phc-discussions - Using multiply (Re: [PHC] A must read...)

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Date: Wed, 15 Jan 2014 21:02:46 -0600 (CST)
From: Steve Thomas <steve@...tu.com>
To: discussions@...sword-hashing.net
Subject: Using multiply (Re: [PHC] A must read...)

> On January 15, 2014 at 11:45 AM Bill Cox <waywardgeek@...il.com> wrote:
>
> I like the idea of floating point, but I doubt it's worth the excess
> trouble we'll run into. 32x32 -> 32 Integer multiply seems solid and
> pervasive enough. Besides that, it's fast in our devices even
> compared to a custom ASIC, and it's a great operation for mixing bits,
> at least when one op is odd.

Have you thought about doing 4 (or more) multiplies in parallel:
4 multiplies with SSE4.1 (PMULLD _mm_mullo_epi32)
8 multiplies with AVX2 (VPMULLD _mm256_mullo_epi32)
16 multiplies with AVX-512 (VPMULLD _mm512_mullo_epi32)

You can reorder the values in any order in SSE2 with PSHUFD
(_mm_shuffle_epi32). Reordering the values in AVX2 and AVX-512 is
trickier and may need multiple instructions.

SSE4.1 has been on pretty much every Intel CPU since 2008. AVX2
just came out last year with Haswell. I think integer AVX-512 will be on
Skylake in 2015. They could delay integer operations until the next
iteration in 2017 like they did with AVX/AVX2. AVX-512 should probably
be considered since this competition will end when AVX-512 is
estimated to be available or on the horizon.
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