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

From: Artur Skawina <[email-masked]>
Date: Fri, 07 Aug 2009 02:53:53 +0200

Linus Torvalds wrote:
> 
> Just out of curiosity, does anything change if you change the
> 
> 	B = SHA_ROR(B,2)
> 
> into a
> 
> 	B = SHA_ROR(SHA_ROR(B,1),1)
> 
> instead? It's very possible that it becomes _much_ worse, but I guess it's 

Did try that yesterday, didn't help. Will recheck now.. yep:

before: linus          0.3554       171.7
after:  linus           0.407         150

still true for the current version.

> So optimizing for P4 is often the wrong thing.
> 
> Secondly, P4's are going away. You may have one, but they are getting 
> rare. So optimizing for them is a losing proposition in the long run.

Sure, no argument; it's just that avoiding the P4 pitfalls is usually
not that hard and the impact on other, non-netburst, archs is low.
There are a lot of P4s out there and they're not going away soon.
(i'm still keeping most of my git trees on a P3...)

For generic C code such as this the difference for your i7 was -2% and
+70% for my P4; all the other (but one, i think) optimizations which
worked on P4 also applied to 32-bit i7. As i happen to have a p4 i can
just as well test the code on it, many improvements will likely apply
to other cpus too. That's all, i doubt anybody seriously considered
"optimizing for P4"; there is a reason intel discontinued them :)

The atom is a more important target, but only the asm versions did well
there so far.

artur