GCC and Floating-Point

[Joseph S. Myers]

On Sat, 28 May 2005, Scott Robert Ladd wrote:

> Toon Moene wrote:
> > Good Luck :-)
> > 
> >     http://gcc.gnu.org/ml/gcc/2001-08/msg00368.html
> > 
> > (it's only four years ago - I can also show you my contributions in this
> > thread in 1999, i.e., the previous millennium).
> 
> Just out of curiosity, why did previous efforts fail in this regard? Was
> it simply too much effort to identify all the transformations? Did the
> GCC community fail to come to a consensus? Or was it simply -- as I'm
> coming to suspect -- that the work involved is not justified by the result?

They didn't fail altogether; -ffast-math was split into multiple options 
(-funsafe-math-optimizations -fno-trapping-math etc.) in March 2001, 
following an analysis of everything that checked flag_fast_math 
<http://gcc.gnu.org/ml/gcc-patches/2000-12/msg00690.html>.

Unfortunately this never got followed up properly by defining separate 
preprocessor macros for each option, so if you use the individual options 
then glibc headers checking __FAST_MATH__ may not enable optimizations 
which would in fact be safe with the subset of options specified.  And as 
the current discussion illustrates, -funsafe-math-optimizations itself has 
become a catch-all for many different optimizations, such as -ffast-math 
was.

-- 
Joseph S. Myers               http://www.srcf.ucam.org/~jsm28/gcc/
    jsm@polyomino.org.uk (personal mail)
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