Xref: utzoo comp.unix.wizards:16705 comp.lang.c:19209
Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cmcl2!edler
From: edler@cmcl2.NYU.EDU (Jan Edler)
Newsgroups: comp.unix.wizards,comp.lang.c
Subject: Re: Optimal for loop on the 68020.
Keywords: for ( i = COUNT; --i >= 0; )
Message-ID: <40063@cmcl2.NYU.EDU>
Date: 6 Jun 89 21:06:02 GMT
References: <11993@well.UUCP> <17891@mimsy.UUCP>
Reply-To: edler@cmcl2.UUCP (Jan Edler)
Followup-To: comp.unix.wizards
Organization: New York University, Ultracomputer project
Lines: 33

In article <17891@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
>In article <11993@well.UUCP> pokey@well.UUCP (Jef Poskanzer) writes:
>>... COUNT was a small (< 127) compile-time constant.
>>    for ( i = COUNT; --i >= 0; )
>
>[all but gcc -O -fstrength-reduce deleted]
>
>>	moveq  #COUNT,d0
>>	jra    tag2
>>tag1:
>>	<loop body>
>>tag2:
>>	dbra   d0,tag1
>>	clrw   d0
>>	subql  #1,d0
>>	jcc    tag1

I prefer
	moveq	#COUNT,d0
	jra	tag3
tag1:	swap	d0
tag2:	<loop body>
tag3:	dbra	d0,tag2
	swap	d0
	dbra	d0,tag1

But it doesn't really make that much difference, I guess.
However, the compiler shouldn't do either of these when d0 is initialized
to a value < 65536.

Jan Edler
NYU Ultracomputer Research Laboratory
edler@nyu.edu