structure alignment question

[Michael Tiemann]

In article <7479 at sun.uucp>, guy at sun.uucp (Guy Harris) writes:
> > The last 68000 compiler I used aligned strings on WORD boundaries.
> > This would cost one byte per string, half the time. But there was a
> > big speed payoff: I could do word operations in my strnlen, strncmp,
> > strncpy, and whatever other string processing functions I happened to
> > write.
> 
> Oh, really?
> 
> 	char	string1[] = "foo";
> 	char	string2[] = "xfoo";
> 
> 	return(strcmp(string1, string2 + 1));
> 
> If you can do this with straightforward word operations ...
> [ ... USW ... ]
> *!NOTHING*!  guarantees that the arguments to the string routines always
> point to the *first* byte of a string.  Furthermore, nothing guarantees that
> "strcat" and "strncat" will always do aligned copies, even if the arguments
> always point to the first byte of a string; the reason for this should be
> obvious.
> -- 
> 	Guy Harris
> 	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
> 	guy at sun.com (or guy at sun.arpa)

What I meant was... There are times when I *can* (can-to can-to can-to)
guarantee that I am refering to the first byte of a string. Sure,
when I am doing SUB-string operations I have to be prepared to
pay the price, but if I happen only to want to do STRING operations,
where I *always* refer *only* to the string as a whole, then it
sure would be nice to be rewarded for passing that extra knowledge
on to my compiler.

When I don't know what I'm dealing with, I use strcmp, strcat, etc,
(and I expect them do make pessimistic decisions).
I wrote my own C routines "fmove", "fzero" which are like
"bcopy" and "bzero", except I only call them when I know I'll
be using word aligned things. Also, "fcmp" depends on the fact that a
word-aligned string is terminated by a null word (too much to
ask of a compiler to do that: possible (2*sizeof(int)-1 byte overhead),
but if they're two of MY strings, I call it.

The point is, a word aligned string does not seem to be too much
to ask for, and the performance improvement IS noticable:
Lets be hypothetical for a moment:

int lookup(s)
     char *s;
{
    int where;

    switch (*(int *)s) {
      case 0x41424300:		/* "abc" */
	return ABC;
      case 0x78797a00:		/* "xyz" */
	return XYZ;
      default:
	/* must look up in symbol table... */
	do { ... };
	return where;
    }
}

Keep your reserved words < 4 characters and off you go!
No flames about "portability": these were handled by a little
emacs function (generates hex and ascii comment for short strings),
and each emacs (VAX, SUN, whatever) has its own version.

"A little knowledge is a dangerous thing"... programmers should
*never* learn about machine architectures: they will only want to
write fast code...

I hope that this time I have clarified my position: everybody can
now say I'm wrong (of course I am) or right.

Michael
tiemann at mcc.com

"I'm that C programmer your mother warned you about"