From 69760c78cff87f2a106ea6bbb5a810e994e453dc Mon Sep 17 00:00:00 2001
From: SpecificProtagonist <specificprotagonist@posteo.org>
Date: Sat, 13 Jan 2024 14:26:43 +0100
Subject: [PATCH] Skip rehashing TypeIds (#11268)

# Objective

`TypeId` contains a high-quality hash. Whenever a lookup based on a
`TypeId` is performed (e.g. to insert/remove components), the hash is
run through a second hash function. This is unnecessary.

## Solution

Skip re-hashing `TypeId`s.

In my
[testing](https://gist.github.com/SpecificProtagonist/4b49ad74c6b82b0aedd3b4ea35121be8),
this improves lookup performance consistently by 10%-15% (of course, the
lookup is only a small part of e.g. a bundle insertion).
---
 crates/bevy_ecs/src/lib.rs | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/crates/bevy_ecs/src/lib.rs b/crates/bevy_ecs/src/lib.rs
index ef0dd07a86..bd0a9f3463 100644
--- a/crates/bevy_ecs/src/lib.rs
+++ b/crates/bevy_ecs/src/lib.rs
@@ -54,7 +54,30 @@ pub mod prelude {
 pub use bevy_utils::all_tuples;
 
 /// A specialized hashmap type with Key of [`TypeId`]
-type TypeIdMap<V> = rustc_hash::FxHashMap<TypeId, V>;
+type TypeIdMap<V> =
+    std::collections::HashMap<TypeId, V, std::hash::BuildHasherDefault<NoOpTypeIdHasher>>;
+
+#[doc(hidden)]
+#[derive(Default)]
+struct NoOpTypeIdHasher(u64);
+
+// TypeId already contains a high-quality hash, so skip re-hashing that hash.
+impl std::hash::Hasher for NoOpTypeIdHasher {
+    fn finish(&self) -> u64 {
+        self.0
+    }
+
+    fn write(&mut self, _bytes: &[u8]) {
+        // This will never be called: TypeId always just calls write_u64 once!
+        // This is unlikely to ever change, but as it isn't officially guaranteed,
+        // panicking will let us detect this as fast as possible.
+        unimplemented!("Hashing of std::any::TypeId changed");
+    }
+
+    fn write_u64(&mut self, i: u64) {
+        self.0 = i;
+    }
+}
 
 #[cfg(test)]
 mod tests {