Add fast path for match checking

compiler/rustc_mir_build/src/thir/pattern/_match.rs

@@ -276,7 +276,7 @@ use self::Usefulness::*;
 use self::WitnessPreference::*;
 
 use rustc_data_structures::captures::Captures;
-use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_index::vec::Idx;
 
 use super::{compare_const_vals, PatternFoldable, PatternFolder};
@@ -504,13 +504,27 @@ impl<'p, 'tcx> FromIterator<&'p Pat<'tcx>> for PatStack<'p, 'tcx> {
     }
 }
 
+/// Depending on the match patterns, the specialization process might be able to use a fast path.
+/// Tracks whether we can use the fast path and the lookup table needed in those cases.
+#[derive(Clone, Debug)]
+enum SpecializationCache {
+    /// Patterns consist of only enum variants.
+    Variants { lookup: FxHashMap<DefId, SmallVec<[usize; 1]>>, wilds: SmallVec<[usize; 1]> },
+    /// Does not belong to the cases above, use the slow path.
+    Incompatible,
+}
+
 /// A 2D matrix.
 #[derive(Clone)]
-crate struct Matrix<'p, 'tcx>(Vec<PatStack<'p, 'tcx>>);
+crate struct Matrix<'p, 'tcx> {
+    patterns: Vec<PatStack<'p, 'tcx>>,
+    cache: SpecializationCache,
+}
 
 impl<'p, 'tcx> Matrix<'p, 'tcx> {
     crate fn empty() -> Self {
-        Matrix(vec![])
+        // Use SpecializationCache::Incompatible as a placeholder; the initialization is in push().
+        Matrix { patterns: vec![], cache: SpecializationCache::Incompatible }
     }
 
     /// Pushes a new row to the matrix. If the row starts with an or-pattern, this expands it.
@@ -522,18 +536,65 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
                 self.push(row)
             }
         } else {
-            self.0.push(row);
+            if self.patterns.is_empty() {
+                self.cache = if row.is_empty() {
+                    SpecializationCache::Incompatible
+                } else {
+                    match *row.head().kind {
+                        PatKind::Variant { .. } => SpecializationCache::Variants {
+                            lookup: FxHashMap::default(),
+                            wilds: SmallVec::new(),
+                        },
+                        // Note: If the first pattern is a wildcard, then all patterns after that is not
+                        // useful. The check is simple enough so we treat it as the same as unsupported
+                        // patterns.
+                        _ => SpecializationCache::Incompatible,
+                    }
+                };
+            }
+            let idx_to_insert = self.patterns.len();
+            match &mut self.cache {
+                SpecializationCache::Variants { ref mut lookup, ref mut wilds } => {
+                    let head = row.head();
+                    match *head.kind {
+                        _ if head.is_wildcard() => {
+                            for (_, v) in lookup.iter_mut() {
+                                v.push(idx_to_insert);
+                            }
+                            wilds.push(idx_to_insert);
+                        }
+                        PatKind::Variant { adt_def, variant_index, .. } => {
+                            lookup
+                                .entry(adt_def.variants[variant_index].def_id)
+                                .or_insert_with(|| wilds.clone())
+                                .push(idx_to_insert);
+                        }
+                        _ => {
+                            self.cache = SpecializationCache::Incompatible;
+                        }
+                    }
+                }
+                SpecializationCache::Incompatible => {}
+            }
+            self.patterns.push(row);
         }
     }
 
     /// Iterate over the first component of each row
     fn heads<'a>(&'a self) -> impl Iterator<Item = &'a Pat<'tcx>> + Captures<'p> {
-        self.0.iter().map(|r| r.head())
+        self.patterns.iter().map(|r| r.head())
     }
 
     /// This computes `D(self)`. See top of the file for explanations.
     fn specialize_wildcard(&self) -> Self {
-        self.0.iter().filter_map(|r| r.specialize_wildcard()).collect()
+        match &self.cache {
+            SpecializationCache::Variants { wilds, .. } => {
+                wilds.iter().filter_map(|&i| self.patterns[i].specialize_wildcard()).collect()
+            }
+            SpecializationCache::Incompatible => {
+                self.patterns.iter().filter_map(|r| r.specialize_wildcard()).collect()
+            }
+        }
     }
 
     /// This computes `S(constructor, self)`. See top of the file for explanations.
@@ -543,10 +604,31 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
         constructor: &Constructor<'tcx>,
         ctor_wild_subpatterns: &Fields<'p, 'tcx>,
     ) -> Matrix<'p, 'tcx> {
-        self.0
-            .iter()
-            .filter_map(|r| r.specialize_constructor(cx, constructor, ctor_wild_subpatterns))
-            .collect()
+        match &self.cache {
+            SpecializationCache::Variants { lookup, wilds } => {
+                if let Constructor::Variant(id) = constructor {
+                    lookup
+                        .get(id)
+                        .unwrap_or(&wilds)
+                        .iter()
+                        .filter_map(|&i| {
+                            self.patterns[i].specialize_constructor(
+                                cx,
+                                constructor,
+                                ctor_wild_subpatterns,
+                            )
+                        })
+                        .collect()
+                } else {
+                    unreachable!()
+                }
+            }
+            SpecializationCache::Incompatible => self
+                .patterns
+                .iter()
+                .filter_map(|r| r.specialize_constructor(cx, constructor, ctor_wild_subpatterns))
+                .collect(),
+        }
     }
 }
 
@@ -568,7 +650,7 @@ impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "\n")?;
 
-        let &Matrix(ref m) = self;
+        let Matrix { patterns: m, .. } = self;
         let pretty_printed_matrix: Vec<Vec<String>> =
             m.iter().map(|row| row.iter().map(|pat| format!("{:?}", pat)).collect()).collect();
 
@@ -1824,7 +1906,7 @@ crate fn is_useful<'p, 'tcx>(
     is_under_guard: bool,
     is_top_level: bool,
 ) -> Usefulness<'tcx> {
-    let &Matrix(ref rows) = matrix;
+    let Matrix { patterns: rows, .. } = matrix;
     debug!("is_useful({:#?}, {:#?})", matrix, v);
 
     // The base case. We are pattern-matching on () and the return value is
@@ -2266,7 +2348,7 @@ fn split_grouped_constructors<'p, 'tcx>(
                 // `borders` is the set of borders between equivalence classes: each equivalence
                 // class lies between 2 borders.
                 let row_borders = matrix
-                    .0
+                    .patterns
                     .iter()
                     .flat_map(|row| {
                         IntRange::from_pat(tcx, param_env, row.head()).map(|r| (r, row.len()))