[Llvm-bgq-discuss] rint() with -ffast-math

[Erik Schnetter]

On Wed, Jun 19, 2013 at 1:24 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> >
> > On Wed, Jun 19, 2013 at 1:15 PM, Hal Finkel < hfinkel at anl.gov >
> > wrote:
> >
> >
> >
> >
> >
> > ----- Original Message -----
> > >
> > > Actually, the Intel AVX instructions have a similar issue: rint()
> > > has
> > > a fast instruction, but round() does not. On this architecture,
> > > round() still does the "right thing", even with -ffast-math, both
> > > with gcc and clang.
> > >
> > >
> > > Actually, as I just see, gcc generates a short sequence of
> > > instructions (five or so) to implement round() properly, whereas
> > > clang calls _round.
> >
> > LLVM only seems to have special handling of:
> > FCEIL, FTRUNC, FRINT, FNEARBYINT, FFLOOR
> > (as there is no FROUND and so round() will always give you the
> > library call).
> >
> >
> > >
> > >
> > > Given this, the behaviour on BGQ is indeed special. I would expect
> > > clang to behave consistently -- to either apply this optimization
> > > across the board, or nowhere. Do you want to raise the issue on the
> > > llvm mailing list?
> >
> > Unfortunately, this optimization (as are many low-level fast-math
> > optimizations) is target-specific. As a result, I'm not sure that
> > you'll even really get the cross-platform consistency that you'd
> > like. That having been said, if this change is too strong, then we
> > should back it out.
> >
> >
> >
> > Yes, it's target specific. Nevertheless, whether rint's tie-breaking
> > can be influenced by __FAST_MATH__ should be a consensus decision.
> > Either BGQ is over-zealous, or Intel is missing a possible
> > optimisation, or llvm makes different speed/accuracy trade-offs on
> > different architectures. And the latter would be bad for users.
>
> Agreed. Would you like to write to the list or should I?
>

I'd prefer if you did it -- I'd hope that your asking has less chance of
being told off via "don't complain, that's what you get when you use
__FAST_MATH__".

-erik

-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.alcf.anl.gov/pipermail/llvm-bgq-discuss/attachments/20130619/89ded18a/attachment.html>