From: Arvind Sankar > Sent: 20 October 2020 15:07 > To: David Laight <David.Laight@xxxxxxxxxx> > > 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@xxxxxxxxxxxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>; linux- > > > crypto@xxxxxxxxxxxxxxx > > > Cc: linux-kernel@xxxxxxxxxxxxxxx > > > 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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
