Make StableHasher::finish return a small hash
#6
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| #[inline] | ||
| pub fn finish128(mut self) -> [u64; 2] { |
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Do we have an idea if this affects performance?
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I extracted finish128 method on Godbolt and look at the diff with the previous version and there was significant increase in "predicted cycles" and instructions, which was due to the ptr::write_bytes call generating a memcpy instead of a small instructions because the size wasn't a constant anymore.
I therefore changed that part (Godbolt) to copy the "last" and "last + 1" in a array as to have the size a constant as before and now the diff is very small, llvm-mca indicates 15100ins -> 15400ins for 100 iterations.
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Thanks for investigating!
Since this code is super hot in the compiler and the u64 version of the finish method probably isn't going to be used much, I'd prefer if we didn't regress things at all.
How about extracting a finish128_inner(nbuf: usize, buf: &mut [MaybeUninit<u64>; BUFFER_WITH_SPILL_CAPACITY], state: State, processed: usize) function? Then Hasher::finish could would only need to copy state and finish128 could keep taking self by value (?_
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I looked at the generated assembly on Godbolt and it's very very similar, mainly some registry naming changes, some instructions moving place and one more stack variable.
Compared to the other version (with the last and last+1 copy), it's roughly similar, in terms of instructions per llvm-mca.
One interesting thing, with the inner variant the number of cycles needed per llvm-mca is dropping significantly from 4410 to 3810, maybe because of less memory pressure. That seems like a win.
Pushed the inner variant.
michaelwoerister
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michaelwoerister
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This looks good to me. I'm wondering if we should have some kind of performance testing as part of this repo. But I'm not sure how to do that in a stable, useful way.
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| fn finish(&self) -> u64 { | ||
| panic!("SipHasher128 cannot provide valid 64 bit hashes") | ||
| let mut buf = self.buf.clone(); |
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Seems like we still need to clone the buffer 🤷
Hasher::finish is just defined in an unfortunate way.
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Thank you, @Urgau! |
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This PR changes
SipHasher128andStableHasherso that theirfinishimplementation are no longer fatal and return a "small hash".Implements #3 (comment)
r? @michaelwoerister