From e60c5c1a77a4bc6a2540437969ec0dc6e213d712 Mon Sep 17 00:00:00 2001 From: Zalathar Date: Thu, 18 Jul 2024 22:11:09 +1000 Subject: [PATCH] Split out `test_remaining_match_pairs_after_or` Only the last candidate can possibly have more match pairs, so this can be separate from the main or-candidate postprocessing loop. --- .../rustc_mir_build/src/build/matches/mod.rs | 101 ++++++++++-------- 1 file changed, 58 insertions(+), 43 deletions(-) diff --git a/compiler/rustc_mir_build/src/build/matches/mod.rs b/compiler/rustc_mir_build/src/build/matches/mod.rs index d18e59759e2..8bba9329085 100644 --- a/compiler/rustc_mir_build/src/build/matches/mod.rs +++ b/compiler/rustc_mir_build/src/build/matches/mod.rs @@ -1598,6 +1598,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> { for subcandidate in candidate.subcandidates.iter_mut() { expanded_candidates.push(subcandidate); } + // Note that the subcandidates have been added to `expanded_candidates`, + // but `candidate` itself has not. If the last candidate has more match pairs, + // they are handled separately by `test_remaining_match_pairs_after_or`. } else { // A candidate that doesn't start with an or-pattern has nothing to // expand, so it is included in the post-expansion list as-is. @@ -1613,12 +1616,20 @@ impl<'a, 'tcx> Builder<'a, 'tcx> { expanded_candidates.as_mut_slice(), ); - // Simplify subcandidates and process any leftover match pairs. - for candidate in candidates_to_expand { + // Postprocess subcandidates, and process any leftover match pairs. + // (Only the last candidate can possibly have more match pairs.) + debug_assert!({ + let mut all_except_last = candidates_to_expand.iter().rev().skip(1); + all_except_last.all(|candidate| candidate.match_pairs.is_empty()) + }); + for candidate in candidates_to_expand.iter_mut() { if !candidate.subcandidates.is_empty() { - self.finalize_or_candidate(span, scrutinee_span, candidate); + self.finalize_or_candidate(candidate); } } + if let Some(last_candidate) = candidates_to_expand.last_mut() { + self.test_remaining_match_pairs_after_or(span, scrutinee_span, last_candidate); + } remainder_start.and(remaining_candidates) } @@ -1642,8 +1653,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> { candidate.subcandidates[0].false_edge_start_block = candidate.false_edge_start_block; } - /// Simplify subcandidates and process any leftover match pairs. The candidate should have been - /// expanded with `create_or_subcandidates`. + /// Simplify subcandidates and remove `is_never` subcandidates. + /// The candidate should have been expanded with `create_or_subcandidates`. /// /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like /// so: @@ -1695,50 +1706,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> { /// | /// ... /// ``` - fn finalize_or_candidate( - &mut self, - span: Span, - scrutinee_span: Span, - candidate: &mut Candidate<'_, 'tcx>, - ) { + fn finalize_or_candidate(&mut self, candidate: &mut Candidate<'_, 'tcx>) { if candidate.subcandidates.is_empty() { return; } self.merge_trivial_subcandidates(candidate); self.remove_never_subcandidates(candidate); - - if !candidate.match_pairs.is_empty() { - let or_span = candidate.or_span.unwrap_or(candidate.extra_data.span); - let source_info = self.source_info(or_span); - // If more match pairs remain, test them after each subcandidate. - // We could add them to the or-candidates before the call to `test_or_pattern` but this - // would make it impossible to detect simplifiable or-patterns. That would guarantee - // exponentially large CFGs for cases like `(1 | 2, 3 | 4, ...)`. - let mut last_otherwise = None; - candidate.visit_leaves(|leaf_candidate| { - last_otherwise = leaf_candidate.otherwise_block; - }); - let remaining_match_pairs = mem::take(&mut candidate.match_pairs); - candidate.visit_leaves(|leaf_candidate| { - assert!(leaf_candidate.match_pairs.is_empty()); - leaf_candidate.match_pairs.extend(remaining_match_pairs.iter().cloned()); - let or_start = leaf_candidate.pre_binding_block.unwrap(); - let otherwise = - self.match_candidates(span, scrutinee_span, or_start, &mut [leaf_candidate]); - // In a case like `(P | Q, R | S)`, if `P` succeeds and `R | S` fails, we know `(Q, - // R | S)` will fail too. If there is no guard, we skip testing of `Q` by branching - // directly to `last_otherwise`. If there is a guard, - // `leaf_candidate.otherwise_block` can be reached by guard failure as well, so we - // can't skip `Q`. - let or_otherwise = if leaf_candidate.has_guard { - leaf_candidate.otherwise_block.unwrap() - } else { - last_otherwise.unwrap() - }; - self.cfg.goto(otherwise, source_info, or_otherwise); - }); - } } /// Try to merge all of the subcandidates of the given candidate into one. This avoids @@ -1814,6 +1788,47 @@ impl<'a, 'tcx> Builder<'a, 'tcx> { } } + /// If more match pairs remain, test them after each subcandidate. + /// We could have added them to the or-candidates during or-pattern expansion, but that + /// would make it impossible to detect simplifiable or-patterns. That would guarantee + /// exponentially large CFGs for cases like `(1 | 2, 3 | 4, ...)`. + fn test_remaining_match_pairs_after_or( + &mut self, + span: Span, + scrutinee_span: Span, + candidate: &mut Candidate<'_, 'tcx>, + ) { + if candidate.match_pairs.is_empty() { + return; + } + + let or_span = candidate.or_span.unwrap_or(candidate.extra_data.span); + let source_info = self.source_info(or_span); + let mut last_otherwise = None; + candidate.visit_leaves(|leaf_candidate| { + last_otherwise = leaf_candidate.otherwise_block; + }); + let remaining_match_pairs = mem::take(&mut candidate.match_pairs); + candidate.visit_leaves(|leaf_candidate| { + assert!(leaf_candidate.match_pairs.is_empty()); + leaf_candidate.match_pairs.extend(remaining_match_pairs.iter().cloned()); + let or_start = leaf_candidate.pre_binding_block.unwrap(); + let otherwise = + self.match_candidates(span, scrutinee_span, or_start, &mut [leaf_candidate]); + // In a case like `(P | Q, R | S)`, if `P` succeeds and `R | S` fails, we know `(Q, + // R | S)` will fail too. If there is no guard, we skip testing of `Q` by branching + // directly to `last_otherwise`. If there is a guard, + // `leaf_candidate.otherwise_block` can be reached by guard failure as well, so we + // can't skip `Q`. + let or_otherwise = if leaf_candidate.has_guard { + leaf_candidate.otherwise_block.unwrap() + } else { + last_otherwise.unwrap() + }; + self.cfg.goto(otherwise, source_info, or_otherwise); + }); + } + /// Pick a test to run. Which test doesn't matter as long as it is guaranteed to fully match at /// least one match pair. We currently simply pick the test corresponding to the first match /// pair of the first candidate in the list.