Replace FxHash with AHash as the default hasher

Cargo.toml

@@ -13,6 +13,9 @@ edition = "2018"
 build = "build.rs"
 
 [dependencies]
+# For the default hasher
+ahash = { version = "0.2", optional = true }
+
 # For external trait impls
 rayon = { version = "1.0", optional = true }
 serde = { version = "1.0.25", default-features = false, optional = true }
@@ -34,7 +37,7 @@ serde_test = "1.0"
 doc-comment = "0.3.1"
 
 [features]
-default = []
+default = ["ahash"]
 nightly = []
 rustc-internal-api = []
 rustc-dep-of-std = ["nightly", "core", "compiler_builtins", "alloc", "rustc-internal-api"]

README.md

@@ -25,39 +25,57 @@ table implementation in the Rust standard library.
 ## Features
 
 - Drop-in replacement for the standard library `HashMap` and `HashSet` types.
-- Uses `FxHash` as the default hasher, which is much faster than SipHash.
-- Around 2x faster than `FxHashMap` and 8x faster than the standard `HashMap`.
+- Uses `AHash` as the default hasher, which is much faster than SipHash.
+- Around 2x faster than the previous standard library `HashMap`.
 - Lower memory usage: only 1 byte of overhead per entry instead of 8.
-- Compatible with `#[no_std]` (currently requires nightly for the `alloc` crate).
+- Compatible with `#[no_std]` (but requires a global allocator with the `alloc` crate).
 - Empty hash maps do not allocate any memory.
 - SIMD lookups to scan multiple hash entries in parallel.
 
 ## Performance
 
-Compared to the previous implementation of `std::collections::HashMap`:
+Compared to the previous implementation of `std::collections::HashMap` (Rust 1.35).
+
+With the hashbrown default AHash hasher:
 
 ```text
- name               stdhash ns/iter  hashbrown ns/iter  diff ns/iter    diff %  speedup
- find_existing      23,831           2,935                   -20,896   -87.68%   x 8.12
- find_nonexisting   25,326           2,283                   -23,043   -90.99%  x 11.09
- get_remove_insert  124              25                          -99   -79.84%   x 4.96
- grow_by_insertion  197              177                         -20   -10.15%   x 1.11
- hashmap_as_queue   72               18                          -54   -75.00%   x 4.00
- new_drop           14               0                           -14  -100.00%    x inf
- new_insert_drop    78               55                          -23   -29.49%   x 1.42
+ name                       oldstdhash ns/iter  hashbrown ns/iter  diff ns/iter   diff %  speedup 
+ insert_ahash_highbits        20,846              7,397                   -13,449  -64.52%   x 2.82 
+ insert_ahash_random          20,515              7,796                   -12,719  -62.00%   x 2.63 
+ insert_ahash_serial          21,668              7,264                   -14,404  -66.48%   x 2.98 
+ insert_erase_ahash_highbits  29,570              17,498                  -12,072  -40.83%   x 1.69 
+ insert_erase_ahash_random    39,569              17,474                  -22,095  -55.84%   x 2.26 
+ insert_erase_ahash_serial    32,073              17,332                  -14,741  -45.96%   x 1.85 
+ iter_ahash_highbits          1,572               2,087                       515   32.76%   x 0.75 
+ iter_ahash_random            1,609               2,074                       465   28.90%   x 0.78 
+ iter_ahash_serial            2,293               2,120                      -173   -7.54%   x 1.08 
+ lookup_ahash_highbits        3,460               4,403                       943   27.25%   x 0.79 
+ lookup_ahash_random          6,377               3,911                    -2,466  -38.67%   x 1.63 
+ lookup_ahash_serial          3,629               3,586                       -43   -1.18%   x 1.01 
+ lookup_fail_ahash_highbits   5,286               3,411                    -1,875  -35.47%   x 1.55 
+ lookup_fail_ahash_random     12,365              4,171                    -8,194  -66.27%   x 2.96 
+ lookup_fail_ahash_serial     4,902               3,240                    -1,662  -33.90%   x 1.51 
 ```
 
-Compared to `rustc_hash::FxHashMap` (standard `HashMap` using `FxHash` instead of `SipHash`):
+With the libstd default SipHash hasher:
 
 ```text
- name               fxhash ns/iter  hashbrown ns/iter  diff ns/iter    diff %  speedup
- find_existing      5,951           2,935                    -3,016   -50.68%   x 2.03
- find_nonexisting   4,637           2,283                    -2,354   -50.77%   x 2.03
- get_remove_insert  29              25                           -4   -13.79%   x 1.16
- grow_by_insertion  160             177                          17    10.62%   x 0.90
- hashmap_as_queue   22              18                           -4   -18.18%   x 1.22
- new_drop           9               0                            -9  -100.00%    x inf
- new_insert_drop    64              55                           -9   -14.06%   x 1.16
+ name                       oldstdhash ns/iter  hashbrown ns/iter  diff ns/iter   diff %  speedup 
+ insert_std_highbits        32,598              20,199                  -12,399  -38.04%   x 1.61 
+ insert_std_random          29,824              20,760                   -9,064  -30.39%   x 1.44 
+ insert_std_serial          33,151              17,256                  -15,895  -47.95%   x 1.92 
+ insert_erase_std_highbits  74,731              48,735                  -25,996  -34.79%   x 1.53 
+ insert_erase_std_random    73,828              47,649                  -26,179  -35.46%   x 1.55 
+ insert_erase_std_serial    73,864              40,147                  -33,717  -45.65%   x 1.84 
+ iter_std_highbits          1,518               2,264                       746   49.14%   x 0.67 
+ iter_std_random            1,502               2,414                       912   60.72%   x 0.62 
+ iter_std_serial            6,361               2,118                    -4,243  -66.70%   x 3.00 
+ lookup_std_highbits        21,705              16,962                   -4,743  -21.85%   x 1.28 
+ lookup_std_random          21,654              17,158                   -4,496  -20.76%   x 1.26 
+ lookup_std_serial          18,726              14,509                   -4,217  -22.52%   x 1.29 
+ lookup_fail_std_highbits   25,852              17,323                   -8,529  -32.99%   x 1.49 
+ lookup_fail_std_random     25,913              17,760                   -8,153  -31.46%   x 1.46 
+ lookup_fail_std_serial     22,648              14,839                   -7,809  -34.48%   x 1.53 
 ```
 
 ## Usage
@@ -80,7 +98,7 @@ map.insert(1, "one");
 
 This crate has the following Cargo features:
 
-- `nightly`: Enables nightly-only features: `no_std` support and `#[may_dangle]`.
+- `nightly`: Enables nightly-only features: `#[may_dangle]`.
 - `serde`: Enables serde serialization support.
 - `rayon`: Enables rayon parallel iterator support.
 

benches/bench.rs

@@ -1,5 +1,5 @@
 // This benchmark suite contains some benchmarks along a set of dimensions:
-//   Hasher: std default (SipHash) and crate default (FxHash).
+//   Hasher: std default (SipHash) and crate default (AHash).
 //   Int key distribution: low bit heavy, top bit heavy, and random.
 //   Task: basic functionality: insert, insert_erase, lookup, lookup_fail, iter
 #![feature(test)]
@@ -15,7 +15,7 @@ use std::collections::hash_map::RandomState;
 const SIZE: usize = 1000;
 
 // The default hashmap when using this crate directly.
-type FxHashMap<K, V> = HashMap<K, V, DefaultHashBuilder>;
+type AHashMap<K, V> = HashMap<K, V, DefaultHashBuilder>;
 // This uses the hashmap from this crate with the default hasher of the stdlib.
 type StdHashMap<K, V> = HashMap<K, V, RandomState>;
 
@@ -41,18 +41,22 @@ impl Iterator for RandomKeys {
 }
 
 macro_rules! bench_suite {
-    ($bench_macro:ident, $bench_fx_serial:ident, $bench_std_serial:ident,
-     $bench_fx_highbits:ident, $bench_std_highbits:ident,
-     $bench_fx_random:ident, $bench_std_random:ident) => {
-        $bench_macro!($bench_fx_serial, FxHashMap, 0..);
+    ($bench_macro:ident, $bench_ahash_serial:ident, $bench_std_serial:ident,
+     $bench_ahash_highbits:ident, $bench_std_highbits:ident,
+     $bench_ahash_random:ident, $bench_std_random:ident) => {
+        $bench_macro!($bench_ahash_serial, AHashMap, 0..);
         $bench_macro!($bench_std_serial, StdHashMap, 0..);
-        $bench_macro!($bench_fx_highbits, FxHashMap, (0..).map(usize::swap_bytes));
+        $bench_macro!(
+            $bench_ahash_highbits,
+            AHashMap,
+            (0..).map(usize::swap_bytes)
+        );
         $bench_macro!(
             $bench_std_highbits,
             StdHashMap,
             (0..).map(usize::swap_bytes)
         );
-        $bench_macro!($bench_fx_random, FxHashMap, RandomKeys::new());
+        $bench_macro!($bench_ahash_random, AHashMap, RandomKeys::new());
         $bench_macro!($bench_std_random, StdHashMap, RandomKeys::new());
     };
 }
@@ -61,56 +65,60 @@ macro_rules! bench_insert {
     ($name:ident, $maptype:ident, $keydist:expr) => {
         #[bench]
         fn $name(b: &mut Bencher) {
+            let mut m = $maptype::with_capacity_and_hasher(SIZE, Default::default());
             b.iter(|| {
-                let mut m = $maptype::default();
+                m.clear();
                 for i in ($keydist).take(SIZE) {
                     m.insert(i, i);
                 }
-                black_box(m);
+                black_box(&mut m);
             })
         }
     };
 }
 
 bench_suite!(
     bench_insert,
-    insert_fx_serial,
+    insert_ahash_serial,
     insert_std_serial,
-    insert_fx_highbits,
+    insert_ahash_highbits,
     insert_std_highbits,
-    insert_fx_random,
+    insert_ahash_random,
     insert_std_random
 );
 
 macro_rules! bench_insert_erase {
     ($name:ident, $maptype:ident, $keydist:expr) => {
         #[bench]
         fn $name(b: &mut Bencher) {
-            let mut m = $maptype::default();
-            let mut add_iter = $keydist;
-            for i in (&mut add_iter).take(SIZE) {
-                m.insert(i, i);
+            let mut base = $maptype::default();
+            for i in ($keydist).take(SIZE) {
+                base.insert(i, i);
             }
-            let mut remove_iter = $keydist;
+            let skip = $keydist.skip(SIZE);
             b.iter(|| {
+                let mut m = base.clone();
+                let mut add_iter = skip.clone();
+                let mut remove_iter = $keydist;
                 // While keeping the size constant,
                 // replace the first keydist with the second.
                 for (add, remove) in (&mut add_iter).zip(&mut remove_iter).take(SIZE) {
                     m.insert(add, add);
                     black_box(m.remove(&remove));
                 }
+                black_box(m);
             })
         }
     };
 }
 
 bench_suite!(
     bench_insert_erase,
-    insert_erase_fx_serial,
+    insert_erase_ahash_serial,
     insert_erase_std_serial,
-    insert_erase_fx_highbits,
+    insert_erase_ahash_highbits,
     insert_erase_std_highbits,
-    insert_erase_fx_random,
+    insert_erase_ahash_random,
     insert_erase_std_random
 );
 
@@ -134,11 +142,11 @@ macro_rules! bench_lookup {
 
 bench_suite!(
     bench_lookup,
-    lookup_fx_serial,
+    lookup_ahash_serial,
     lookup_std_serial,
-    lookup_fx_highbits,
+    lookup_ahash_highbits,
     lookup_std_highbits,
-    lookup_fx_random,
+    lookup_ahash_random,
     lookup_std_random
 );
 
@@ -163,11 +171,11 @@ macro_rules! bench_lookup_fail {
 
 bench_suite!(
     bench_lookup_fail,
-    lookup_fail_fx_serial,
+    lookup_fail_ahash_serial,
     lookup_fail_std_serial,
-    lookup_fail_fx_highbits,
+    lookup_fail_ahash_highbits,
     lookup_fail_std_highbits,
-    lookup_fail_fx_random,
+    lookup_fail_ahash_random,
     lookup_fail_std_random
 );
 
@@ -191,10 +199,10 @@ macro_rules! bench_iter {
 
 bench_suite!(
     bench_iter,
-    iter_fx_serial,
+    iter_ahash_serial,
     iter_std_serial,
-    iter_fx_highbits,
+    iter_ahash_highbits,
     iter_std_highbits,
-    iter_fx_random,
+    iter_ahash_random,
     iter_std_random
 );

ci/run.sh

@@ -21,6 +21,9 @@ fi
 
 export RUSTFLAGS="$RUSTFLAGS --cfg hashbrown_deny_warnings"
 
+# Make sure we can compile without the default hasher
+"${CARGO}" -vv check --target="${TARGET}" --no-default-features
+
 "${CARGO}" -vv test --target="${TARGET}"
 "${CARGO}" -vv test --target="${TARGET}" --features "${FEATURES}"