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From: david@sun.UUCP
Newsgroups: comp.sys.m68k
Subject: Re: Incidentally ... (dbra loops)
Message-ID: <16360@sun.uucp>
Date: Wed, 8-Apr-87 13:55:27 EST
Article-I.D.: sun.16360
Posted: Wed Apr  8 13:55:27 1987
Date-Received: Sat, 11-Apr-87 07:41:54 EST
References: <362@sbcs.UUCP> <1466@ncr-sd.SanDiego.NCR.COM> <580@plx.UUCP> <6139@mimsy.UUCP> <75@eps2.UUCP>
Organization: Sun Microsystems, Mountain View
Lines: 51

In article <75@eps2.UUCP> jon@eps2.UUCP (Jonathan Hue) writes:
>On this Sun-3/160 running 3.2, the compiler won't generate the dbra.  However,
>the object code optimizer will.  But the only way I know of to find this out
>is to adb the executable or a .o (I know I am not as clever as you, am I
>missing something?  I didn't think you could get an optimized .s).

cc -O -S foo.c

In article <6139@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes:
> In article <6134@mimsy.UUCP> I wrote:
> >[use] while (--j >= 0) [rather than while (j--)]
> 
> I poked around today and discovered that Sun's compiler, at least,
> will turn
> 
> 	register short j;	/* but not `register int j' */
> 	while (--j != -1)
> 		...
> 
> into a `dbra' loop.  If you are willing to put machine dependent
> source optimisations into your C code, this might be something to
> consider (at least for inner loops).

Be a good citizen and hide it with macros...

#ifdef mc68000
typedef short LOOP_T;
#define LOOP_DECR(var)	(--(var) != -1)
#else
typedef int LOOP_T;
#define LOOP_DECR(var)	(--(var) >= 0)
#endif

	register LOOP_T j;

	while (LOOP_DECR(j))
		something;

which leads us to the mystic loop macro ...

#define	LOOP(count, op)	do { 
		register LOOP_T _loop = (count); 

		if (--_loop >= 0) 
			do { op; } while (LOOP_DECR(_loop));
	} while (0)

(end of line backslashes omitted for clarity)
-- 
David DiGiacomo, Sun Microsystems, Mt. View, CA  sun!david david@sun.com
Disclaimer: blah blah blah