Initial value of `FxHasher` leads to avoidable collisions

[eggyal]

It's easy to see that FxHasher { hash: 0 }.add_to_hash(0) is a fixed point (i.e. does not result in any change to the internal state self.hash):

rustc-hash/src/lib.rs

Lines 76 to 81 in 5e09ea0

	impl FxHasher {
	#[inline]
	fn add_to_hash(&mut self, i: usize) {
	self.hash = self.hash.rotate_left(5).bitxor(i).wrapping_mul(K);
	}
	}

In particular 0.rotate_left(5).bitxor(0).wrapping_mul(K) == 0.

Therefore initialising FxHasher with a hash value of 0 is a bit undesirable, as sequences of 0s (written to the hash before any non-zero value) cannot be distinguished by their hashes.

rustc-hash/src/lib.rs

Lines 69 to 74 in 5e09ea0

	impl Default for FxHasher {
	#[inline]
	fn default() -> FxHasher {
	FxHasher { hash: 0 }
	}
	}

Of course, there's always the possibility that a value of self.hash.rotate_left(5) could be written to the hasher, which will result in the internal state reaching 0—but in practice writing values of 0 can arise quite often, and choosing some other initial value would be quite beneficial? Perhaps even K?

[eggyal]

Of course, resolving #15 would make this moot.

[workingjubilee]

@Nilstrieb Should this be closed?

[FeldrinH]

I would say that this should not be closed. While there are now API-s for setting initial seeds manually or randomly it would still be a good idea to change the default seed to avoid collisions in cases where the default FxHasher is used (for example the FxHashMap type alias).

[workingjubilee]

Ah. Very well. PRs welcome!

[WaffleLapkin]

I wanted to check some different values on rustc, to know if some values miraculously make it faster, but hadn't gotten to it yet

[nnethercote]

I previously tried changing the initial value away from zero, along with lots of other things, see here for details.

The strength of FxHasher lies entirely in its incredible speed at hashing small inputs, and the value zero contributes to that. Consider the main part of the code:

#[inline]
fn default() -> FxHasher {
    FxHasher { hash: 0 }
}

const K: usize = 0x517cc1b727220a95;

#[inline]
fn add_to_hash(&mut self, i: usize) {
    self.hash = self.hash.rotate_left(5).bitxor(i).wrapping_mul(K);
}

#[inline]
fn finish(&self) -> u64 {
    self.hash as u64
}

If you are hashing a single integer, it's just three operations: rol, xor, mul. Except the rol is on the initial hash value, which is zero, so the result of that rol is zero. And then xor'ing i onto zero just gives i. And then you multiply. All that can be constant folded away by the compiler and the actual hashing operation is just to multiply i by K, which is about the fastest hash function imaginable.

If you change the initial value away from zero you can probably constant fold the rol but not the xor, so it becomes two instructions instead of one.

[WaffleLapkin]

@nnethercote have you tried the starting constant other than 1?

The fact that 0 allows to constant fold more is interesting, although I'm surprised it actually makes a difference 🤔

[workingjubilee]

I wonder if using usize::MAX bears any fruit.

[nnethercote]

I don't remember if I tried anything other than 1. I do remember trying many different things to improve FxHasher performance and none of them working, enough that I have sworn off trying to improve it further. (That's kind of the point of the blog post.)

I wonder if using usize::MAX bears any fruit.

Non-tiny integers might increase binary size.

[workingjubilee]

Mmmh. I don't think so in this case, as usize::MAX is a bit special: it's the all-1s bitpattern.

[the8472]

I previously tried changing the initial value away from zero, along with lots of other things, see here for details.

Out of curiosity, do you know why it's xor and not wrapping add? According to instruction tables they have mostly the same latency and throughput (varies slightly between processors) and I'd naively assume that the carries in the add would help a little with the quality.

[nnethercote]

Out of curiosity, do you know why it's xor and not wrapping add?

My best guess is that xor is a something of a traditional choice?

FxHasher was copied from Firefox's source code, see here for some design discussion, though it doesn't touch upon the use of xor.