Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!joyce!sri-unix!quintus!ok
From: ok@quintus.uucp (Richard A. O'Keefe)
Newsgroups: comp.arch
Subject: Re: RISC bashing at USENIX (really RISCs as X servers)
Message-ID: <186@quintus.UUCP>
Date: 22 Jul 88 02:51:35 GMT
References: <6965@ico.ISC.COM> <936@garth.UUCP> <202@baka.stan.UUCP> <1681@gofast.camcon.uucp>
Sender: news@quintus.UUCP
Reply-To: ok@quintus.UUCP (Richard A. O'Keefe)
Organization: Quintus Computer Systems, Inc.
Lines: 56

In article <1681@gofast.camcon.uucp> anc@camcon.uucp (Adrian Cockcroft) writes:
>> The loop which does most of the work for a bit blt looks like this for the
>> common copy case:
>> register long count;
>> register long *src, *dst;
>>    while( --count )
>>    {
>>       *dst++ = *src++;
>>    }
>Are you using the DBRA instruction for this? Has anyone ever seen a compiler
>generate a DBRA? Maybe SUNs bcopy library routine is in assembler and uses it.

The SunOS 3.2 C compiler is perfectly happy to generate a DBRA; you just
have to write some suprising C to do it.

void move(dst, src, len)
    register long *dst, *src;
    register short len;
    {
	do *dst++ = *src++;
	while (--len != -1);
    }

compiled with -O yielded

	movl	a6@(8),a5
	movl	a6@(12),a4
	movw	a6@(18),d7
L16:
	movl	a4@+,a5@+
	dbra	d7,L16

Remember that dbra operates on *16-bit* registers (boo hiss).
A reasonably good strcpy for 68010s can be done thus:

void strmov(dst, src)
    register char *dst, *src;
    {
	register short len = -2;
	while ((*dst++ = *src++) && --len != -1);
    }

which turns into 

	movl	a6@(8),a5
	movl	a6@(12),a4
	moveq	#-2,d7
L14:
	movb	a4@+,a5@+
	dbeq	d7,L14

Frankly, I'd rather write clearer C and put up with not getting DBcc.
(Oops, just forfeited my junior hacker's badge...)

That, of course, is the snag with CISCy instructions: if they're not
*exactly* what you want, you might as well not have them.