Rollup merge of rust-lang#37229 - nnethercote:FxHasher, r=nikomatsakis

Replace FNV with a faster hash function.

Hash table lookups are very hot in rustc profiles and the time taken within `FnvHash` itself is a big part of that. Although FNV is a simple hash, it processes its input one byte at a time. In contrast, Firefox has a homespun hash function that is also simple but works on multiple bytes at a time. So I tried it out and the results are compelling:

```
futures-rs-test  4.326s vs  4.212s --> 1.027x faster (variance: 1.001x, 1.007x)
helloworld       0.233s vs  0.232s --> 1.004x faster (variance: 1.037x, 1.016x)
html5ever-2016-  5.397s vs  5.210s --> 1.036x faster (variance: 1.009x, 1.006x)
hyper.0.5.0      5.018s vs  4.905s --> 1.023x faster (variance: 1.007x, 1.006x)
inflate-0.1.0    4.889s vs  4.872s --> 1.004x faster (variance: 1.012x, 1.007x)
issue-32062-equ  0.347s vs  0.335s --> 1.035x faster (variance: 1.033x, 1.019x)
issue-32278-big  1.717s vs  1.622s --> 1.059x faster (variance: 1.027x, 1.028x)
jld-day15-parse  1.537s vs  1.459s --> 1.054x faster (variance: 1.005x, 1.003x)
piston-image-0. 11.863s vs 11.482s --> 1.033x faster (variance: 1.060x, 1.002x)
regex.0.1.30     2.517s vs  2.453s --> 1.026x faster (variance: 1.011x, 1.013x)
rust-encoding-0  2.080s vs  2.047s --> 1.016x faster (variance: 1.005x, 1.005x)
syntex-0.42.2   32.268s vs 31.275s --> 1.032x faster (variance: 1.014x, 1.022x)
syntex-0.42.2-i 17.629s vs 16.559s --> 1.065x faster (variance: 1.013x, 1.021x)
```

(That's a stage1 compiler doing debug builds. Results for a stage2 compiler are similar.)

The attached commit is not in a state suitable for landing because I changed the implementation of FnvHasher without changing its name (because that would have required touching many lines in the compiler). Nonetheless, it is a good place to start discussions.

Profiles show very clearly that this new hash function is a lot faster to compute than FNV. The quality of the new hash function is less clear -- it seems to do better in some cases and worse in others (judging by the number of instructions executed in `Hash{Map,Set}::get`).

CC @brson, @arthurprs

Author: eddyb

src/librustc/dep_graph/dep_tracking_map.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 use hir::def_id::DefId;
-use rustc_data_structures::fnv::FnvHashMap;
+use rustc_data_structures::fx::FxHashMap;
 use std::cell::RefCell;
 use std::ops::Index;
 use std::hash::Hash;
@@ -24,7 +24,7 @@ use super::{DepNode, DepGraph};
 pub struct DepTrackingMap<M: DepTrackingMapConfig> {
     phantom: PhantomData<M>,
     graph: DepGraph,
-    map: FnvHashMap<M::Key, M::Value>,
+    map: FxHashMap<M::Key, M::Value>,
 }
 
 pub trait DepTrackingMapConfig {
@@ -38,7 +38,7 @@ impl<M: DepTrackingMapConfig> DepTrackingMap<M> {
         DepTrackingMap {
             phantom: PhantomData,
             graph: graph,
-            map: FnvHashMap()
+            map: FxHashMap()
         }
     }
 

src/librustc/dep_graph/edges.rs

@@ -8,15 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use rustc_data_structures::fnv::{FnvHashMap, FnvHashSet};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use std::fmt::Debug;
 use std::hash::Hash;
 use super::{DepGraphQuery, DepNode};
 
 pub struct DepGraphEdges<D: Clone + Debug + Eq + Hash> {
     nodes: Vec<DepNode<D>>,
-    indices: FnvHashMap<DepNode<D>, IdIndex>,
-    edges: FnvHashSet<(IdIndex, IdIndex)>,
+    indices: FxHashMap<DepNode<D>, IdIndex>,
+    edges: FxHashSet<(IdIndex, IdIndex)>,
     open_nodes: Vec<OpenNode>,
 }
 
@@ -46,8 +46,8 @@ impl<D: Clone + Debug + Eq + Hash> DepGraphEdges<D> {
     pub fn new() -> DepGraphEdges<D> {
         DepGraphEdges {
             nodes: vec![],
-            indices: FnvHashMap(),
-            edges: FnvHashSet(),
+            indices: FxHashMap(),
+            edges: FxHashSet(),
             open_nodes: Vec::new()
         }
     }