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 11:23:52PM +0000, David Laight wrote:
> 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).

I know it's wrong, the point is that it's impossible to gain very much
from optimizing the LOAD loop because it doesn't account for much of the
total time.

> 
> 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.
> 

I am not sure about what's going on inside the hardware, but even with
a straightforward assembly version that just reads out of memory the way
the calculation is specified, unrolling the BLEND loop 8x speeds up the
performance by 7-8%.

The compiler is actually pretty bad here, just translating everything
into assembler with no attempt to optimize anything gets a 10-12%
speedup over the C version.