Duff's Device

I've been wondering sometimes:
Anyone know why Duff's device is usually written like this:

void duff(const char *str, int len) {
int n = (len + 7) / 8;
switch (len % 8) {
case 0: do{ foo(*str++);
case 7: foo(*str++);
case 6: foo(*str++);
...
case 1: foo(*str++);
} while (--n > 0);
}
}

instead of this?

void duff2(const char *str, int len) {
switch (len % 8) {
case 0: while ((len -= 8) >= 0) {
foo(*str++);
case 7: foo(*str++);
case 6: foo(*str++);
...
case 1: foo(*str++);
}
}
}

The original has an extra '+' and doesn't handle len=0.
Nor does it need the divide by 8, though I realize n-=1
may be cheaper than n-=8 on some architectures.
People have had 18 years to notice now:-)

--
Hallvard
.

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