Re: cleaner/better zlib sources?

[Davide Libenzi <davidel@xmailserver.org>]

On Thu, 15 Mar 2007, Davide Libenzi wrote:

> On Thu, 15 Mar 2007, Linus Torvalds wrote:
> 
> > 
> > I looked at git profiles yesterday, and some of them are pretty scary. We 
> > spend about 50% of the time under some loads in just zlib uncompression, 
> > and when I actually looked closer at the zlib sources I can kind of 
> > understand why. That thing is horrid.
> > 
> > The sad part is that it looks like it should be quite possible to make 
> > zlib simply just perform better. The profiles seem to say that a lot of 
> > the cost is literally in the "inflate()" state machine code (and by that I 
> > mean *not* the code itself, but literally in the indirect jump generated 
> > by the case-statement).
> > 
> > Now, on any high-performance CPU, doing state-machines by having
> > 
> > 	for (;;)
> > 		switch (data->state) {
> > 			...
> > 			data->state = NEW_STATE;
> > 			continue;
> > 		}
> > 
> > (which is what zlib seems to be doing) is just about the worst possible 
> > way to code things.
> 
> A quick hack would be to just define:
> 
> #define SWITCH_LBL(n) \
> 	case n: \
> 	lbl_##n:
> 
> #define STATE_CHANGE(s) \
> 	state->mode = s; \
> 	goto lbl_##s;
> 
> Then replace all the "state->mode = STATE; break;" into STATE_CHANGE(STATE);
> I'm giving it a try as we speak ...

I get about 5-6% boost with it AFAICS ...



- Davide