Disturbing C: interleaved Switch and While

[King Harold]

Instead of:

Code: Select all

do {
    *to = *from++;
} while (--count > 0);

this: (automatic loop unrolling)

Code: Select all

    register n = (count + 7) / 8;
    switch (count % 8) {
    case 0: do { *to = *from++;
    case 7:      *to = *from++;
    case 6:      *to = *from++;
    case 5:      *to = *from++;
    case 4:      *to = *from++;
    case 3:      *to = *from++;
    case 2:      *to = *from++;
    case 1:      *to = *from++;
               } while (--n > 0);
    }

I find it.. disturbing.
So, what's everyone else's opinion?
(btw, this is called "Duff's device")

[Spencer]

I like it. Why stop there? 16 cases = $$$

These days, I think I'll stick with -funroll-loops. There my have been a time when these "manual C optimizations" where beneficial, but now they would just confuse your compiler.

[Dwedit]

I like how that code writes repeatedly to the same destination address.

[CoBB]

I like how that code writes repeatedly to the same destination address.

It’s a memory mapped register.

A much nicer example of abusing switch and C macros is implementing coroutines. I think Contiki uses something similar for its protothreads.

[King Harold]

I like it. Why stop there? 16 cases = $$$

These days, I think I'll stick with -funroll-loops. There my have been a time when these "manual C optimizations" where beneficial, but now they would just confuse your compiler.

It could end up doing the same
Or it could just use 2 loops with the second one doing the "rest" (but that's boring)

edit: those coroutine macro's scare me a bit..

[Dwedit]

The best method for practical programming is to just call memset or memcpy, those functions are ASM optimized like crazy.

[King Harold]

They update their 'to' pointer as well though (or even only their to pointer in the case of memset) so they do something different

[Timendus]

A much nicer example of abusing switch and C macros is implementing coroutines. I think Contiki uses something similar for its protothreads.

You are correct, it's exactely the same principle... I still think it's creepy though