[Bug c++/64497] std::scalbln does not round correctly for long doubles

["walter.mascarenhas at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64497

--- Comment #2 from Walter Mascarenhas <walter.mascarenhas at gmail dot com> ---
What if there is a difference in the expected behavior
for this function in  C and C++11? Is it not up to g++
for implementing what is mandated in C++11? (This
is not a rhetorical question, I really do not know the answer.)

In http://man7.org/linux/man-pages/man3/scalbn.3.html it
is written that scalbln should return 0 in case of underflow:

"If the result underflows, a range error occurs, and the functions
return zero, with a sign the same as *x*."

On the other hand,
http://en.cppreference.com/w/cpp/numeric/math/scalbn states that

"If a range error due to underflow occurs, the correct result (after
rounding) is returned."

I looked at the standard (N3797.pdf) but did not find anything
specific about std::scalbln.
If there is indeed a discrepancy in the definitions of scalbln in C
and C++11 then there
may be no bug in libm, and my vendor will not change it.

I do not have a copy of the ISO 60599 standard, and I do not know whether
the content of the pages http://man7.org/linux/man-pages/man3/scalbn.3.html and
http://en.cppreference.com/w/cpp/numeric/math/scalbn are compatible with
any standards. Therefore I am in no position to argue,
but maybe you could think a bit longer about this..














On Mon, Jan 5, 2015 at 10:29 AM, redi at gcc dot gnu.org <
gcc-bugzilla@gcc.gnu.org> wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64497
>
> --- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
> GCC just calls the scalnlnl() function in libm, so it's not a GCC bug, and
> is
> not specific to C++ either. I suggest you report it to your libc vendor.
>
> Complete testcase in C:
>
> #include <stdio.h>
> #include <math.h>
> #include <assert.h>
>
> int main()
> {
>   long double di = scalbnl(1.1L, -16446);
>   assert( di != 0.0L );
>   long double dl = scalblnl(1.1L, -16446L);
>   assert( dl != 0.0L );
> }
>
> --
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