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

[Arvind Sankar <nivedita@alum.mit.edu>]

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.

> 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?

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?

> 
> Interestingly I had to write an asm ror32() to get reasonable
> code (in userspace). The C version the kernel uses didn't work.
> 
> 	David
> 
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