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Diffstat (limited to '')
-rw-r--r-- | compiler/rustc_trait_selection/src/solve/search_graph/cache.rs | 123 |
1 files changed, 123 insertions, 0 deletions
diff --git a/compiler/rustc_trait_selection/src/solve/search_graph/cache.rs b/compiler/rustc_trait_selection/src/solve/search_graph/cache.rs new file mode 100644 index 000000000..730a8e612 --- /dev/null +++ b/compiler/rustc_trait_selection/src/solve/search_graph/cache.rs @@ -0,0 +1,123 @@ +//! This module both handles the global cache which stores "finished" goals, +//! and the provisional cache which contains partially computed goals. +//! +//! The provisional cache is necessary when dealing with coinductive cycles. +//! +//! For more information about the provisional cache and coinduction in general, +//! check out the relevant section of the rustc-dev-guide. +//! +//! FIXME(@lcnr): Write that section, feel free to ping me if you need help here +//! before then or if I still haven't done that before January 2023. +use super::overflow::OverflowData; +use super::StackDepth; +use crate::solve::{CanonicalGoal, QueryResult}; +use rustc_data_structures::fx::FxHashMap; +use rustc_index::vec::IndexVec; +use rustc_middle::ty::TyCtxt; + +rustc_index::newtype_index! { + pub struct EntryIndex {} +} + +#[derive(Debug, Clone)] +pub(super) struct ProvisionalEntry<'tcx> { + // In case we have a coinductive cycle, this is the + // the currently least restrictive result of this goal. + pub(super) response: QueryResult<'tcx>, + // In case of a cycle, the position of deepest stack entry involved + // in that cycle. This is monotonically decreasing in the stack as all + // elements between the current stack element in the deepest stack entry + // involved have to also be involved in that cycle. + // + // We can only move entries to the global cache once we're complete done + // with the cycle. If this entry has not been involved in a cycle, + // this is just its own depth. + pub(super) depth: StackDepth, + + // The goal for this entry. Should always be equal to the corresponding goal + // in the lookup table. + pub(super) goal: CanonicalGoal<'tcx>, +} + +pub(super) struct ProvisionalCache<'tcx> { + pub(super) entries: IndexVec<EntryIndex, ProvisionalEntry<'tcx>>, + // FIXME: This is only used to quickly check whether a given goal + // is in the cache. We should experiment with using something like + // `SsoHashSet` here because in most cases there are only a few entries. + pub(super) lookup_table: FxHashMap<CanonicalGoal<'tcx>, EntryIndex>, +} + +impl<'tcx> ProvisionalCache<'tcx> { + pub(super) fn empty() -> ProvisionalCache<'tcx> { + ProvisionalCache { entries: Default::default(), lookup_table: Default::default() } + } + + pub(super) fn is_empty(&self) -> bool { + self.entries.is_empty() && self.lookup_table.is_empty() + } + + /// Adds a dependency from the current leaf to `target` in the cache + /// to prevent us from moving any goals which depend on the current leaf + /// to the global cache while we're still computing `target`. + /// + /// Its important to note that `target` may already be part of a different cycle. + /// In this case we have to ensure that we also depend on all other goals + /// in the existing cycle in addition to the potentially direct cycle with `target`. + pub(super) fn add_dependency_of_leaf_on(&mut self, target: EntryIndex) { + let depth = self.entries[target].depth; + for provisional_entry in &mut self.entries.raw[target.index()..] { + // The depth of `target` is the position of the deepest goal in the stack + // on which `target` depends. That goal is the `root` of this cycle. + // + // Any entry which was added after `target` is either on the stack itself + // at which point its depth is definitely at least as high as the depth of + // `root`. If it's not on the stack itself it has to depend on a goal + // between `root` and `leaf`. If it were to depend on a goal deeper in the + // stack than `root`, then `root` would also depend on that goal, at which + // point `root` wouldn't be the root anymore. + debug_assert!(provisional_entry.depth >= depth); + provisional_entry.depth = depth; + } + + // We only update entries which were added after `target` as no other + // entry should have a higher depth. + // + // Any entry which previously had a higher depth than target has to + // be between `target` and `root`. Because of this we would have updated + // its depth when calling `add_dependency_of_leaf_on(root)` for `target`. + if cfg!(debug_assertions) { + self.entries.iter().all(|e| e.depth <= depth); + } + } + + pub(super) fn depth(&self, entry_index: EntryIndex) -> StackDepth { + self.entries[entry_index].depth + } + + pub(super) fn provisional_result(&self, entry_index: EntryIndex) -> QueryResult<'tcx> { + self.entries[entry_index].response.clone() + } +} + +pub(super) fn try_move_finished_goal_to_global_cache<'tcx>( + tcx: TyCtxt<'tcx>, + overflow_data: &mut OverflowData, + stack: &IndexVec<super::StackDepth, super::StackElem<'tcx>>, + goal: CanonicalGoal<'tcx>, + response: QueryResult<'tcx>, +) { + // We move goals to the global cache if we either did not hit an overflow or if it's + // the root goal as that will now always hit the same overflow limit. + // + // NOTE: We cannot move any non-root goals to the global cache even if their final result + // isn't impacted by the overflow as that goal still has unstable query dependencies + // because it didn't go its full depth. + // + // FIXME(@lcnr): We could still cache subtrees which are not impacted by overflow though. + // Tracking that info correctly isn't trivial, so I haven't implemented it for now. + let should_cache_globally = !overflow_data.did_overflow() || stack.is_empty(); + if should_cache_globally { + // FIXME: move the provisional entry to the global cache. + let _ = (tcx, goal, response); + } +} |