|Does Duff Device break C compilers Lambert.Lum@eng.efi.com (Lambert Lum) (1996-05-06)|
|Re: Does Duff Device break C compilers email@example.com (1996-05-07)|
|Re: Does Duff Device break C compilers krste@ICSI.Berkeley.EDU (1996-05-08)|
|Re: Does Duff Device break C compilers firstname.lastname@example.org (1996-05-10)|
|Re: Does Duff Device break C compilers email@example.com (1996-05-10)|
|Re: Does Duff Device break C compilers firstname.lastname@example.org (1996-05-10)|
|Re: Does Duff Device break C compilers email@example.com (1996-05-13)|
|Re: Does Duff Device break C compilers firstname.lastname@example.org (1996-05-19)|
|From:||email@example.com (Mark Brader)|
|Date:||10 May 1996 01:36:06 -0400|
|Organization:||SoftQuad Inc., Toronto, Canada|
|References:||96-05-050 96-05-054 96-05-057|
> > On the other hand, wouldn't be surprised if the resulting code wasn't
> > that great. Why? It makes an irreducible loop ...
Yep. Tom Duff was hand-optimizing for a particular machine and a particular
compiler when he originally wrote it, one where it did work.
> > Better is to rewrite it, like this
> > /* no assumptions made about count */
> > for (n = 0; n < count; n++)
> > to[n] = from[n];
> > which is correct, obvious, maintainable, parallelizable, vectorizable,
> > software pipelinable, and in all other ways superior.
In all other ways superior except that it *does the wrong thing*!
The correct translation is
for (n = 0; n < count; n++)
*to = from[n];
because "to" was pointing to an output register, specifically "the Pro-
grammed IO data register of an Evans & Sutherland Picture System II".
Note that this also means that the loop really has to be executed serially!
> Better yet...
> memcpy(from, to, count*(sizeof *from));
Worse yet -- it cannot be trivially corrected as the "for" version can.
If we were talking about a block copy, THEN memcpy() or memmove() would
of course generally be best.
> [Oh, right. Duh. It was still a great hack on the PDP-11. -John]
Tom was working on a VAX, actually. The *to = *from++ compiled to
a single instruction.
For greater clarity: Tom wrote in 1988 "I do not claim to have invented
loop unrolling, merely this particular expression of it in C." And it
was in 1983 that he did it.
SoftQuad Inc., Toronto
My text in this article is in the public domain.
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