RE: [PATCH v4 6/6] crypto: lib/sha256 - Unroll LOAD and BLEND loops

[David Laight <David.Laight@ACULAB.COM>]

From: Arvind Sankar
> Sent: 25 October 2020 20:18
> 
> On Sun, Oct 25, 2020 at 06:51:18PM +0000, David Laight wrote:
> > From: Arvind Sankar
> > > Sent: 25 October 2020 14:31
> > >
> > > Unrolling the LOAD and BLEND loops improves performance by ~8% on x86_64
> > > (tested on Broadwell Xeon) while not increasing code size too much.
> >
> > I can't believe unrolling the BLEND loop makes any difference.
> 
> It's actually the BLEND loop that accounts for almost all of the
> difference. The LOAD loop doesn't matter much in general: even replacing
> it with a plain memcpy() only increases performance by 3-4%. But
> unrolling it is low cost in code size terms, and clang actually does it
> without being asked.

(memcpy is wrong - misses the byte swaps).

That's odd, the BLEND loop is about 20 instructions.
I wouldn't expect unrolling to help - unless you manage
to use 16 registers for the active W[] values.

> > WRT patch 5.
> > On the C2758 the original unrolled code is slightly faster.
> > On the i7-7700 the 8 unroll is a bit faster 'hot cache',
> > but slower 'cold cache' - probably because of the d-cache
> > loads for K[].
> >
> > Non-x86 architectures might need to use d-cache reads for
> > the 32bit 'K' constants even in the unrolled loop.
> > X86 can use 'lea' with a 32bit offset to avoid data reads.
> > So the cold-cache case for the old code may be similar.
> 
> Not sure I follow: in the old code, the K's are 32-bit immediates, so
> they should come from the i-cache whether an add or an lea is used?

I was thinking of other instruction sets that end up using pc-relative
addressing for constants.
Might only happen for 64bint ones though.

> Why is the cold-cache case relevant anyway? If the code is only being
> executed a couple of times or so, i.e. you're hashing a single say
> 64-128 byte input once in a blue moon, the performance of the hash
> doesn't really matter, no?

I was measuring the cold cache one because I could.
I didn't note the actual figures but it was 8-10 times slower
that the hot-cache case.
While sha256 is likely to be run hot-cache (on a big buffer)
the cold-cache timing are probably relevant for things like memcpy().
I remember seeing a very long divide function for sparc32 that
was probably only a gain in a benchmark loop - it would have
displaced a lot of the working set from the i-cache!

	David

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