Re: x87 float truncation/accuracy (gcc vs. icc/msvc)

[Jan Hubicka <jh at suse dot cz>]

> 
> On Thu, 18 Mar 2004, Jan Hubicka wrote:
> > You can just cut&paste the extendsfdf implementation.  Basically it
> > immitate move pattern for x87 but do proper conversions for SSE.
> > I was very tempted to do this for a while (and sent patch back in 98
> > or so) but there appeared to be consensus that the truncations are
> > very important, but it does not seem to be the practice.
> 
> If you can find the original posting and/or refresh this patch against
> mainline, I'll be happy to review it for you, provided any change in
> current functionality is guarded by flag_unsafe_math_optimizations.

It will need probably complette rewrite as the backend has been rewriten
since then too.  I will try to re-implement it, but I won'd be able to
do much work till 20th april, so probably i will have to delay it after
that.

Honza
> 
> 
> > If you want to get really good about elliminating the truncations, you
> > will need to play the games with combiner patterns containing truncates,
> > silimarly as we do for extensions but this is tricky (you will face
> > pattern explosion).
> 
> Indeed.  This was my major concern about modelling truncation like
> extension.  i386.md already contains a large number of patterns intended
> purely to provide "*ext" variants of floating point operations.  I
> wasn't sure if these were required by the i386 backend's extend?f?f2
> implementation.
> 
> Thanks for your help,
> 
> Roger
> --