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<ol class="chapter"><li class="chapter-item expanded "><a href="../about_salsa.html"><strong aria-hidden="true">1.</strong> About salsa</a></li><li class="chapter-item expanded affix "><li class="part-title">How to use Salsa</li><li class="chapter-item expanded "><a href="../overview.html"><strong aria-hidden="true">2.</strong> Overview</a></li><li class="chapter-item expanded "><a href="../tutorial.html"><strong aria-hidden="true">3.</strong> Tutorial: calc language</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tutorial/structure.html"><strong aria-hidden="true">3.1.</strong> Basic structure</a></li><li class="chapter-item expanded "><a href="../tutorial/jar.html"><strong aria-hidden="true">3.2.</strong> Jars and databases</a></li><li class="chapter-item expanded "><a href="../tutorial/db.html"><strong aria-hidden="true">3.3.</strong> Defining the database struct</a></li><li class="chapter-item expanded "><a href="../tutorial/ir.html"><strong aria-hidden="true">3.4.</strong> Defining the IR: the various "salsa structs"</a></li><li class="chapter-item expanded "><a href="../tutorial/parser.html"><strong aria-hidden="true">3.5.</strong> Defining the parser: memoized functions and inputs</a></li><li class="chapter-item expanded "><a href="../tutorial/accumulators.html"><strong aria-hidden="true">3.6.</strong> Defining the parser: reporting errors</a></li><li class="chapter-item expanded "><a href="../tutorial/debug.html"><strong aria-hidden="true">3.7.</strong> Defining the parser: debug impls and testing</a></li><li class="chapter-item expanded "><a href="../tutorial/checker.html"><strong aria-hidden="true">3.8.</strong> Defining the checker</a></li><li class="chapter-item expanded "><a href="../tutorial/interpreter.html"><strong aria-hidden="true">3.9.</strong> Defining the interpreter</a></li></ol></li><li class="chapter-item expanded "><a href="../reference.html"><strong aria-hidden="true">4.</strong> Reference</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../reference/algorithm.html"><strong aria-hidden="true">4.1.</strong> Algorithm</a></li></ol></li><li class="chapter-item expanded "><a href="../common_patterns.html"><strong aria-hidden="true">5.</strong> Common patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../common_patterns/selection.html"><strong aria-hidden="true">5.1.</strong> Selection</a></li><li class="chapter-item expanded "><a href="../common_patterns/on_demand_inputs.html"><strong aria-hidden="true">5.2.</strong> On-demand (Lazy) inputs</a></li></ol></li><li class="chapter-item expanded "><a href="../tuning.html"><strong aria-hidden="true">6.</strong> Tuning</a></li><li class="chapter-item expanded "><a href="../cycles.html"><strong aria-hidden="true">7.</strong> Cycle handling</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../cycles/fallback.html"><strong aria-hidden="true">7.1.</strong> Recovering via fallback</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">How Salsa works internally</li><li class="chapter-item expanded "><a href="../how_salsa_works.html"><strong aria-hidden="true">8.</strong> How Salsa works</a></li><li class="chapter-item expanded "><a href="../videos.html"><strong aria-hidden="true">9.</strong> Videos</a></li><li class="chapter-item expanded "><a href="../plumbing.html"><strong aria-hidden="true">10.</strong> Plumbing</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../plumbing/jars_and_ingredients.html"><strong aria-hidden="true">10.1.</strong> Jars and ingredients</a></li><li class="chapter-item expanded "><a href="../plumbing/database_and_runtime.html"><strong aria-hidden="true">10.2.</strong> Databases and runtime</a></li><li class="chapter-item expanded "><a href="../plumbing/tracked_structs.html" class="active"><strong aria-hidden="true">10.3.</strong> Tracked structures</a></li><li class="chapter-item expanded "><a href="../plumbing/query_ops.html"><strong aria-hidden
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<main>
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<h1 id="tracked-structs"><a class="header" href="#tracked-structs">Tracked structs</a></h1>
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<p>Tracked structs are stored in a special way to reduce their costs.</p>
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<p>Tracked structs are created via a <code>new</code> operation.</p>
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<h2 id="the-tracked-struct-and-tracked-field-ingredients"><a class="header" href="#the-tracked-struct-and-tracked-field-ingredients">The tracked struct and tracked field ingredients</a></h2>
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<p>For a single tracked struct we create multiple ingredients.
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The <strong>tracked struct ingredient</strong> is the ingredient created first.
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It offers methods to create new instances of the struct and therefore
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has unique access to the interner and hashtables used to create the struct id.
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It also shares access to a hashtable that stores the <code>TrackedStructValue</code> that
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contains the field data.</p>
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<p>For each field, we create a <strong>tracked field ingredient</strong> that moderates access
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to a particular field. All of these ingredients use that same shared hashtable
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to access the <code>TrackedStructValue</code> instance for a given id. The <code>TrackedStructValue</code>
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contains both the field values but also the revisions when they last changed value.</p>
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<h2 id="each-tracked-struct-has-a-globally-unique-id"><a class="header" href="#each-tracked-struct-has-a-globally-unique-id">Each tracked struct has a globally unique id</a></h2>
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<p>This will begin by creating a <em>globally unique, 32-bit id</em> for the tracked struct. It is created by interning a combination of</p>
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<ul>
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<li>the currently executing query;</li>
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<li>a u64 hash of the <code>#[id]</code> fields;</li>
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<li>a <em>disambiguator</em> that makes this hash unique within the current query. i.e., when a query starts executing, it creates an empty map, and the first time a tracked struct with a given hash is created, it gets disambiguator 0. The next one will be given 1, etc.</li>
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</ul>
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<h2 id="each-tracked-struct-has-a-trackedstructvalue-storing-its-data"><a class="header" href="#each-tracked-struct-has-a-trackedstructvalue-storing-its-data">Each tracked struct has a <code>TrackedStructValue</code> storing its data</a></h2>
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<p>The struct and field ingredients share access to a hashmap that maps
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each field id to a value struct:</p>
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<pre><code class="language-rust ignore">#[derive(Debug)]
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struct TrackedStructValue<C>
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where
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C: Configuration,
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{
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/// The durability minimum durability of all inputs consumed
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/// by the creator query prior to creating this tracked struct.
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/// If any of those inputs changes, then the creator query may
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/// create this struct with different values.
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durability: Durability,
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/// The revision when this entity was most recently created.
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/// Typically the current revision.
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/// Used to detect "leaks" outside of the salsa system -- i.e.,
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/// access to tracked structs that have not (yet?) been created in the
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/// current revision. This should be impossible within salsa queries
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/// but it can happen through "leaks" like thread-local data or storing
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/// values outside of the root salsa query.
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created_at: Revision,
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/// Fields of this tracked struct. They can change across revisions,
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/// but they do not change within a particular revision.
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fields: C::Fields,
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/// The revision information for each field: when did this field last change.
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/// When tracked structs are re-created, this revision may be updated to the
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/// current revision if the value is different.
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revisions: C::Revisions,
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}
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</code></pre>
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<p>The value struct stores the values of the fields but also the revisions when
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that field last changed. Each time the struct is recreated in a new revision,
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the old and new values for its fields are compared and a new revision is created.</p>
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<h2 id="the-macro-generates-the-tracked-struct-configuration"><a class="header" href="#the-macro-generates-the-tracked-struct-configuration">The macro generates the tracked struct <code>Configuration</code></a></h2>
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<p>The "configuration" for a tracked struct defines not only the types of the fields,
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but also various important operations such as extracting the hashable id fields
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and updating the "revisions" to track when a field last changed:</p>
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<pre><code class="language-rust ignore">/// Trait that defines the key properties of a tracked struct.
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/// Implemented by the `#[salsa::tracked]` macro when applied
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/// to a struct.
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pub trait Configuration {
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/// The id type used to define instances of this struct.
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/// The [`TrackedStructIngredient`][] contains the interner
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/// that will create the id values.
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type Id: InternedId;
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/// A (possibly empty) tuple of the fields for this struct.
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type Fields;
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/// A array of [`Revision`][] values, one per each of the value fields.
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/// When a struct is re-recreated in a new revision, the corresponding
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/// entries for each field are updated to the new revision if their
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/// values have changed (or if the field is marked as `#[no_eq]`).
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type Revisions;
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fn id_fields(fields: &Self::Fields) -> impl Hash;
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/// Access the revision of a given value field.
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/// `field_index` will be between 0 and the number of value fields.
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fn revision(revisions: &Self::Revisions, field_index: u32) -> Revision;
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/// Create a new value revision array where each element is set to `current_revision`.
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fn new_revisions(current_revision: Revision) -> Self::Revisions;
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/// Update an existing value revision array `revisions`,
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/// given the tuple of the old values (`old_value`)
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/// and the tuple of the values (`new_value`).
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/// If a value has changed, then its element is
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/// updated to `current_revision`.
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fn update_revisions(
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current_revision: Revision,
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old_value: &Self::Fields,
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new_value: &Self::Fields,
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revisions: &mut Self::Revisions,
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);
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}
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</code></pre>
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