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From: rolfe@dsuvax.uucp (Timothy J. Rolfe)
Newsgroups: alt.folklore.urban,comp.lang.c,alt.folklore.computers
Subject: Beautiful side-effects! (q = q++)
Message-ID: <1991Jun26.154734.14439@dsuvax.uucp>
Date: 26 Jun 91 15:47:34 GMT
References: <902@adimail.UUCP> <S9958KB@taronga.hackercorp.com> <7079@gara.une.oz.au> <15520@exodus.Eng.Sun.COM> <1991Jun25.151408.1024@ux1.cso.uiuc.edu>
Organization: Dakota State University
Lines: 60

Beautify, beautiful side effect!!

As it stands, "q = q++;" SHOULD leave q unchanged:  evaluating the
right hand side we get a value (q BEFORE the increment).  That is
the value that is supposed to go to the left hand side.  The question
is WHEN the post-increment is done.

On a MicroVAX-II under Ultrix we get it BOTH ways, depending on whether
we use the cc, gcc, or vcc compiler --- plain cc, GNU's cc, and VAX
native C.

We can even go better:

        main()
        {
           int q=5, inc=10;
           void kludge();
        
           kludge (q, inc);
        }
        
        void kludge(q, inc)
        int q, inc;
        {
           q = q++ + inc;
           printf ("%d\n", q);
        }

The extra level is to get around the #@$%#@ vcc compiler's optimizing
q = q++ + inc; into a simple q = 15; (since it knows the values!).

cc- and gcc-compiled versions generate an answer of 16; vcc-compiled
version generates an answer of 15.

gcc-generated machine code for kludge:

.globl _kludge
_kludge:
	.word 0x0
	addl2 8(ap),4(ap)      ; <== calculated straight into q
	incl 4(ap)             ; <== then do the post-increment
	pushl 4(ap)            ; Assorted stuff for printf
	pushab LC0
	calls $2,_printf
	ret

vcc-generated machine code for kludge:

kludge:
	.entry	kludge,^m<>
	movl	4(ap),r2       ; <== get the q value
	incl	4(ap)          ; <== then do the post-increment	
	addl3	8(ap),r2,4(ap) ; <== finally calculated into q
	pushl	4(ap)          ; Assorted stuff for printf
	pushal	$CHAR_STRING_CONSTANTS
	calls	#2,printf
	ret	

                                                        --- Tim Rolfe
                                             <a.k.a. ROLFE@SDNET.BITNET>