Use fxhash in TypeIdMap. (#1119)

Relying on TypeId being some hash internally isn't future-proof because there is no guarantee about internal layout or structure of TypeId. I benchmarked TypeId noop hasher vs fxhash and found that there is very little difference.
Also fxhash is likely to be better supported because it is widely used in rustc itself.
[Benchmarks of hashers](#1097)
[Engine wide benchmarks](#1119 (comment))

Author: AngelicosPhosphoros

crates/bevy_ecs/Cargo.toml

@@ -20,6 +20,7 @@ trace = []
 bevy_tasks = { path = "../bevy_tasks", version = "0.4.0" }
 bevy_utils = { path = "../bevy_utils", version = "0.4.0" }
 bevy_ecs_macros = { path = "macros", version = "0.4.0" }
+fxhash = "0.2"
 rand = "0.7.3"
 serde = "1.0"
 thiserror = "1.0"

crates/bevy_ecs/src/core/archetype.rs

@@ -15,14 +15,12 @@
 // modified by Bevy contributors
 
 use crate::{AtomicBorrow, Component, Entity};
-use bevy_utils::AHasher;
 use bitflags::bitflags;
 use std::{
     alloc::{alloc, dealloc, Layout},
     any::{type_name, TypeId},
     cell::UnsafeCell,
     collections::HashMap,
-    hash::{BuildHasherDefault, Hasher},
     mem,
     ptr::{self, NonNull},
 };
@@ -557,44 +555,8 @@ fn align(x: usize, alignment: usize) -> usize {
 
 /// A hasher optimized for hashing a single TypeId.
 ///
-/// TypeId is already thoroughly hashed, so there's no reason to hash it again.
-/// Just leave the bits unchanged.
-#[derive(Default)]
-pub(crate) struct TypeIdHasher {
-    hash: u64,
-}
-
-impl Hasher for TypeIdHasher {
-    fn write_u64(&mut self, n: u64) {
-        // Only a single value can be hashed, so the old hash should be zero.
-        debug_assert_eq!(self.hash, 0);
-        self.hash = n;
-    }
-
-    // Tolerate TypeId being either u64 or u128.
-    fn write_u128(&mut self, n: u128) {
-        debug_assert_eq!(self.hash, 0);
-        self.hash = n as u64;
-    }
-
-    fn write(&mut self, bytes: &[u8]) {
-        debug_assert_eq!(self.hash, 0);
-
-        // This will only be called if TypeId is neither u64 nor u128, which is not anticipated.
-        // In that case we'll just fall back to using a different hash implementation.
-        let mut hasher = AHasher::default();
-        hasher.write(bytes);
-        self.hash = hasher.finish();
-    }
-
-    fn finish(&self) -> u64 {
-        self.hash
-    }
-}
-
-/// A HashMap with TypeId keys
-///
-/// Because TypeId is already a fully-hashed u64 (including data in the high seven bits,
-/// which hashbrown needs), there is no need to hash it again. Instead, this uses the much
-/// faster no-op hash.
-pub(crate) type TypeIdMap<V> = HashMap<TypeId, V, BuildHasherDefault<TypeIdHasher>>;
+/// We don't use RandomState from std or Random state from Ahash
+/// because fxhash is [proved to be faster](https://github.com/bevyengine/bevy/pull/1119#issuecomment-751361215)
+/// and we don't need Hash Dos attack protection here
+/// since TypeIds generated during compilation and there is no reason to user attack himself.
+pub(crate) type TypeIdMap<V> = HashMap<TypeId, V, fxhash::FxBuildHasher>;