Xref: utzoo comp.lang.c:16368 comp.sys.m68k:1104
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!lll-lcc!pyramid!cbmvax!ditto
From: ditto@cbmvax.UUCP (Michael "Ford" Ditto)
Newsgroups: comp.lang.c,comp.sys.m68k
Subject: Re: Efficientness of Compiler Generated Code
Summary: GCC on 68020 is better
Keywords: bit-fields operations; HLL instructions; m68000; i8086; C
Message-ID: <6007@cbmvax.UUCP>
Date: 18 Feb 89 20:49:05 GMT
References: <7756@ihlpf.ATT.COM>
Reply-To: ditto@cbmvax.UUCP (Michael "Ford" Ditto)
Organization: Commodore Technology, West Chester, PA
Lines: 53

In article <7756@ihlpf.ATT.COM> pcl@ihlpf.UUCP (pcl) writes:
>Anyway, the following C program generates the corresponding 8086 and 68k
>instructions which seem quite inefficient.  Any optimization suggestions?
>
>********* C ***********
>
>struct DHDR {
>	unsigned short	gfi: 4;
>	unsigned short	lci:12;
>	unsigned short	pr : 3;
>	unsigned short	m  : 1;
>	unsigned short	ps : 3;
>	unsigned short	x  : 1;
>};
>
>char pktbuf[1024];
>struct DHDR *dhp;
>main()
>{
>	dhp = (struct DHDR *)pktbuf;
>	dhp->pr++;
>	dhp->ps++;
>}

> [ really awful compiler output deleted ]

The 68020 has bit field instructions which help quite a bit.

Here's the 68020 code that GCC generates:
(using GCC v1.19: gcc -m68020 -fomit-frame-pointer -O -S T.c)

	text
	even
	global main
main:
	mov.l &pktbuf,dhp
	bfextu pktbuf+2{&0:&3},%d0
	add.l &1,%d0
	bfins %d0,pktbuf+2{&0:&3}
	bfextu pktbuf+2{&4:&3},%d0
	add.l &1,%d0
	bfins %d0,pktbuf+2{&4:&3}
	rts
	comm dhp,4
	comm pktbuf,1024

-- 
					-=] Ford [=-

"The number of Unix installations	(In Real Life:  Mike Ditto)
has grown to 10, with more expected."	ford@kenobi.cts.com
- The Unix Programmer's Manual,		...!sdcsvax!crash!kenobi!ford
  2nd Edition, June, 1972.		ditto@cbmvax.commodore.com