Path: utzoo!utgpu!water!watmath!clyde!rutgers!sri-unix!quintus!ok
From: ok@quintus.UUCP (Richard A. O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: what C needs -- packed structures
Summary: bogus 68000 code
Message-ID: <519@cresswell.quintus.UUCP>
Date: 11 Jan 88 03:12:00 GMT
References: <8801071842.AA04669@decwrl.dec.com> <2083@chinet.UUCP>
Organization: Quintus Computer Systems, Mountain View, CA
Lines: 25

In article <2083@chinet.UUCP>, dag@chinet.UUCP (Daniel A. Glasser) writes:
> struct yerks { char y_foo; int *y_fie; char y_poo; };
> struct yerks *w;
> int i;
> i = *w->y_fie;
He claims that this assignment would turn into 9 M680x0 instructions.
Wrong.  It can be coded in three:
	movl	_w, a0
	movl	1(a0), a0
	movl	(a0), _i
The /370, and the 80386 can also handle misaligned data.  The R2000
can handle misaligned data at the cost of one extra instruction *if*
the compiler knows the data are misaligned, which in this case it
would.  None of this spoils Glasser's real point, namely that packing
is machine-dependent, and doesn't come free on any machine.
Even the machines that will handle his example in few instructions tend
to take lots more *cycles* to handle misaligned data.  (A loop like
	register int n = {something};
	register long *d = {something};
	register long *s = {something};
	while (--n >= 0) *d++ = *s++;
takes about 30% longer for misaligned than for aligned data on a 68020.)
Glasser says
> No, packed structures do not belong in the standard.
He's absolutely right.  We already *have* bitfields, after all.