Re: crc32() optimization

Wolfgang Denk
2002-11-11T01:07:23Z

In message <20021111013114.GB27214@buici.com> you wrote:
>
> > > What's "Duff's Device"?
> >
> > It's a tricky way to implement general loop unrolling directly in C.
> > Applied to your problem, code that looks like this (instead of 8 any
> > other loop count may be used, but you need to adjust the "case"
> > statements then):
> >
> > register int n = (len + (8-1)) / 8;
> >
> > switch (len % 8) {
> > case 0: do { val = crc32_table ... ;
> > case 7: val = crc32_table ... ;
> > case 6: val = crc32_table ... ;
> > case 5: val = crc32_table ... ;
> > case 4: val = crc32_table ... ;
> > case 3: val = crc32_table ... ;
> > case 2: val = crc32_table ... ;
> > case 1: val = crc32_table ... ;
> > } while (--n > 0);
> > }
>
> This doesn't look right to me. You are decrementing n but using the
> modulus of len in the switch. The len modulus is correct when n == 1,
> but not when n > 1. The idea makes sense, but the implementation
> appears to be missing a detail.

You don't understand. The switch is only needed for the first,
partial loop where we want less than N statements; then we're nunning
the remaining fully unrolled loos in the do{}while loop.

> As for performance problems, I believe that the trouble is evident
> from the assembler output. The reason that the unrolled loop is more
> efficient than the simple loop is mainly because you don't jump as
> often. We all know that jumps tend to perturb the instruction fetch
> queue and cache.

Did you enable optimization?

> Here is an example. I know it is wrong, but it shows how the compiler
> implements the switch. I've marked the problem jump. It is executed

Irrelevant here.

> So, the version I posted may not be significantly better that the
> original one that unrolled 6 times. It is just that unrolling to a

You save the extra while{} loop.

Best regards,

Wolfgang Denk

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