Re: [PATCH v2] siphash: add cryptographically secure hashtable function

[Jason A. Donenfeld]

Hey Linus,

On Mon, Dec 12, 2016 at 5:01 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> The above is extremely inefficient. Considering that most kernel data
> would be expected to be smallish, that matters (ie the usual benchmark
> would not be about hashing megabytes of data, but instead millions of
> hashes of small data).
>
> I think this could be rewritten (at least for 64-bit architectures) as
>
>     #ifdef CONFIG_DCACHE_WORD_ACCESS
>
>         if (left)
>                 b |= le64_to_cpu(load_unaligned_zeropad(data) &
> bytemask_from_count(left));
>
>     #else
>
>         .. do the duff's device thing with the switch() ..
>
>     #endif
>
> which should give you basically perfect code generation (ie a single
> 64-bit load and a byte mask).

I modified the test to hash data of size 0 through 7 repeatedly
100000000 times, and benchmarked that a few times on a Skylake laptop.
The `load_unaligned_zeropad & bytemask_from_count` version was
consistently 7% slower.

I then modified it again to simply hash a 4 byte constant repeatedly
1000000000 times. The `load_unaligned_zeropad & bytemask_from_count`
version was around 6% faster. I tried again with a 7 byte constant and
got more or less a similar result.

Then I tried with a 1 byte constant, and found that the
`load_unaligned_zeropad & bytemask_from_count` version was slower.

So, it would seem that between the `if (left)` and the `switch
(left)`, there's the same number of branches. But for small values of
`left`, the duff's device just has simpler arithmetic, whereas for
large values of `left`, the `load_unaligned_zeropad` prevails. If
micro-optimization is really appealing, one could imagine a hybrid of
the two:

    switch (left) {
    case 7:
    case 6:
    case 5:
    case 4:
        b |= le64_to_cpu(load_unaligned_zeropad(data) &
bytemask_from_count(left));
        break;
    case 3: b |= ((u64)data[2]) << 16;
    case 2: b |= ((u64)data[1]) <<  8;
    case 1: b |= ((u64)data[0]); break;
    case 0: break;
    }

But I'm not sure this complication is worth it, and it might be more
likely that the left-over size is 4 bytes most of the time, so we
should just use your trick on platforms that support it.

Jason