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Re: [rfc] enable non-trivial complex division


On Sat, 19 Feb 2005, Richard Henderson wrote:

> I'm not sure that I agree that other languages want the, somewhat odd,
> behaviour wrt nans and infinities.  Certainly there doesn't seem to be
> anything in the C++ or Fortran standards that specify this.
> 
> The worst part of this, IMO, being that C99 considers (+inf, nan) to
> be an infinity and not a nan.  This makes no sense to me.
> 
> Anyway, I've enabled the new library routine for C99, made everyone
> else default to the more stable inline implementation.  I do add the
> new option described above for C, and for C allow -ffast-math to 
> change the default to the trivial inline implementation.  I don't let
> other languages change to the trivial inline implementation with 
> -fast-math, because of the lack of an option to change it back.
> 
> Comments?

If we are to have three different implementations then long-term I think 
they should be (a) trivial, (b) logarithmic reduction to avoid undue 
overflow for both multiplication and division, (c) (b) plus special nan 
and infinity handling.  With -fcx-limited-range being a 
language-independent switch to change from (b) to (a).  We don't have (b) 
at present, though flag_complex_method == 1 substitutes for it.  That 
suggests for now having -fcx-limited-range as a language-independent flag, 
included in -ffast-math for all languages, which switches from the default 
of 1 (2 for C99) to 0 (and -ffast-math -fno-cx-limited-range keeping the 
default of 1 or 2) with C99 only changing the default from 1 to 2; and in 
future adding logarithmic reduction for both 1 and 2.

-- 
Joseph S. Myers               http://www.srcf.ucam.org/~jsm28/gcc/
    jsm@polyomino.org.uk (personal mail)
    joseph@codesourcery.com (CodeSourcery mail)
    jsm28@gcc.gnu.org (Bugzilla assignments and CCs)


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