At Tue, 22 Apr 2008 11:50:15 +0200,
Julian Seward wrote:
> Ok. So it's clear that is what the original authors intended.
> And I fully sympathise with wanting to avoid unsigned integer
> wraparound problems in such loops.
>
> However, regarding
>
http://www.gnu.org/software/gsl/design/gsl-design.html#SEC31
>
> not only is the clever version
>
> for (i = N; i > 0 && i--;) { ... }
>
> confusing, it also generates worse code than the simple
> version:
>
> for (i = 0; i < N; i++) { j = N - i; ... }
>
Thanks for that interesting analysis! The loop unrolling point is a
good one which I must have overlooked at the time (although I don't
know if gcc did loop unrolling back then - it was almost ten years ago
I think).
That part as written when I was translating a lot of code from fortran
and I was trying to avoid introducing additional variables, to
minimise the number of indices I mentally needed to keep track of.
I've played with a few variations and found that the alternative
for (i = N; i-- > 0;)
does get unrolled (with gcc-4.2), while
for (i = N; i > 0 && i--; )
does not. Unfortunately the former is perhaps still somewhat
unintuitive, but I think I will update the design document and code to
use it.
I never really liked code that depends on the distinction between --i
and i--, being burned a few times by subtle bugs from not noticing the
wrong one being used, hence the tendency to write things like i>0 &&
i--.
--
Brian Gough
