Re: cleaner/better zlib sources?

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