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Date: Wed, 26 Feb 2014 16:18:26 -0500
From: Bill Cox <>
Subject: Re: [PHC] multiply-hardening (Re: NoelKDF ready for submission)

On Wed, Feb 26, 2014 at 12:22 PM, Bill Cox <> wrote:
> I'm going to reintegrate multiplication back into the memory hashing
> threads and eliminate the multiplication hardening thread.  At least
> for Haswell, a single scalar multiply and XOR seem to run nicely in
> parallel with AVX2 memory hashing, even at L1 hashing speeds.
> I'm going to make an option for between 0 and 8 multiplications per
> 256-bits of memory hashing.  0 would be useful for applications where
> multipliers are very slow or not available and have to be emulated, or
> if the CPU has no multiple instruction issue capability, meaning any
> multiplies add directly to user's runtime.  1 seems like a good match
> for AVX2 running in L1 cache, and I'm guessing 2 will be good for SSE.
>  For hashing into external memory, up to 8 seems reasonable.

I've succeeded!

It took some crazy optimization for Haswell, but I now get 0 to 8
scalar multiplies per inner loop of memory hashing - 32 bytes worth
(just 3 AVX2 instructions in the body of a for loop), without slowing
down the memory hashing unless I set the number of multiplications too
high.  For running out of L1 cache, Haswell is so fast, it didn't have
time for even 1 multiplication in the loop, and an if-statement around
the multiplication loop slowed it down too much.  So, wrote a wrapper
function around the block hash algorithm with a switch statement on
the number of multiplies, and I passed a constant to hashBlocks in
each case.  The compiler was smart enough to in-line the multiplies
the correct number of times, with 9 different versions of the
subroutine.  Now 0 multiplies runs as fast as before I integrated the
multiplication loop into hashBlocks.


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