Re: [PATCH 0/7] block-sha1: improved SHA1 hashing

[Artur Skawina <art.08.09@gmail.com>]

Linus Torvalds wrote:
> 
> On Thu, 6 Aug 2009, Artur Skawina wrote:
>> Does this make any difference for you? For me it's the best one so far
>> (the linusas2 number clearly shows that for me the register renaming does
>> nothing; other than that the functions should be very similar)
> 
> Nope. If anything, it's bit slower, but it might be in the noise. I 
> generally got 330MB/s with my "cpp renaming" on Nehalem (32-bit - the 
> 64-bit numbers are ~400MB/s), but with this I got 325MB/s twice in a row, 
> which matches the linusas2 numbers pretty exactly.
> 
> But it seems to make a big difference for you.

It seems to do well on P2 and P4 here, if it works for core2 this could
be a good generic candidate. It only does 62% on an Atom, but the best C
version so far exceeds it only by ~2%.

> Btw, _what_ P4 do you have (Northwood or Prescott)?

northwood

> The Intel optimization manuals very much talk about avoiding rotates. And 
> they mention "with a CPUID signature corresponding to family 15 and model 
> encoding of 0, 1, or 2" specifically as being longer latency. That's 
> basically pre-prescott P4, I think.

cpu family      : 15
model           : 2
model name      : Intel(R) Pentium(R) 4 CPU 2.80GHz
stepping        : 5

> Anyway, on P4 I think you have two double-speed integer issue ports (ie 
> max four ops per cycle), but only one of them takes a rotate, and only in 
> the first half of the cycle (ie just one shift per cycle).
> 
> And afaik, that is actually the _improved_ state in Prescott. The older 
> P4's didn't have a full shifter unit at all, iirc: shifts were "complex 
> instructions" in Northwood and weren't even single-clock.
> 
> In Core 2, I think there's still just one shifter unit, but at least it's 
> as fast as all the other units. So P4 really does stand out as sucking as 
> far as shifts are concerned, and if you have an older P4, it will be even 
> worse.

hmm, I might be able to try it on some old willamette, but my prescott's
mobo died, so i can't verify that right now.

I'll upload an updated sha1bench, maybe somebody else feels like checking...

artur