Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!van-bc!rsoft!mindlink!a563
From: a563@mindlink.UUCP (Dave Kirsch)
Newsgroups: comp.lang.c
Subject: Re: problems/risks due to programming language, stories requ
Message-ID: <1310@mindlink.UUCP>
Date: 15 Mar 90 01:10:16 GMT
Organization: MIND LINK! - British Columbia, Canada
Lines: 78

> raymond writes:
> 
> Msg-ID: <1990Mar15.205909.7882@agate.berkeley.edu>
> Posted: 15 Mar 90 20:59:09 GMT
> 
> Org.  : U.C. Berkeley
> Person: Raymond Chen
> 
> In article <1306@mindlink.UUCP> a563@mindlink.UUCP (Dave Kirsch) writes:
> >C flaws?  Do this in Pascal:
> >
> >switch (i) {
> >  case 2 :  /* Add 2 to j */
> >    j++;
> >  case 1 :  /* Add 1 to j */
> >    j++;
> >  case 0 :  /* Print j out */
> >    printf("%d\n", j);
> >    break;
> >  default :
> >    printf("Illegal value in switch.\n");
> >    abort();
> >}
> 
> Sure
> 
>         case i of
>           2 : begin j := j + 1; goto 1; end;
>           1 : begin
> 1:                  j := j + 1; goto 0; end;
>           0 : begin
> 0:                  writeln(j); end;
>           else begin writeln('Illegal value in case.'); Halt; end;
>         end;
> 
> Remember:  C's switch statement is a thinly-disguised computed goto.
>   If you're going to use goto's you may as well use them clearly
>   and explicitly.
> 
> I never use fallthrough%.  If I need fallthrough, I make the
> fallthrough EXPLICIT via goto's.  That way EVERYBODY knows what I'm
> doing:  You, me, and lint.
> 
> And of course it incurs absolutely NO performance penalty.  (The
> compiler can't cache register values across "case" labels since
> they are just labels for the computed goto.)  The only possible
> penalty is from a compiler which lacks a peephole optimizer.  (You
> just have to elide the redundant jump statement.)
> --
> % Well, rarely.  I'll use fallthrough when writing my entries into the
>   International Obfuscated C Code Contest.

Yuck!  Let's do a bit on readability, that Pascal code is gross!  Aye, it
works, but doesn't do much for readability.  I can scan through the C code in
one pass and see what it does, I have to look at the Pascal example you have
there a few times just to figure out what your doing.  Fall through DOES have
it's benefits, eg if I have a 100 line code piece in a case, but the case
before wants to do 10 lines before it, I don't want to copy the whole 100 lines
over to the other case [though I could, as most compilers (TC and MSC do) will
optimize and one compile one copy].  Jumping between cases with goto's doesn't
agree with MY programming style.  Only time I EVER use goto is when I want to
jump out of two or more inner loops, rather than using a flag variable.

I guess it's a choice of taste, although the C code is still more effecient
than the Pascal code, as the C code 'falls through', and the Pascal code
generates two uncessary jumps with the gotos.

The fall through is NOT a computed goto.  The actual code is linear.  The C
compiler I use makes a jump table for each of the cases, loads up the correct
spot to jump to, and then jumps there.  Once it's jumped to the 'case 2:' it
will execute 'j++;' then fall through to the 'case 1:'.  There is no jump
between them.  When you put a break in, it generates a jump.

--
_____________________________________________________________________
Dave Kirsch           UUCP: {uunet,ubc-cs}!van-bc!rsoft!mindlink!a563
Voice: (604) 327-4404       a563@mindlink.UUCP
                      Vancouver, British Columbia