RE: [PATCH 4/5] crypto: lib/sha256 - Unroll SHA256 loop 8 times intead of 64

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

From: Arvind Sankar
> Sent: 20 October 2020 15:07
> To: David Laight <David.Laight@ACULAB.COM>
> 
> On Tue, Oct 20, 2020 at 07:41:33AM +0000, David Laight wrote:
> > From: Arvind Sankar> Sent: 19 October 2020 16:30
> > > To: Herbert Xu <herbert@gondor.apana.org.au>; David S. Miller <davem@davemloft.net>; linux-
> > > crypto@vger.kernel.org
> > > Cc: linux-kernel@vger.kernel.org
> > > Subject: [PATCH 4/5] crypto: lib/sha256 - Unroll SHA256 loop 8 times intead of 64
> > >
> > > This reduces code size substantially (on x86_64 with gcc-10 the size of
> > > sha256_update() goes from 7593 bytes to 1952 bytes including the new
> > > SHA256_K array), and on x86 is slightly faster than the full unroll.
> >
> > The speed will depend on exactly which cpu type is used.
> > It is even possible that the 'not unrolled at all' loop
> > (with the all the extra register moves) is faster on some x86-64 cpu.
> 
> Yes, I should have mentioned this was tested on a Broadwell Xeon, at
> least on that processor, no unrolling is a measurable performance loss.
> But the hope is that 8x unroll should be generally enough unrolling that
> 64x is unlikely to speed it up more, and so has no advantage over 8x.

(I've just looked at the actual code, not just the patch.)

Yes I doubt the 64x unroll was ever a significant gain.
Unrolling completely requires a load of register moves/renames;
probably too many to be 'zero cost'.

With modern cpu you can often get the loop control instructions
'for free' so unrolling just kills the I-cache.
Some of the cpu have loop buffers for decoded instructions,
unroll beyond that size and you suddenly get the decoder costs
hitting you again.

...
> > If you can put SHA256_K[] and W[] into a struct then the
> > compiler can use the same register to address into both
> > arrays (using an offset of 64*4 for the second one).
> > (ie keep the two arrays, not an array of struct).
> > This should reduce the register pressure slightly.
> 
> I can try that, could copy the data in sha256_update() so it's amortized
> over the whole input.

Having looked more closely the extra copy needed is probably
bigger than any saving.

What that code needs is some special 3-input instructions :-)
It would work a lot better written in VHDL for an fpga.

	David

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