From 1aceaaa969d85195966e453590cbfc68771bf904 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <nnethercote@mozilla.com>
Date: Fri, 1 Nov 2019 08:52:14 +1100
Subject: Avoid hashing the key twice in `get_query()`.

For a single-threaded parallel compiler, this reduces instruction counts
across several benchmarks, by up to 2.8%.

The commit also adds documentation about `Sharded`'s use of `FxHasher`.
---
 src/librustc_data_structures/sharded.rs | 6 ++++++
 1 file changed, 6 insertions(+)

(limited to 'src/librustc_data_structures')

diff --git a/src/librustc_data_structures/sharded.rs b/src/librustc_data_structures/sharded.rs
index 2f972eeccdc..a28a5e0f041 100644
--- a/src/librustc_data_structures/sharded.rs
+++ b/src/librustc_data_structures/sharded.rs
@@ -60,6 +60,7 @@ impl<T> Sharded<T> {
         }
     }
 
+    /// The shard is selected by hashing `val` with `FxHasher`.
     #[inline]
     pub fn get_shard_by_value<K: Hash + ?Sized>(&self, val: &K) -> &Lock<T> {
         if SHARDS == 1 {
@@ -69,6 +70,11 @@ impl<T> Sharded<T> {
         }
     }
 
+    /// Get a shard with a pre-computed hash value. If `get_shard_by_value` is
+    /// ever used in combination with `get_shard_by_hash` on a single `Sharded`
+    /// instance, then `hash` must be computed with `FxHasher`. Otherwise,
+    /// `hash` can be computed with any hasher, so long as that hasher is used
+    /// consistently for each `Sharded` instance.
     #[inline]
     pub fn get_shard_by_hash(&self, hash: u64) -> &Lock<T> {
         let hash_len = mem::size_of::<usize>();
-- 
cgit 1.4.1-3-g733a5