Implement LazyBTreeMap and use it in a few places.

This is a thin wrapper around BTreeMap that avoids allocating upon
creation. It speeds up some rustc-perf benchmarks by up to 3.6%.

2 files changed, 109 insertions, 0 deletions

diff --git a/src/librustc_data_structures/lazy_btree_map.rs b/src/librustc_data_structures/lazy_btree_map.rs
new file mode 100644
index 00000000000..74f91af10fe
--- /dev/null
+++ b/src/librustc_data_structures/lazy_btree_map.rs
@@ -0,0 +1,108 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::collections::btree_map;
+use std::collections::BTreeMap;
+
+/// A thin wrapper around BTreeMap that avoids allocating upon creation.
+///
+/// Vec, HashSet and HashMap all have the nice feature that they don't do any
+/// heap allocation when creating a new structure of the default size. In
+/// contrast, BTreeMap *does* allocate in that situation. The compiler uses
+/// B-Tree maps in some places such that many maps are created but few are
+/// inserted into, so having a BTreeMap alternative that avoids allocating on
+/// creation is a performance win.
+///
+/// Only a fraction of BTreeMap's functionality is currently supported.
+/// Additional functionality should be added on demand.
+#[derive(Debug)]
+pub struct LazyBTreeMap<K, V>(Option<BTreeMap<K, V>>);
+
+impl<K, V> LazyBTreeMap<K, V> {
+    pub fn new() -> LazyBTreeMap<K, V> {
+        LazyBTreeMap(None)
+    }
+
+    pub fn iter(&self) -> Iter<K, V> {
+        Iter(self.0.as_ref().map(|btm| btm.iter()))
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.as_ref().map_or(true, |btm| btm.is_empty())
+    }
+}
+
+impl<K: Ord, V> LazyBTreeMap<K, V> {
+    fn instantiate(&mut self) -> &mut BTreeMap<K, V> {
+        if let Some(ref mut btm) = self.0 {
+            btm
+        } else {
+            let btm = BTreeMap::new();
+            self.0 = Some(btm);
+            self.0.as_mut().unwrap()
+        }
+    }
+
+    pub fn insert(&mut self, key: K, value: V) -> Option<V> {
+        self.instantiate().insert(key, value)
+    }
+
+    pub fn entry(&mut self, key: K) -> btree_map::Entry<K, V> {
+        self.instantiate().entry(key)
+    }
+
+    pub fn values<'a>(&'a self) -> Values<'a, K, V> {
+        Values(self.0.as_ref().map(|btm| btm.values()))
+    }
+}
+
+impl<K: Ord, V> Default for LazyBTreeMap<K, V> {
+    fn default() -> LazyBTreeMap<K, V> {
+        LazyBTreeMap::new()
+    }
+}
+
+impl<'a, K: 'a, V: 'a> IntoIterator for &'a LazyBTreeMap<K, V> {
+    type Item = (&'a K, &'a V);
+    type IntoIter = Iter<'a, K, V>;
+
+    fn into_iter(self) -> Iter<'a, K, V> {
+        self.iter()
+    }
+}
+
+pub struct Iter<'a, K: 'a, V: 'a>(Option<btree_map::Iter<'a, K, V>>);
+
+impl<'a, K: 'a, V: 'a> Iterator for Iter<'a, K, V> {
+    type Item = (&'a K, &'a V);
+
+    fn next(&mut self) -> Option<(&'a K, &'a V)> {
+        self.0.as_mut().and_then(|iter| iter.next())
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.as_ref().map_or_else(|| (0, Some(0)), |iter| iter.size_hint())
+    }
+}
+
+pub struct Values<'a, K: 'a, V: 'a>(Option<btree_map::Values<'a, K, V>>);
+
+impl<'a, K, V> Iterator for Values<'a, K, V> {
+    type Item = &'a V;
+
+    fn next(&mut self) -> Option<&'a V> {
+        self.0.as_mut().and_then(|values| values.next())
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.as_ref().map_or_else(|| (0, Some(0)), |values| values.size_hint())
+    }
+}
+
diff --git a/src/librustc_data_structures/lib.rs b/src/librustc_data_structures/lib.rs
index ba1d73dc268..154c086614c 100644
--- a/src/librustc_data_structures/lib.rs
+++ b/src/librustc_data_structures/lib.rs
@@ -56,6 +56,7 @@ pub mod bitvec;
 pub mod graph;
 pub mod indexed_set;
 pub mod indexed_vec;
+pub mod lazy_btree_map;
 pub mod obligation_forest;
 pub mod sip128;
 pub mod snapshot_map;