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 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.

Unrolling the LOAD one might - but you don't need 8 times,
once should be more than enough.
The LOAD loop needs a memory read, memory write and BSWAP per iteration.
The loop control is add + compare + jmp.
On sandy bridge and later the compare and jmp become a single u-op.
So the loop has the read, write (can happen together) and 3 other u-ops.
That won't run at 1 clock per iteration on Sandy Bridge.
However just unroll once and you need 4 non-memory u-op per loop iteration.
That might run at 2 clocks per 8 bytes.

Fiddling the loop to remove the compare (ie run from -64 to 0)
should merge the 'add' and 'jnz' into a single u-op.
That might be enough to get the 'rolled up' loop to run in 1 clock
on sandy bridge, certainly on slightly later cpu.

That is theoretical for intel cpu sandy bridge onwards.
I've an i7-7700 (Kaby Lake?) that I belive has an extra
instruction pipeline and might run the initial loop in 1 clock.

I don't have any recent AMD cpu, nor any ARM or PPC ones.
But fully out-of-order cpu are likely to be similar.

One of the other test systems I've got is an Atom C2758.
This 8 core but mostly in-order.
Running sha256_transform() on that tend to give one of two
TSC counts, one of which is double the other!
That is pretty consistent even for 100 iterations.

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.

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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