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| <h1 id="caching-in-the-new-trait-solver"><a class="header" href="#caching-in-the-new-trait-solver">Caching in the new trait solver</a></h1> |
| <p>Caching results of the trait solver is necessary for performance. |
| We have to make sure that it is sound. Caching is handled by the |
| <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_trait_selection/src/solve/search_graph.rs#L102-L117"><code>SearchGraph</code></a></p> |
| <h2 id="the-global-cache"><a class="header" href="#the-global-cache">The global cache</a></h2> |
| <p>At its core, the cache is fairly straightforward. When evaluating a goal, we |
| check whether it’s in the global cache. If so, we reuse that entry. If not, we |
| compute the goal and then store its result in the cache.</p> |
| <p>To handle incremental compilation the computation of a goal happens inside of |
| <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_trait_selection/src/solve/search_graph.rs#L391"><code>DepGraph::with_anon_task</code></a> which creates a new <code>DepNode</code> which depends on all queries |
| used inside of this computation. When accessing the global cache we then read this |
| <code>DepNode</code>, manually adding a dependency edge to all the queries used: <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_middle/src/traits/solve/cache.rs#L78">source</a>.</p> |
| <h3 id="dealing-with-overflow"><a class="header" href="#dealing-with-overflow">Dealing with overflow</a></h3> |
| <p>Hitting the recursion limit is not fatal in the new trait solver but instead simply |
| causes it to return ambiguity: <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_trait_selection/src/solve/search_graph.rs#L276">source</a>. Whether we hit the recursion limit |
| can therefore change the result without resulting in a compilation failure. This |
| means we must consider the remaining available depth when accessing a cache result.</p> |
| <p>We do this by storing more information in the cache entry. For goals whose evaluation |
| did not reach the recursion limit, we simply store its reached depth: <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_middle/src/traits/solve/cache.rs#L102">source</a>. |
| These results can freely be used as long as the current <code>available_depth</code> is higher than |
| its <code>reached_depth</code>: <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_middle/src/traits/solve/cache.rs#L76-L86">source</a>. We then update the reached depth of the |
| current goal to make sure that whether we’ve used the global cache entry is not |
| observable: <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_trait_selection/src/solve/search_graph.rs#L308">source</a>.</p> |
| <p>For goals which reach the recursion limit we currently only use the cached result if the |
| available depth <em>exactly matches</em> the depth of the entry. The cache entry for each goal |
| therefore contains a separate result for each remaining depth: <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_middle/src/traits/solve/cache.rs#L124">source</a>.<sup class="footnote-reference" id="fr-1-1"><a href="#footnote-1">1</a></sup></p> |
| <h2 id="handling-cycles"><a class="header" href="#handling-cycles">Handling cycles</a></h2> |
| <p>The trait solver has to support cycles. These cycles are either inductive or coinductive, |
| depending on the participating goals. See the <a href="./coinduction.html">chapter on coinduction</a> for more details. |
| We distinguish between the cycle heads and the cycle root: a stack entry is a |
| cycle head if it recursively accessed. The <em>root</em> is the deepest goal on the stack which |
| is involved in any cycle. Given the following dependency tree, <code>A</code> and <code>B</code> are both cycle |
| heads, while only <code>A</code> is a root.</p> |
| <pre class="mermaid">graph TB |
| A --> B |
| B --> C |
| C --> B |
| C --> A |
| </pre> |
| |
| <p>The result of cycle participants depends on the result of goals still on the stack. |
| However, we are currently computing that result, so its result is still unknown. This is |
| handled by evaluating cycle heads until we reach a fixpoint. In the first iteration, we |
| return either success or overflow with no constraints, depending on whether the cycle is |
| coinductive: <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_trait_selection/src/solve/search_graph.rs#L366-L370">source</a>. After evaluating the head of a cycle, we |
| check whether its <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_trait_selection/src/solve/search_graph.rs#L57"><code>provisional_result</code></a> is equal to the result of this iteration. If so, |
| we’ve finished evaluating this cycle and return its result. If not, we update the provisional |
| result and reevaluate the goal: <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_trait_selection/src/solve/search_graph.rs#L425-L446">source</a>. After the first iteration it does not |
| matter whether cycles are coinductive or inductive. We always use the provisional result.</p> |
| <h3 id="only-caching-cycle-roots"><a class="header" href="#only-caching-cycle-roots">Only caching cycle roots</a></h3> |
| <p>We cannot move the result of any cycle participant to the global cache until we’ve |
| finished evaluating the cycle root. However, even after we’ve completely evaluated the |
| cycle, we are still forced to discard the result of all participants apart from the root |
| itself.</p> |
| <p>We track the query dependencies of all global cache entries. This causes the caching of |
| cycle participants to be non-trivial. We cannot simply reuse the <code>DepNode</code> of the cycle |
| root.<sup class="footnote-reference" id="fr-2-1"><a href="#footnote-2">2</a></sup> If we have a cycle <code>A -> B -> A</code>, then the <code>DepNode</code> for <code>A</code> contains a dependency |
| from <code>A -> B</code>. Reusing this entry for <code>B</code> may break if the source is changed. The <code>B -> A</code> |
| edge may not exist anymore and <code>A</code> may have been completely removed. This can easily result |
| in an ICE.</p> |
| <p>However, it’s even worse as the result of a cycle can change depending on which goal is |
| the root: <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.rs#L4-L16">example</a>. This forces us to weaken caching even further. |
| We must not use a cache entry of a cycle root, if there exists a stack entry, which was |
| a participant of its cycle involving that root. We do this by storing all cycle participants |
| of a given root in its global cache entry and checking that it contains no element of the |
| stack: <a href="https://github.com/rust-lang/rust/blob/7606c13961ddc1174b70638e934df0439b7dc515/compiler/rustc_middle/src/traits/solve/cache.rs#L72-L74">source</a>.</p> |
| <h3 id="the-provisional-cache"><a class="header" href="#the-provisional-cache">The provisional cache</a></h3> |
| <p>TODO: write this :3</p> |
| <ul> |
| <li>stack dependence of provisional results</li> |
| <li>edge case: provisional cache impacts behavior</li> |
| </ul> |
| <hr> |
| <ol class="footnote-definition"> |
| <li id="footnote-1"> |
| <p>This is overly restrictive: if all nested goals return the overflow response with some |
| available depth <code>n</code>, then their result should be the same for any depths smaller than <code>n</code>. |
| We can implement this optimization in the future. <a href="#fr-1-1">↩</a></p> |
| </li> |
| <li id="footnote-2"> |
| <p>summarizing the relevant <a href="https://rust-lang.zulipchat.com/#narrow/stream/364551-t-types.2Ftrait-system-refactor/topic/global.20cache">Zulip thread</a> <a href="#fr-2-1">↩</a></p> |
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| </ol> |
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