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

[Linus Torvalds <torvalds@linux-foundation.org>]

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.

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

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.

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.

		Linus