Date: Fri, 9 May 1997 02:50:01 -0700 (PDT) From: j@uriah.heep.sax.de (J Wunsch) To: freebsd-bugs Subject: Re: gnu/3554: cc failed on deafult <= default Message-ID: <199705090950.CAA24080@hub.freebsd.org>
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The following reply was made to PR gnu/3554; it has been noted by GNATS.
From: j@uriah.heep.sax.de (J Wunsch)
To: jin@iss-p1.lbl.gov
Cc: FreeBSD-gnats-submit@FreeBSD.ORG
Subject: Re: gnu/3554: cc failed on deafult <= default
Date: Fri, 9 May 1997 10:44:31 +0200
As Jin Guojun[ITG] wrote:
> >Description:
>
> switch (x) {
> case ...
> ...
> deafult: ... /* no error reported, but fails */
> }
This is legal C code. deafult: is just a label as any other case
labels, or a plain label as in:
for (i = 0; i < max; i++) {
someptr = &array[i];
defualt:
do_again:
...
if (some_condition) {
goto do_again; /* both statements are the same */
goto defualt;
}
...
}
You should get a warning about an unused label with -Wall.
Or to quote some more twisted example here, take Duff's device:
:Duff's device: n. The most dramatic use yet seen of {fall
through} in C, invented by Tom Duff when he was at Lucasfilm.
Trying to {bum} all the instructions he could out of an inner
loop that copied data serially onto an output port, he decided to
{unroll} it. He then realized that the unrolled version could
be implemented by *interlacing* the structures of a switch and
a loop:
register n = (count + 7) / 8; /* count > 0 assumed */
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);
}
Having verified that the device is valid portable C, Duff announced
it. C's default {fall through} in case statements has long been
its most controversial single feature; Duff observed that "This
code forms some sort of argument in that debate, but I'm not sure
whether it's for or against."
(Courtesy the Jargon file, of course.)
--
cheers, J"org
joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/ -- NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)
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