From f860873983d1db578aea09d47cd594d9da512e49 Mon Sep 17 00:00:00 2001 From: lcnr Date: Tue, 23 Jul 2024 23:19:29 +0200 Subject: [PATCH] split provisional cache and stack lookup this makes it easier to maintain and modify going forward. There may be a small performance cost as we now need to access the provisional cache *and* walk through the stack to detect cycles. However, the provisional cache should be mostly empty and the stack should only have a few elements so the performance impact is likely minimal. Given the complexity of the search graph maintainability trumps linear performance improvements. --- .../rustc_type_ir/src/search_graph/mod.rs | 204 +++++++++--------- .../src/search_graph/validate.rs | 18 +- 2 files changed, 104 insertions(+), 118 deletions(-) diff --git a/compiler/rustc_type_ir/src/search_graph/mod.rs b/compiler/rustc_type_ir/src/search_graph/mod.rs index bd93c86b0860b..a1aa63c2e7d75 100644 --- a/compiler/rustc_type_ir/src/search_graph/mod.rs +++ b/compiler/rustc_type_ir/src/search_graph/mod.rs @@ -224,8 +224,8 @@ struct DetachedEntry { result: X::Result, } -/// Stores the stack depth of a currently evaluated goal *and* already -/// computed results for goals which depend on other goals still on the stack. +/// Stores the provisional result of already computed results for goals which +/// depend on other goals still on the stack. /// /// The provisional result may depend on whether the stack above it is inductive /// or coinductive. Because of this, we store separate provisional results for @@ -238,16 +238,13 @@ struct DetachedEntry { /// see tests/ui/traits/next-solver/cycles/provisional-cache-impacts-behavior.rs. #[derive_where(Default; X: Cx)] struct ProvisionalCacheEntry { - stack_depth: Option, with_inductive_stack: Option>, with_coinductive_stack: Option>, } impl ProvisionalCacheEntry { fn is_empty(&self) -> bool { - self.stack_depth.is_none() - && self.with_inductive_stack.is_none() - && self.with_coinductive_stack.is_none() + self.with_inductive_stack.is_none() && self.with_coinductive_stack.is_none() } } @@ -378,71 +375,26 @@ impl, X: Cx> SearchGraph { None }; - // Check whether the goal is in the provisional cache. - // The provisional result may rely on the path to its cycle roots, - // so we have to check the path of the current goal matches that of - // the cache entry. - let cache_entry = self.provisional_cache.entry(input).or_default(); - if let Some(entry) = cache_entry - .with_coinductive_stack - .as_ref() - .filter(|p| Self::stack_coinductive_from(cx, &self.stack, p.head)) - .or_else(|| { - cache_entry - .with_inductive_stack - .as_ref() - .filter(|p| !Self::stack_coinductive_from(cx, &self.stack, p.head)) - }) - { - debug!("provisional cache hit"); - // We have a nested goal which is already in the provisional cache, use - // its result. We do not provide any usage kind as that should have been - // already set correctly while computing the cache entry. - debug_assert!(self.stack[entry.head].has_been_used.is_some()); - Self::tag_cycle_participants(&mut self.stack, entry.head); - return entry.result; - } else if let Some(stack_depth) = cache_entry.stack_depth { - debug!("encountered cycle with depth {stack_depth:?}"); - // We have a nested goal which directly relies on a goal deeper in the stack. - // - // We start by tagging all cycle participants, as that's necessary for caching. - // - // Finally we can return either the provisional response or the initial response - // in case we're in the first fixpoint iteration for this goal. - let is_coinductive_cycle = Self::stack_coinductive_from(cx, &self.stack, stack_depth); - let cycle_kind = - if is_coinductive_cycle { CycleKind::Coinductive } else { CycleKind::Inductive }; - let usage_kind = UsageKind::Single(cycle_kind); - self.stack[stack_depth].has_been_used = Some( - self.stack[stack_depth] - .has_been_used - .map_or(usage_kind, |prev| prev.merge(usage_kind)), - ); - Self::tag_cycle_participants(&mut self.stack, stack_depth); - - // Return the provisional result or, if we're in the first iteration, - // start with no constraints. - return if let Some(result) = self.stack[stack_depth].provisional_result { - result - } else { - D::initial_provisional_result(cx, cycle_kind, input) - }; - } else { - // No entry, we push this goal on the stack and try to prove it. - let depth = self.stack.next_index(); - let entry = StackEntry { - input, - available_depth, - reached_depth: depth, - non_root_cycle_participant: None, - encountered_overflow: false, - has_been_used: None, - nested_goals: Default::default(), - provisional_result: None, - }; - assert_eq!(self.stack.push(entry), depth); - cache_entry.stack_depth = Some(depth); + if let Some(result) = self.lookup_provisional_cache(cx, input) { + return result; + } + + if let Some(result) = self.check_cycle_on_stack(cx, input) { + return result; + } + + let depth = self.stack.next_index(); + let entry = StackEntry { + input, + available_depth, + reached_depth: depth, + non_root_cycle_participant: None, + encountered_overflow: false, + has_been_used: None, + nested_goals: Default::default(), + provisional_result: None, }; + assert_eq!(self.stack.push(entry), depth); // This is for global caching, so we properly track query dependencies. // Everything that affects the `result` should be performed within this @@ -474,8 +426,7 @@ impl, X: Cx> SearchGraph { debug_assert!(validate_cache.is_none()); let coinductive_stack = Self::stack_coinductive_from(cx, &self.stack, head); - let entry = self.provisional_cache.get_mut(&input).unwrap(); - entry.stack_depth = None; + let entry = self.provisional_cache.entry(input).or_default(); if coinductive_stack { entry.with_coinductive_stack = Some(DetachedEntry { head, result }); } else { @@ -534,35 +485,52 @@ impl, X: Cx> SearchGraph { }) } - /// When encountering a cycle, both inductive and coinductive, we only - /// move the root into the global cache. We also store all other cycle - /// participants involved. - /// - /// We must not use the global cache entry of a root goal if a cycle - /// participant is on the stack. This is necessary to prevent unstable - /// results. See the comment of `StackEntry::nested_goals` for - /// more details. - fn insert_global_cache( - &mut self, - cx: X, - input: X::Input, - final_entry: StackEntry, - result: X::Result, - dep_node: X::DepNodeIndex, - ) { - let additional_depth = final_entry.reached_depth.as_usize() - self.stack.len(); - debug!(?final_entry, ?result, "insert global cache"); - cx.with_global_cache(self.mode, |cache| { - cache.insert( - cx, - input, - result, - dep_node, - additional_depth, - final_entry.encountered_overflow, - &final_entry.nested_goals, - ) - }) + fn lookup_provisional_cache(&mut self, cx: X, input: X::Input) -> Option { + let cache_entry = self.provisional_cache.get(&input)?; + let &DetachedEntry { head, result } = cache_entry + .with_coinductive_stack + .as_ref() + .filter(|p| Self::stack_coinductive_from(cx, &self.stack, p.head)) + .or_else(|| { + cache_entry + .with_inductive_stack + .as_ref() + .filter(|p| !Self::stack_coinductive_from(cx, &self.stack, p.head)) + })?; + + debug!("provisional cache hit"); + // We have a nested goal which is already in the provisional cache, use + // its result. We do not provide any usage kind as that should have been + // already set correctly while computing the cache entry. + Self::tag_cycle_participants(&mut self.stack, head); + debug_assert!(self.stack[head].has_been_used.is_some()); + Some(result) + } + + fn check_cycle_on_stack(&mut self, cx: X, input: X::Input) -> Option { + let (head, _stack_entry) = self.stack.iter_enumerated().find(|(_, e)| e.input == input)?; + debug!("encountered cycle with depth {head:?}"); + // We have a nested goal which directly relies on a goal deeper in the stack. + // + // We start by tagging all cycle participants, as that's necessary for caching. + // + // Finally we can return either the provisional response or the initial response + // in case we're in the first fixpoint iteration for this goal. + let is_coinductive_cycle = Self::stack_coinductive_from(cx, &self.stack, head); + let cycle_kind = + if is_coinductive_cycle { CycleKind::Coinductive } else { CycleKind::Inductive }; + let usage_kind = UsageKind::Single(cycle_kind); + self.stack[head].has_been_used = + Some(self.stack[head].has_been_used.map_or(usage_kind, |prev| prev.merge(usage_kind))); + Self::tag_cycle_participants(&mut self.stack, head); + + // Return the provisional result or, if we're in the first iteration, + // start with no constraints. + if let Some(result) = self.stack[head].provisional_result { + Some(result) + } else { + Some(D::initial_provisional_result(cx, cycle_kind, input)) + } } /// When we encounter a coinductive cycle, we have to fetch the @@ -613,13 +581,43 @@ impl, X: Cx> SearchGraph { StepResult::Done(stack_entry, result) } else { debug!(?result, "fixpoint changed provisional results"); - let depth = self.stack.push(StackEntry { + self.stack.push(StackEntry { has_been_used: None, provisional_result: Some(result), ..stack_entry }); - debug_assert_eq!(self.provisional_cache[&input].stack_depth, Some(depth)); StepResult::HasChanged } } + + /// When encountering a cycle, both inductive and coinductive, we only + /// move the root into the global cache. We also store all other cycle + /// participants involved. + /// + /// We must not use the global cache entry of a root goal if a cycle + /// participant is on the stack. This is necessary to prevent unstable + /// results. See the comment of `StackEntry::nested_goals` for + /// more details. + fn insert_global_cache( + &mut self, + cx: X, + input: X::Input, + final_entry: StackEntry, + result: X::Result, + dep_node: X::DepNodeIndex, + ) { + let additional_depth = final_entry.reached_depth.as_usize() - self.stack.len(); + debug!(?final_entry, ?result, "insert global cache"); + cx.with_global_cache(self.mode, |cache| { + cache.insert( + cx, + input, + result, + dep_node, + additional_depth, + final_entry.encountered_overflow, + &final_entry.nested_goals, + ) + }) + } } diff --git a/compiler/rustc_type_ir/src/search_graph/validate.rs b/compiler/rustc_type_ir/src/search_graph/validate.rs index 1ae806834ba7d..b4802811b0f57 100644 --- a/compiler/rustc_type_ir/src/search_graph/validate.rs +++ b/compiler/rustc_type_ir/src/search_graph/validate.rs @@ -23,8 +23,6 @@ impl, X: Cx> SearchGraph { ref nested_goals, provisional_result, } = *entry; - let cache_entry = provisional_cache.get(&entry.input).unwrap(); - assert_eq!(cache_entry.stack_depth, Some(depth)); if let Some(head) = non_root_cycle_participant { assert!(head < depth); assert!(nested_goals.is_empty()); @@ -45,19 +43,9 @@ impl, X: Cx> SearchGraph { } } - for (&input, entry) in &self.provisional_cache { - let ProvisionalCacheEntry { stack_depth, with_coinductive_stack, with_inductive_stack } = - entry; - assert!( - stack_depth.is_some() - || with_coinductive_stack.is_some() - || with_inductive_stack.is_some() - ); - - if let &Some(stack_depth) = stack_depth { - assert_eq!(stack[stack_depth].input, input); - } - + for (&_input, entry) in &self.provisional_cache { + let ProvisionalCacheEntry { with_coinductive_stack, with_inductive_stack } = entry; + assert!(with_coinductive_stack.is_some() || with_inductive_stack.is_some()); let check_detached = |detached_entry: &DetachedEntry| { let DetachedEntry { head, result: _ } = *detached_entry; assert_ne!(stack[head].has_been_used, None);