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From: Eddie.Wyatt@ius2.cs.cmu.edu.UUCP
Newsgroups: comp.lang.c
Subject: Re: jumptables
Message-ID: <1063@ius2.cs.cmu.edu>
Date: Wed, 11-Mar-87 15:15:03 EST
Article-I.D.: ius2.1063
Posted: Wed Mar 11 15:15:03 1987
Date-Received: Fri, 13-Mar-87 00:19:46 EST
References: <9929@sri-spam.istc.sri.com> <649@vu-vlsi.UUCP>
Sender: edw@ius2.cs.cmu.edu
Organization: Carnegie-Mellon University, CS/RI
Lines: 97


   To follow is the assemble language generated by a C compiler running
on a Vax 780 (Unix os of course) for a case statement.


LL0:
	.data
	.text
	.align	1
	.globl	_main
_main:
	.word	L12
	jbr 	L14
L15:
	movl	-4(fp),r0
	jbr 	L17
L18:
	movl	-12(fp),-8(fp)
	jbr 	L16
L19:
	movl	-16(fp),-20(fp)
	jbr 	L16
L20:
	movl	-20(fp),-16(fp)
	jbr 	L16
L21:
	movl	-8(fp),-16(fp)
	jbr 	L16
L22:
	movl	-8(fp),-24(fp)
	jbr 	L16
L23:
	movl	-24(fp),-16(fp)
	jbr 	L16
L17:
	casel	r0,$0,$5
L24:
	.word	L18-L24
	.word	L19-L24
	.word	L20-L24
	.word	L21-L24
	.word	L22-L24
	.word	L23-L24
L25:
L16:
	ret
	.set	L12,0x0
L14:
	subl2	$24,sp
	jbr 	L15
	.data


Original code

main()
    {
    int c;
    int h, k, j, i, m;

    switch (c)
	{
        case 0:
	    h = k;
	    break;
        case 1:
	    i = j;
	    break;
        case 2:
	    j = i;
	    break;
        case 3:
	    j  = h;
	    break;
        case 4:
	    m  = h;
	    break;
        case 5:
	    j  = m;
	    break;
	}
    }


Note that the vax has a special case instruction.  

  The answer to your question of why you can't have jump tables is
that it is up to the C compiler to make such optimizations that may be machine
depended.
  May I also ask have you checked the code that your C compiler has generated
to make sure it is not generating direct threaded code for case statements?
As I have been told, the C compiler for the Suns does.


					Eddie Wyatt
					Research Programming (CMU)
					edw@ius1.cs.cmu.edu