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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>.</p> |
| <p>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 either |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.Providers.html"><code>Providers</code></a> struct (for local crate queries) or |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.ExternProviders.html"><code>ExternProviders</code></a> struct (for external crate queries) |
| during compiler initialization. |
| The macro system generates both structs, |
| which act as function tables for all query implementations, where each |
| field is a function pointer to the actual provider.</p> |
| <p><strong>Note:</strong> Both the <code>Providers</code> and <code>ExternProviders</code> structs are generated by macros and act as function tables for all query implementations. |
| They are <strong>not</strong> Rust traits, but plain structs 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. |
| There are two sets of providers:</p> |
| <ul> |
| <li>The <code>Providers</code> struct for queries about the <strong>local crate</strong> (that is, the one being compiled)</li> |
| <li>The <code>ExternProviders</code> struct for queries about <strong>external crates</strong> (that is, |
| dependencies of the local crate)</li> |
| </ul> |
| <p>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 both the local and external providers by its creator using |
| the <code>Providers</code> struct from <code>rustc_middle::util</code>. |
| This struct contains both the local and external providers:</p> |
| <pre><code class="language-rust ignore">pub struct Providers { |
| pub queries: crate::query::Providers, // Local crate providers |
| pub extern_queries: crate::query::ExternProviders, // External crate providers |
| pub hooks: crate::hooks::Providers, |
| }</code></pre> |
| <p>Each of these provider structs is generated by the macros and contains function pointers for their respective queries.</p> |
| <h4 id="how-are-providers-registered"><a class="header" href="#how-are-providers-registered">How are providers registered?</a></h4> |
| <p>The <code>util::Providers</code> struct is filled in during compiler initialization, by the <code>rustc_interface</code> crate from the <code>DEFAULT_QUERY_PROVIDERS</code> static. |
| The actual provider functions are defined across 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 query::Providers) { |
| *providers = query::Providers { |
| type_of, |
| // ... add more providers here |
| ..*providers |
| }; |
| }</code></pre> |
| <p>Note that this function accepts <code>query::Providers</code> not <code>util::Providers</code>. |
| It is exceedingly rare to need a <code>provide</code> function that doesn’t just accept <code>query::Providers</code>. |
| If more than the <code>queries</code> field of <code>util::Providers</code> is being updated then <code>util::Providers</code> can be accepted instead:</p> |
| <pre><code class="language-rust ignore">pub fn provide(providers: &mut rustc_middle::util::Providers) { |
| providers.queries.type_of = type_of; |
| // ... add more local providers here |
| |
| providers.extern_queries.type_of = extern_type_of; |
| // ... add more external providers here |
| |
| providers.hooks.some_hook = some_hook; |
| // ... add more hooks here |
| }</code></pre> |
| <p>Note that <code>util::Providers</code> implements <code>DerefMut</code> to <code>query::Providers</code> so callers of the <code>provide</code> functions can pass in a <code>util::Providers</code> and it will just work for provider functions that accept <code>query::Providers</code> too</p> |
| <ul> |
| <li>This function takes a mutable reference to the <code>query::Providers</code> struct and sets the fields to point to the correct provider functions.</li> |
| <li>You can also assign queries individually, e.g. <code>providers.type_of = type_of;</code>.</li> |
| <li>You can assign fields individually for each provider type (local, external, and hooks).</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>. |
| This section focuses on wiring up the providers; for how to declare the query itself in the big <code>rustc_queries!</code> macro, see <a href="#adding-a-new-query">Adding a new query</a> below.</p> |
| <p>In practice you usually:</p> |
| <ol> |
| <li>Decide which crate “owns” the query (for example <code>rustc_hir_analysis</code>, <code>rustc_mir_build</code>, or another <code>rustc_*</code> crate).</li> |
| <li>In that crate, look for an existing <code>provide</code> function: |
| <pre><code class="language-rust ignore">pub fn provide(providers: &mut query::Providers) { |
| // existing assignments |
| }</code></pre> |
| If it exists, you will extend it to set the field for your new query. |
| If the crate does not yet have a <code>provide</code> function, add one and make sure it is included in <code>DEFAULT_QUERY_PROVIDERS</code> in the <code>rustc_interface</code> crate so that it actually gets called during initialization (see the discussion above).</li> |
| <li>Implement the provider function itself: |
| <pre><code class="language-rust ignore">fn fubar<'tcx>(tcx: TyCtxt<'tcx>, key: LocalDefId) -> Fubar<'tcx> { ... }</code></pre> |
| </li> |
| <li>Register it in the crate’s <code>provide</code> function: |
| <pre><code class="language-rust ignore">pub fn provide(providers: &mut query::Providers) { |
| *providers = query::Providers { |
| fubar, |
| ..*providers |
| }; |
| }</code></pre> |
| </li> |
| </ol> |
| <h3 id="how-queries-interact-with-external-crate-metadata"><a class="header" href="#how-queries-interact-with-external-crate-metadata">How queries interact with external crate metadata</a></h3> |
| <p>When a query is made for an external crate (i.e., a dependency), the query system needs to load the information from that crate’s metadata. |
| This is handled by the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/index.html"><code>rustc_metadata</code> crate</a>, which is responsible for decoding and providing the information stored in the <code>.rmeta</code> files.</p> |
| <p>The process works like this:</p> |
| <ol> |
| <li> |
| <p>When a query is made, the query system first checks if the <code>DefId</code> refers to a local or external crate by checking if <code>def_id.krate == LOCAL_CRATE</code>. |
| This determines whether to use the local provider from <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.Providers.html"><code>Providers</code></a> or the external provider from <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.ExternProviders.html"><code>ExternProviders</code></a>.</p> |
| </li> |
| <li> |
| <p>For external crates, the query system will look for a provider in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.ExternProviders.html"><code>ExternProviders</code></a> struct. |
| The <code>rustc_metadata</code> crate registers these external providers through the <code>provide_extern</code> function in <code>rustc_metadata/src/rmeta/decoder/cstore_impl.rs</code>. |
| Just like:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>pub fn provide_extern(providers: &mut ExternProviders) { |
| providers.foo = |tcx, def_id| { |
| // Load and decode metadata for external crate |
| let cdata = CStore::from_tcx(tcx).get_crate_data(def_id.krate); |
| cdata.foo(def_id.index) |
| }; |
| // Register other external providers... |
| } |
| <span class="boring">}</span></code></pre> |
| </li> |
| <li> |
| <p>The metadata is stored in a binary format in <code>.rmeta</code> files that contains pre-computed information about the external crate, such as types, function signatures, trait implementations, and other information needed by the compiler. |
| When an external query is made, the <code>rustc_metadata</code> crate:</p> |
| <ul> |
| <li>Loads the <code>.rmeta</code> file for the external crate</li> |
| <li>Decodes the metadata using the <code>Decodable</code> trait</li> |
| <li>Returns the decoded information to the query system</li> |
| </ul> |
| </li> |
| </ol> |
| <p>This approach avoids recompiling external crates, allows for faster compilation of dependent crates, and enables incremental compilation to work across crate boundaries. |
| Here is a simplified example, when you call <code>tcx.type_of(def_id)</code> for a type defined in an external crate, the query system will:</p> |
| <ol> |
| <li>Detect that the <code>def_id</code> refers to an external crate by checking <code>def_id.krate != LOCAL_CRATE</code></li> |
| <li>Call the appropriate provider from <code>ExternProviders</code> which was registered by <code>rustc_metadata</code></li> |
| <li>The provider will load and decode the type information from the external crate’s metadata</li> |
| <li>Return the decoded type to the caller</li> |
| </ol> |
| <p>This is why most <code>rustc_*</code> crates only need to provide local providers - the external providers are handled by the metadata system. |
| The only exception is when a crate needs to provide special handling for external queries, in which case it would implement both local and external providers.</p> |
| <p>When we define a new query that should work across crates, it does not automatically become cross-crate just because it is listed in <code>rustc_queries!</code>. |
| You will typically need to:</p> |
| <ul> |
| <li>Add the query to <code>rustc_queries!</code> with appropriate modifiers (for example whether it is cached on disk).</li> |
| <li>Implement a local provider in the owning crate and register it via that crate’s <code>provide</code> function.</li> |
| <li>Add an external provider in <code>rustc_metadata</code> via <code>provide_extern</code>, and ensure the query’s result is encoded and decoded in the crate metadata.</li> |
| </ul> |
| <p>An example of introducing such a cross-crate query can be found in commit <a href="https://github.com/rust-lang/rust/commit/996a185"><code>996a185</code></a> in the <code>rust-lang/rust</code> repository.</p> |
| <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> |
| </li> |
| </ol> |
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