra data, see below.) Every |
| <code>Allocation</code> has a globally unique <code>AllocId</code> assigned in <code>Memory</code>. With that, a |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/struct.Pointer.html"><code>Pointer</code></a> consists of a pair of an <code>AllocId</code> (indicating the allocation) and |
| an offset into the allocation (indicating which byte of the allocation the |
| pointer points to). It may seem odd that a <code>Pointer</code> is not just an integer |
| address, but remember that during const evaluation, we cannot know at which |
| actual integer address the allocation will end up -- so we use <code>AllocId</code> as |
| symbolic base addresses, which means we need a separate offset. (As an aside, |
| it turns out that pointers at run-time are |
| <a href="https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#pointer-provenance">more than just integers, too</a>.)</p> |
| <p>These allocations exist so that references and raw pointers have something to |
| point to. There is no global linear heap in which things are allocated, but each |
| allocation (be it for a local variable, a static or a (future) heap allocation) |
| gets its own little memory with exactly the required size. So if you have a |
| pointer to an allocation for a local variable <code>a</code>, there is no possible (no |
| matter how unsafe) operation that you can do that would ever change said pointer |
| to a pointer to a different local variable <code>b</code>. |
| Pointer arithmetic on <code>a</code> will only ever change its offset; the <code>AllocId</code> stays the same.</p> |
| <p>This, however, causes a problem when we want to store a <code>Pointer</code> into an |
| <code>Allocation</code>: we cannot turn it into a sequence of <code>u8</code> of the right length! |
| <code>AllocId</code> and offset together are twice as big as a pointer "seems" to be. This |
| is what the <code>relocation</code> field of <code>Allocation</code> is for: the byte offset of the |
| <code>Pointer</code> gets stored as a bunch of <code>u8</code>, while its <code>AllocId</code> gets stored |
| out-of-band. The two are reassembled when the <code>Pointer</code> is read from memory. |
| The other bit of extra data an <code>Allocation</code> needs is <code>undef_mask</code> for keeping |
| track of which of its bytes are initialized.</p> |
| <h3 id="global-memory-and-exotic-allocations"><a class="header" href="#global-memory-and-exotic-allocations">Global memory and exotic allocations</a></h3> |
| <p><code>Memory</code> exists only during the Miri evaluation; it gets destroyed when the |
| final value of the constant is computed. In case that constant contains any |
| pointers, those get "interned" and moved to a global "const eval memory" that is |
| part of <code>TyCtxt</code>. These allocations stay around for the remaining computation |
| and get serialized into the final output (so that dependent crates can use |
| them).</p> |
| <p>Moreover, to also support function pointers, the global memory in <code>TyCtxt</code> can |
| also contain "virtual allocations": instead of an <code>Allocation</code>, these contain an |
| <code>Instance</code>. That allows a <code>Pointer</code> to point to either normal data or a |
| function, which is needed to be able to evaluate casts from function pointers to |
| raw pointers.</p> |
| <p>Finally, the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/enum.GlobalAlloc.html"><code>GlobalAlloc</code></a> type used in the global memory also contains a |
| variant <code>Static</code> that points to a particular <code>const</code> or <code>static</code> item. This is |
| needed to support circular statics, where we need to have a <code>Pointer</code> to a |
| <code>static</code> for which we cannot yet have an <code>Allocation</code> as we do not know the |
| bytes of its value.</p> |
| <h3 id="pointer-values-vs-pointer-types"><a class="header" href="#pointer-values-vs-pointer-types">Pointer values vs Pointer types</a></h3> |
| <p>One common cause of confusion in Miri is that being a pointer <em>value</em> and having |
| a pointer <em>type</em> are entirely independent properties. By "pointer value", we |
| refer to a <code>Scalar::Ptr</code> containing a <code>Pointer</code> and thus pointing somewhere into |
| Miri's virtual memory. This is in contrast to <code>Scalar::Raw</code>, which is just some |
| concrete integer.</p> |
| <p>However, a variable of pointer or reference <em>type</em>, such as <code>*const T</code> or <code>&T</code>, |
| does not have to have a pointer <em>value</em>: it could be obtained by casting or |
| transmuting an integer to a pointer. |
| And similarly, when casting or transmuting a reference to some |
| actual allocation to an integer, we end up with a pointer <em>value</em> |
| (<code>Scalar::Ptr</code>) at integer <em>type</em> (<code>usize</code>). This is a problem because we |
| cannot meaningfully perform integer operations such as division on pointer |
| values.</p> |
| <h2 id="interpretation"><a class="header" href="#interpretation">Interpretation</a></h2> |
| <p>Although the main entry point to constant evaluation is the <code>tcx.const_eval_*</code> |
| functions, there are additional functions in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/index.html">rustc_const_eval/src/const_eval</a> |
| that allow accessing the fields of a <code>ConstValue</code> (<code>ByRef</code> or otherwise). You should |
| never have to access an <code>Allocation</code> directly except for translating it to the |
| compilation target (at the moment just LLVM).</p> |
| <p>Miri starts by creating a virtual stack frame for the current constant that is |
| being evaluated. There's essentially no difference between a constant and a |
| function with no arguments, except that constants do not allow local (named) |
| variables at the time of writing this guide.</p> |
| <p>A stack frame is defined by the <code>Frame</code> type in |
| <a href="https://github.com/rust-lang/rust/blob/master/compiler/rustc_const_eval/src/interpret/eval_context.rs">rustc_const_eval/src/interpret/eval_context.rs</a> |
| and contains all the local |
| variables memory (<code>None</code> at the start of evaluation). Each frame refers to the |
| evaluation of either the root constant or subsequent calls to <code>const fn</code>. The |
| evaluation of another constant simply calls <code>tcx.const_eval_*</code>, which produce an |
| entirely new and independent stack frame.</p> |
| <p>The frames are just a <code>Vec<Frame></code>, there's no way to actually refer to a |
| <code>Frame</code>'s memory even if horrible shenanigans are done via unsafe code. The only |
| memory that can be referred to are <code>Allocation</code>s.</p> |
| <p>Miri now calls the <code>step</code> method (in |
| <a href="https://github.com/rust-lang/rust/blob/master/compiler/rustc_const_eval/src/interpret/step.rs">rustc_const_eval/src/interpret/step.rs</a> |
| ) until it either returns an error or has no further statements to execute. Each |
| statement will now initialize or modify the locals or the virtual memory |
| referred to by a local. This might require evaluating other constants or |
| statics, which just recursively invokes <code>tcx.const_eval_*</code>.</p> |
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