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| <h1 id="queries-demand-driven-compilation"><a class="header" href="#queries-demand-driven-compilation">Queries: demand-driven compilation</a></h1> |
| <p>As described in <a href="overview.html#queries">Overview of the compiler</a>, the Rust compiler |
| is still (as of <!-- date-check --> July 2021) transitioning from a |
| traditional "pass-based" setup to a "demand-driven" system. The compiler query |
| system is the key to rustc's demand-driven organization. |
| The idea is pretty simple. Instead of entirely independent passes |
| (parsing, type-checking, etc.), a set of function-like <em>queries</em> |
| compute information about the input source. For example, |
| there is a query called <code>type_of</code> that, given the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/def_id/struct.DefId.html"><code>DefId</code></a> of |
| some item, will compute the type of that item and return it to you.</p> |
| <p>Query execution is <em>memoized</em>. The first time you invoke a |
| query, it will go do the computation, but the next time, the result is |
| returned from a hashtable. Moreover, query execution fits nicely into |
| <em>incremental computation</em>; the idea is roughly that, when you invoke a |
| query, the result <em>may</em> be returned to you by loading stored data |
| from disk.<sup class="footnote-reference" id="fr-incr-comp-detail-1"><a href="#footnote-incr-comp-detail">1</a></sup></p> |
| <p>Eventually, we want the entire compiler |
| control-flow to be query driven. There will effectively be one |
| top-level query (<code>compile</code>) that will run compilation on a crate; this |
| will in turn demand information about that crate, starting from the |
| <em>end</em>. For example:</p> |
| <ul> |
| <li>The <code>compile</code> query might demand to get a list of codegen-units |
| (i.e. modules that need to be compiled by LLVM).</li> |
| <li>But computing the list of codegen-units would invoke some subquery |
| that returns the list of all modules defined in the Rust source.</li> |
| <li>That query in turn would invoke something asking for the HIR.</li> |
| <li>This keeps going further and further back until we wind up doing the |
| actual parsing.</li> |
| </ul> |
| <p>Although this vision is not fully realized, large sections of the |
| compiler (for example, generating <a href="mir/index.html">MIR</a>) currently work exactly like this.</p> |
| <h2 id="invoking-queries"><a class="header" href="#invoking-queries">Invoking queries</a></h2> |
| <p>Invoking a query is simple. The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html"><code>TyCtxt</code></a> ("type context") struct offers a method |
| for each defined query. For example, to invoke the <code>type_of</code> |
| query, you would just do this:</p> |
| <pre><code class="language-rust ignore">let ty = tcx.type_of(some_def_id);</code></pre> |
| <h2 id="how-the-compiler-executes-a-query"><a class="header" href="#how-the-compiler-executes-a-query">How the compiler executes a query</a></h2> |
| <p>So you may be wondering what happens when you invoke a query |
| method. The answer is that, for each query, the compiler maintains a |
| cache – if your query has already been executed, then, the answer is |
| simple: we clone the return value out of the cache and return it |
| (therefore, you should try to ensure that the return types of queries |
| are cheaply cloneable; insert an <code>Rc</code> if necessary).</p> |
| <h3 id="providers"><a class="header" href="#providers">Providers</a></h3> |
| <p>If, however, the query is <em>not</em> in the cache, then the compiler will |
| call the corresponding <strong>provider</strong> function. A provider is a function |
| implemented in a specific module and <strong>manually registered</strong> into the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.Providers.html"><code>Providers</code></a> struct during compiler initialization. |
| The macro system generates the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.Providers.html"><code>Providers</code></a> struct, |
| which acts as a function table for all query implementations, where each |
| field is a function pointer to the actual provider.</p> |
| <p><strong>Note:</strong> The <code>Providers</code> struct is generated by macros and acts as a function table for all query implementations. |
| It is <strong>not</strong> a Rust trait, but a plain struct with function pointer fields.</p> |
| <p><strong>Providers are defined per-crate.</strong> The compiler maintains, |
| internally, a table of providers for every crate, at least |
| conceptually. Right now, there are really two sets: the providers for |
| queries about the <strong>local crate</strong> (that is, the one being compiled) |
| and providers for queries about <strong>external crates</strong> (that is, |
| dependencies of the local crate). Note that what determines the crate |
| that a query is targeting is not the <em>kind</em> of query, but the <em>key</em>. |
| For example, when you invoke <code>tcx.type_of(def_id)</code>, that could be a |
| local query or an external query, depending on what crate the <code>def_id</code> |
| is referring to (see the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/keys/trait.Key.html"><code>self::keys::Key</code></a> trait for more |
| information on how that works).</p> |
| <p>Providers always have the same signature:</p> |
| <pre><code class="language-rust ignore">fn provider<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| key: QUERY_KEY, |
| ) -> QUERY_RESULT { |
| ... |
| }</code></pre> |
| <p>Providers take two arguments: the <code>tcx</code> and the query key. |
| They return the result of the query.</p> |
| <p>N.B. Most of the <code>rustc_*</code> crates only provide <strong>local |
| providers</strong>. Almost all <strong>extern providers</strong> wind up going through the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/index.html"><code>rustc_metadata</code> crate</a>, which loads the information |
| from the crate metadata. But in some cases there are crates that |
| provide queries for <em>both</em> local and external crates, in which case |
| they define both a <code>provide</code> and a <code>provide_extern</code> function, through |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/back/symbol_export/fn.wasm_import_module_map.html"><code>wasm_import_module_map</code></a>, that <code>rustc_driver</code> can invoke.</p> |
| <h3 id="how-providers-are-set-up"><a class="header" href="#how-providers-are-set-up">How providers are set up</a></h3> |
| <p>When the tcx is created, it is given the providers by its creator using |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.Providers.html"><code>Providers</code></a> struct. This struct is generated by |
| the macros here, but it is basically a big list of function pointers:</p> |
| <pre><code class="language-rust ignore">struct Providers { |
| type_of: for<'tcx> fn(TyCtxt<'tcx>, DefId) -> Ty<'tcx>, |
| // ... one field for each query |
| }</code></pre> |
| <h4 id="how-are-providers-registered"><a class="header" href="#how-are-providers-registered">How are providers registered?</a></h4> |
| <p>The <code>Providers</code> struct is filled in during compiler initialization, mainly by the <code>rustc_driver</code> crate.<br /> |
| But the actual provider functions are implemented in various <code>rustc_*</code> crates (like <code>rustc_middle</code>, <code>rustc_hir_analysis</code>, etc).</p> |
| <p>To register providers, each crate exposes a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/hir/fn.provide.html"><code>provide</code></a> function that looks like this:</p> |
| <pre><code class="language-rust ignore">pub fn provide(providers: &mut Providers) { |
| *providers = Providers { |
| type_of, |
| // ... add more providers here |
| ..*providers |
| }; |
| }</code></pre> |
| <ul> |
| <li>This function takes a mutable reference to the <code>Providers</code> struct and sets the fields to point to the correct provider functions.</li> |
| <li>You can also assign fields individually, e.g. <code>providers.type_of = type_of;</code>.</li> |
| </ul> |
| <h4 id="adding-a-new-provider"><a class="header" href="#adding-a-new-provider">Adding a new provider</a></h4> |
| <p>Suppose you want to add a new query called <code>fubar</code>. You would:</p> |
| <ol> |
| <li>Implement the provider function: |
| <pre><code class="language-rust ignore">fn fubar<'tcx>(tcx: TyCtxt<'tcx>, key: DefId) -> Fubar<'tcx> { ... }</code></pre> |
| </li> |
| <li>Register it in the <code>provide</code> function: |
| <pre><code class="language-rust ignore">pub fn provide(providers: &mut Providers) { |
| *providers = Providers { |
| fubar, |
| ..*providers |
| }; |
| }</code></pre> |
| </li> |
| </ol> |
| <hr /> |
| <h2 id="adding-a-new-query"><a class="header" href="#adding-a-new-query">Adding a new query</a></h2> |
| <p>How do you add a new query? |
| Defining a query takes place in two steps:</p> |
| <ol> |
| <li>Declare the query name, its arguments and description.</li> |
| <li>Supply query providers where needed.</li> |
| </ol> |
| <p>To declare the query name and arguments, you simply add an entry to |
| the big macro invocation in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/index.html"><code>compiler/rustc_middle/src/query/mod.rs</code></a>. |
| Then you need to add a documentation comment to it with some <em>internal</em> description. |
| Then, provide the <code>desc</code> attribute which contains a <em>user-facing</em> description of the query. |
| The <code>desc</code> attribute is shown to the user in query cycles.</p> |
| <p>This looks something like:</p> |
| <pre><code class="language-rust ignore">rustc_queries! { |
| /// Records the type of every item. |
| query type_of(key: DefId) -> Ty<'tcx> { |
| cache_on_disk_if { key.is_local() } |
| desc { |tcx| "computing the type of `{}`", tcx.def_path_str(key) } |
| } |
| ... |
| }</code></pre> |
| <p>A query definition has the following form:</p> |
| <pre><code class="language-rust ignore">query type_of(key: DefId) -> Ty<'tcx> { ... } |
| ^^^^^ ^^^^^^^ ^^^^^ ^^^^^^^^ ^^^ |
| | | | | | |
| | | | | query modifiers |
| | | | result type |
| | | query key type |
| | name of query |
| query keyword</code></pre> |
| <p>Let's go over these elements one by one:</p> |
| <ul> |
| <li><strong>Query keyword:</strong> indicates a start of a query definition.</li> |
| <li><strong>Name of query:</strong> the name of the query method |
| (<code>tcx.type_of(..)</code>). Also used as the name of a struct |
| (<code>ty::queries::type_of</code>) that will be generated to represent |
| this query.</li> |
| <li><strong>Query key type:</strong> the type of the argument to this query. |
| This type must implement the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/keys/trait.Key.html"><code>ty::query::keys::Key</code></a> trait, which |
| defines (for example) how to map it to a crate, and so forth.</li> |
| <li><strong>Result type of query:</strong> the type produced by this query. This type |
| should (a) not use <code>RefCell</code> or other interior mutability and (b) be |
| cheaply cloneable. Interning or using <code>Rc</code> or <code>Arc</code> is recommended for |
| non-trivial data types.<sup class="footnote-reference" id="fr-steal-1"><a href="#footnote-steal">2</a></sup></li> |
| <li><strong>Query modifiers:</strong> various flags and options that customize how the |
| query is processed (mostly with respect to <a href="queries/incremental-compilation-in-detail.html#query-modifiers">incremental compilation</a>).</li> |
| </ul> |
| <p>So, to add a query:</p> |
| <ul> |
| <li>Add an entry to <code>rustc_queries!</code> using the format above.</li> |
| <li>Link the provider by modifying the appropriate <code>provide</code> method; |
| or add a new one if needed and ensure that <code>rustc_driver</code> is invoking it.</li> |
| </ul> |
| <h2 id="external-links"><a class="header" href="#external-links">External links</a></h2> |
| <p>Related design ideas, and tracking issues:</p> |
| <ul> |
| <li>Design document: <a href="https://github.com/nikomatsakis/rustc-on-demand-incremental-design-doc/blob/master/0000-rustc-on-demand-and-incremental.md">On-demand Rustc incremental design doc</a></li> |
| <li>Tracking Issue: <a href="https://github.com/rust-lang/rust/issues/42293">"Red/Green" dependency tracking in compiler</a></li> |
| </ul> |
| <p>More discussion and issues:</p> |
| <ul> |
| <li><a href="https://github.com/rust-lang/rust/issues/42633">GitHub issue #42633</a></li> |
| <li><a href="https://internals.rust-lang.org/t/incremental-compilation-beta/4721">Incremental Compilation Beta</a></li> |
| <li><a href="https://blog.rust-lang.org/2016/09/08/incremental.html">Incremental Compilation Announcement</a></li> |
| </ul> |
| <hr> |
| <ol class="footnote-definition"><li id="footnote-incr-comp-detail"> |
| <p>The <a href="queries/incremental-compilation-in-detail.html">Incremental compilation in detail</a> chapter gives a more |
| in-depth description of what queries are and how they work. |
| If you intend to write a query of your own, this is a good read. <a href="#fr-incr-comp-detail-1">↩</a></p> |
| </li> |
| <li id="footnote-steal"> |
| <p>The one exception to those rules is the <code>ty::steal::Steal</code> type, |
| which is used to cheaply modify MIR in place. See the definition |
| of <code>Steal</code> for more details. New uses of <code>Steal</code> should <strong>not</strong> be |
| added without alerting <code>@rust-lang/compiler</code>. <a href="#fr-steal-1">↩</a></p> |
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