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Postings from December 2004
Re: modf chainsaw?
From: Ed Allen Smith
December 14, 2004 03:24
Re: modf chainsaw?
Message ID: mid+200412141129.iBEBT1Bt1007214@dogberry.rutgers.edu
In message <20041214111430.GA2756@efn.org> (on 14 December 2004 03:14:30
-0800), email@example.com (Yitzchak Scott-Thoennes) wrote:
>On Tue, Dec 14, 2004 at 05:59:00AM -0500, Ed Allen Smith
>> In message <20041214094930.GB756@efn.org> (on 14 December 2004 01:49:30
>> -0800), firstname.lastname@example.org (Yitzchak Scott-Thoennes) wrote:
>> >It looks to me like the only use of modfl? is now in pp_pack.c, and that
>> >looks like a bizarre kind of way to do something by someone who didn't
>> >know that fmod existed.
>> >Can someone who has some idea what the uuencode unpacking code is doing
>> >comment on whether this:
>> > cdouble = Perl_modf(cdouble / adouble, &trouble) * adouble;
>> >can't be this instead:
>> > cdouble = Perl_fmod(cdouble, adouble);
>> >Then the Perl_modfl stuff in numeric.c can be ripped out, and the
>> >requirement for modfl for using long doubles can be removed from
>Yeah, I saw that, but couldn't make heads or tails of when an (IV) cast
>would produce a different number than modf. Both should truncate zeroward.
>Neither should involve any rounding at all.
I've seen what I suspected to be wacked long double casting results in IRIX,
although that was when casting to a double and I never did trace back enough
to find out what the exact problem was (I was attempting some tinkering on
gsl and concluded that I was more likely to mess it up than help it be more
accurate by lessening internal rounding).
>But I had meant to note it and that pp_int used to be another modf
>consumer (5.8.0 and earlier).
>> dSP; dTARGET; tryAMAGICun(int);
>> NV value;
>> IV iv = TOPi; /* attempt to convert to IV if possible. */
>> /* XXX it's arguable that compiler casting to IV might be subtly
>> different from modf (for numbers inside (IV_MIN,UV_MAX)) in which
>> else preferring IV has introduced a subtle behaviour change
>> bug. OTOH relying on floating point to be accurate is a bug. */
Allen Smith http://cesario.rutgers.edu/easmith/
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