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Speed up DefaultHasher, SipHasher, and SipHasher13. #69152

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Speed up DefaultHasher, SipHasher, and SipHasher13. #69152

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Commits on Feb 23, 2020

  1. Optimize sip::Hasher::short_write. 

    This commit changes `sip::Hasher` to use the faster `short_write` approach
that was used for `SipHasher128` in rust-lang#68914.

This has no effect because `sip::Hasher::short_write` is currently
unused. See the next commit for more details, and a fix.

(One difference with rust-lang#68914 is that this commit doesn't apply the
`u8to64_le` change from that PR, because I found it is slower, because
it introduces `memcpy` calls with non-statically-known lengths.
Therefore, this commit also undoes the `u8to64_le` change in
`SipHasher128` for this reason. This doesn't affect `SipHasher128` much
because it doesn't use `u8to64_le` much, but I am making the change to
keep the two implementations consistent.)
    @nnethercote
    nnethercote committed Feb 23, 2020
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  2. Add write_* methods for DefaultHasher, SipHasher, SipHasher13… 

    …, `sip::Hasher`.

`sip::Hasher::short_write` is currently unused. It is called by
`sip::Hasher::write_{u8,usize}`, but those methods are also unused,
because `DefaultHasher`, `SipHasher` and `SipHasher13` don't implement
any of the `write_xyz` methods, so all their write operations end up
calling `sip::Hasher::write`.

I confirmed this by inserting a `panic!` in `sip::Hasher::short_write`
and running the tests -- they all passed.

This commit adds all the `write_xyz` methods to `DefaultHasher`,
`SipHasher`, `SipHasher13` and `sip::Hasher`, which means that
`short_write` will be used for all of them.

(I haven't properly implemented 128-bit writes in `sip::Hasher` and
`SipHasher128`. That could be a follow-up if something thinks it is
important.)

Some microbenchmarks I wrote show that this commit speeds up default
hashing of integers by about 2.5x, and can speed up hash tables that use
`DefaultHasher` and have keys that contain integers by up to 30%, though
the exact amount depends heavily on the details of the keys and how the
table is used. This makes default `Hash{Map,Set}` much more likely to be
competitive with `FxHash{Map,Set}`.
    @nnethercote
    nnethercote committed Feb 23, 2020
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