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

[Hal Finkel]

----- Original Message -----
> From
> http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Optimize-Options.html:
> ===========================================================================
> -ffast-math
> 
> Sets -fno-math-errno, -funsafe-math-optimizations,
> -fno-trapping-math,
> -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and
> fcx-limited-range.
> 
> This option causes the preprocessor macro __FAST_MATH__ to be
> defined.
> 
> This option should never be turned on by any -O option since it can
> result in incorrect output for programs which depend on an exact
> implementation of IEEE or ISO rules/specifications for math
> functions.
> ===========================================================================
> 
> Based upon this, I would not count on accurate results when fast-math
> is defined unless explicitly verified.
> 
> Assuming LLVM behaves like GCC and defines __FAST_MATH__, you could
> also do this:

Yes, Clang also defines __FAST_MATH__ -- however, I think that if a user is forced to manually implement rint() then that is not a good outcome.

 -Hal

> 
> #ifdef __FAST_MATH__
>   slower_rounding_function_that_is_always_correct(stuff);
> #else
>   rint(stuff);
> #endif
> 
> Jeff
> 
> On Wed, Jun 19, 2013 at 10:48 AM, Erik Schnetter
> <schnetter at cct.lsu.edu> wrote:
> > On Wed, Jun 19, 2013 at 11:12 AM, Hal Finkel <hfinkel at anl.gov>
> > wrote:
> >>
> >> ----- Original Message -----
> >> >
> >> >
> >> >
> >> > The function rint() is supposed to round to the nearest integer,
> >> > breaking ties to even. With -ffast-math, it breaks ties away
> >> > from
> >> > zero. That is, in corner cases the result is incorrectly
> >> > rounded.
> >> >
> >> >
> >> > Is this intended? This (BGQ with Clang) is the first system that
> >> > does
> >> > so. (I understand why one would do this given the machine
> >> > instructions available.)
> >>
> >> Yes, this is the intended behavior (and LLVM will currently do
> >> this on all
> >> PPC systems). It is a function of the (odd) way in which the PPC
> >> frin
> >> instruction is defined. The upside is that it is much faster than
> >> the libc
> >> function call. That having been said, I put this optimization in,
> >> and I can
> >> take it out again ;) [or make it require some other flag]. Is the
> >> behavior
> >> too different for you?
> >
> >
> > I can live with this optimization, I just want to know where
> > -ffast-math has
> > its boundaries...
> >
> > -erik
> >
> > --
> > Erik Schnetter <schnetter at cct.lsu.edu>
> > http://www.perimeterinstitute.ca/personal/eschnetter/
> >
> > _______________________________________________
> > llvm-bgq-discuss mailing list
> > llvm-bgq-discuss at lists.alcf.anl.gov
> > https://lists.alcf.anl.gov/mailman/listinfo/llvm-bgq-discuss
> >
> 
> 
> 
> --
> Jeff Hammond
> Argonne Leadership Computing Facility
> University of Chicago Computation Institute
> jhammond at alcf.anl.gov / (630) 252-5381
> http://www.linkedin.com/in/jeffhammond
> https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond
> ALCF docs: http://www.alcf.anl.gov/user-guides
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory