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From: edw@IUS1.CS.CMU.EDU (Eddie Wyatt)
Newsgroups: comp.unix.questions
Subject: Re: Wierd Compilers
Message-ID: <282@PT.CS.CMU.EDU>
Date: Mon, 2-Nov-87 11:38:24 EST
Article-I.D.: PT.282
Posted: Mon Nov  2 11:38:24 1987
Date-Received: Fri, 6-Nov-87 03:27:14 EST
References: <367@white.gcm>
Sender: netnews@PT.CS.CMU.EDU
Organization: Carnegie-Mellon University, CS/RI
Lines: 48
Keywords: Sun

In article <367@white.gcm>, dc@gcm (Dave Caswell) writes:
> 
> /* Program 1 */
> main()
> {
> 	int 	i = 500000;
> 
> 	while(--i);
> }
> 
> /* Program 2 */
> main()
> {
> 	int 	i = 500000;
> 
> 	while(i--);
> }
> 
> Does anyone have a guess why program one runs in half the time of program two?
> BTW this is a SUN 3.


Do a cc -S and you will see

Program one has an inter-loop like:

L14:
        subql   #0x1,a6@(-0x4)
        jeq     L15
        jra     L14

Program two has an inter-loop like:

L14:
        movl    a6@(-0x4),d0
        subql   #0x1,a6@(-0x4)
        tstl    d0
        jeq     L15
        jra     L14
L15:

  I actually go out of my way to write my loops like number one.

-- 

					Eddie Wyatt

e-mail: edw@ius1.cs.cmu.edu