Re: [9fans] Rounding off by one

[tlaronde@polynum.com]

On Mon, Apr 19, 2010 at 06:11:43PM +0200, Martin Neubauer wrote:
> * tlaronde@polynum.com (tlaronde@polynum.com) wrote:
> > While verifying that I have the same result under Plan9 than under some
> > Unix (NetBSD), it happens that METAFONT compiles 110 of the 125 fontes,
> > but hence fails on some (and it should not) with something that smells
> > like a round off by one error on negative integer division. There is no
> > problem with TeX, but TeX doesn't do the same things.
> This might not be related to your problem, but during the recent
> discussion of Plan 9's future /sys/src/cmd/cc/c99 was mentioned.
> Reference 25 (in section "Not done") at least seems to be a good
> candidate for a test case between Plan 9 and NetBSD compilers.

Thanks for the note. But I think I have found the problem: a division by
2 supposed to be "floor" and that slips as (x + y) / 2: is there
anything that guarantees that the result shall be floor, ceil or
nearest? I find nothing in C89.

If this is the case, (x + y) >> 1 will be the solution (suggested by
Donald E. Knuth in the METAFONTbook by the way), and that could be an
explanation of why it works with gcc, because it optimizes this.

I hope this is it, since I will finally be able to publish...

Cheers,
--
        Thierry Laronde <tlaronde +AT+ polynum +dot+ com>
                      http://www.kergis.com/
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