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

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

On Fri, 7 Aug 2009, Artur Skawina wrote:
> 
> Actually that's even more of a reason to make sure the code doesn't suck :)
> The difference on less perverse cpus will usually be small, but on P4 it
> can be huge.

No. First off, the things you have to do on P4 are just insane. See the 
email I just sent out asking you to test whether two 1-bit rotates might 
be faster than 1 2-bit rotate.

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.

> A few years back I found my old ip checksum microbenchmark, and when I ran
> it on a P4 (prescott iirc) i didn't believe my eyes. The straightforward 
> 32-bit C implementation was running circles around the in-kernel one...
> And a few tweaks to the assembler version got me another ~100% speedup.[1]

Yeah, not very surprising. The P4 is very good at the simplest possible 
kind of code that does _nothing_ fancy.

But then it completely chokes on some code. I mean _totally_. It slows 
down by a huge amount if there is anything but the most trivial kinds of 
instructions. And by "trivial", I mean _really_ trivial. Shifts (as in 
SHA1), but iirc also things like "adc" (add with carry) etc.

So it's not hard to write code that works well on other uarchs, and then 
totally blow up on P4. I think it doesn't rename the flags at all, so any 
flag dependency (carry being the most common one) will stall things 
horrible.

There's also a very subtle store forwarding failure thing (and a lot of 
other events) that causes a nasty micro-architectural replay trap, and 
again you go from "running like a bat out of hell" to "slower than a i486 
at a tenth the frequency".

Really. It's disgusting. Perfectly fine code can run really slowly on the 
P4 just because it hits some random internal micro-architectural flaw. And 
there's a _lot_ of those "glass jaw" issues.

The best way to avoid them is to use _only_ simple ALU instructions (add, 
sub, and/or/not), and to be _very_ careful with loads and stores. 

		Linus