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

[Erik Schnetter]

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.

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?

-erik



On Wed, Jun 19, 2013 at 11:52 AM, Jeff Hammond <jhammond at alcf.anl.gov>wrote:

> 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:
>
> #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
>



-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/
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