From mboxrd@z Thu Jan  1 00:00:00 1970
From: Davide Libenzi <davidel@xmailserver.org>
Subject: Re: cleaner/better zlib sources?
Date: Thu, 15 Mar 2007 18:33:44 -0700 (PDT)
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To: Linus Torvalds <torvalds@linux-foundation.org>
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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 ...




- Davide