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

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Date:   Tue, 20 Oct 2020 10:07:26 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Arvind Sankar' <nivedita@...m.mit.edu>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] crypto: lib/sha256 - Unroll SHA256 loop 8 times
 intead of 64

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@...dor.apana.org.au>; David S. Miller <davem@...emloft.net>; linux-
> > crypto@...r.kernel.org
> > Cc: linux-kernel@...r.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.

> 
> > 
> > Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> > ---
> >  lib/crypto/sha256.c | 164 ++++++++------------------------------------
> >  1 file changed, 28 insertions(+), 136 deletions(-)
> > 
> > diff --git a/lib/crypto/sha256.c b/lib/crypto/sha256.c
> > index c6bfeacc5b81..9f0b71d41ea0 100644
> > --- a/lib/crypto/sha256.c
> > +++ b/lib/crypto/sha256.c
> > @@ -18,6 +18,17 @@
> >  #include <crypto/sha.h>
> >  #include <asm/unaligned.h>
> ...
> > 
> > +#define SHA256_ROUND(i, a, b, c, d, e, f, g, h) do {		\
> > +	u32 t1, t2;						\
> > +	t1 = h + e1(e) + Ch(e, f, g) + SHA256_K[i] + W[i];	\
> > +	t2 = e0(a) + Maj(a, b, c);    d += t1;    h = t1 + t2;	\
> 
> Split to 3 lines.

This was the way the code was before, but I can reformat it, sure.

> 
> 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.

> 
> 	David
> 
> -
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