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From: molly@killer.UUCP
Newsgroups: comp.lang.c
Subject: Re: goto's and switch statements -- mild proposal
Message-ID: <1335@killer.UUCP>
Date: Fri, 14-Aug-87 04:42:05 EDT
Article-I.D.: killer.1335
Posted: Fri Aug 14 04:42:05 1987
Date-Received: Sun, 16-Aug-87 08:42:27 EDT
References: <855@tjalk.cs.vu.nl> <2683@hoptoad.uucp> <916@haddock.ISC.COM> <1032@ius1.cs.cmu.edu>
Organization: The Unix(tm) Connection, Dallas, Texas
Lines: 67
Summary: The label table already has plenty of info about the case labels

In article <1032@ius1.cs.cmu.edu>, edw@ius1.cs.cmu.edu (Eddie Wyatt) writes:
> In article <916@haddock.ISC.COM>, karl@haddock.ISC.COM (Karl Heuer) writes:
> > In article <2683@hoptoad.uucp> gnu@hoptoad.uucp (John Gilmore) writes:
> > >What bothers me is that I can't say ["goto default" and "goto case N"].
> 
>    Consider the problems "goto case N" cause when there are two switch
> statements each with a case N label in the function block.
> 
>    Also just consider how you would implement the two switch statements
> each with a case N label.  Since each label is a posible target of 
> a goto, it must be put into a label table for that function block.
> Now you have  multiple defined labels (label N) unless you do a few
> ugly things to uniquely define each label - but then look up on
> labels become more expensive.

The case statement labels already have to go into a label table to keep track
of what values have been entered so the default can be handled correctly.
Some compilers generate more than one type of code to handle a switch.  The
first variety of code is the

	cmpl	#value,d0
	beq	label

and the next is the word array (kinda maybe like ...)

	subl	#smallest,d0
	cmpl	#largest-smallest,d0
	bgt	default
	addl	d0,d0
	movw	6(pc,d0),d0
	jmp	2(pc,d0:w)

.word	label1
.word	labelN

and so on.  You can't discard the switch label info until the `}' that
closes the switch statement if you are generating code in this fashion.
(I invent a label that will appear after the last code from the switch
and jump there so I can defer the descision - I always hold onto my
switch tables)

The big bastard in this plan is the multiple labels.  You could add a
flag `Number of default: labels in this block' and allow goto default;
as a statement.  Or you might even allow it to be syntactically legal
inside of a switch.  Now that would be useful, except it's too easy
already.  But then, the goto case N is also easy -

	N = function to get value;
goto_case_N;
	switch (N) {
		.
		.
		.
	}

	N = some new value;
	goto goto_case_N;

Now that's an example of bogus code.

Molly

-- 
       Molly Fredericks       UUCP: { any place real }!ihnp4!killer!molly
    Disclaimer:  Neither me, nor my cat, had anything to do with any of this
  "I love giving my cat a bath, except for all those hairs I get on my tongue"
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