[LCP]Address of a label

[linuxcprogramming@lists.linux.org.au (Chirag Kantharia)]

On Fri, Feb 22, 2002 at 11:38:51AM +0000, David Spencer wrote:
| The only disadvantage to a switch that I can think of is that I *think* 
| it is implemented as an if/else if ladder, so for a switch statement 
| with 200 equally probably cases you will have on average 100 comparisons 
| per bytecode, which could be a little inefficient.  An optimising 
| compiler might spot the fact that it is sequential and implement a 
| computed goto and/or jumptable but you'd have to examine the assembler 
| output to be sure.

Hi David,

Thanx for your mail. Yes, what you say is true. The above is the only
disadvantage of using a switch(), and that's exactly, what made me think
of alternatives. I'm not advocating usage of gotos, but I also think,
that, there might be very few situations, where goto might be just more
efficient.

But as you say, an optimizing compiler might spot this sequential cases,
and implement a jumptable, instead of a switch(), which is what I found
out yesterday. At least on Linux, gcc -S for a huge switch, reports only
one cmp. I'm yet to check on other platforms.

| A computed goto equivalent could be done with an array of structs:
| struct bytecodes
| {
|   void (*func)();
| } multiplexer[200];

Yes. I'd thought of this as well. I'd written about it, in my mail.

chyrag.
-- 
Chirag Kantharia, symonds.net/~chyrag/
Linux scrooge 2.4.17 #1 Wed Jan 16 17:07:25 IST 2002 i686 unknown