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* [Bug c++/64497] New: std::scalbln does not round correctly for long doubles
@ 2015-01-05 10:40 walter.mascarenhas at gmail dot com
2015-01-05 12:29 ` [Bug c++/64497] " redi at gcc dot gnu.org
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: walter.mascarenhas at gmail dot com @ 2015-01-05 10:40 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64497
Bug ID: 64497
Summary: std::scalbln does not round correctly for long doubles
Product: gcc
Version: 4.8.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: walter.mascarenhas at gmail dot com
The overload std::scalbln(long double, long) may not round the result correctly
when the exponent is very small. For instance, with round mode = near,
std::scalbln(1.1L, -16446)
returns 0, whereas std::scalbn(1.1L, -16446) and std::ldexp(1.1L, -16446)
return std::numeric_limits<long double>::denorm_min(), which I believe
is the correct result.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/64497] std::scalbln does not round correctly for long doubles
2015-01-05 10:40 [Bug c++/64497] New: std::scalbln does not round correctly for long doubles walter.mascarenhas at gmail dot com
@ 2015-01-05 12:29 ` redi at gcc dot gnu.org
2015-01-05 13:11 ` walter.mascarenhas at gmail dot com
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: redi at gcc dot gnu.org @ 2015-01-05 12:29 UTC (permalink / raw)
To: gcc-bugs
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 );
}
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/64497] std::scalbln does not round correctly for long doubles
2015-01-05 10:40 [Bug c++/64497] New: std::scalbln does not round correctly for long doubles walter.mascarenhas at gmail dot com
2015-01-05 12:29 ` [Bug c++/64497] " redi at gcc dot gnu.org
@ 2015-01-05 13:11 ` walter.mascarenhas at gmail dot com
2015-01-05 14:07 ` redi at gcc dot gnu.org
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: walter.mascarenhas at gmail dot com @ 2015-01-05 13:11 UTC (permalink / raw)
To: gcc-bugs
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 );
> }
>
> --
> You are receiving this mail because:
> You are on the CC list for the bug.
> You reported the bug.
>
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/64497] std::scalbln does not round correctly for long doubles
2015-01-05 10:40 [Bug c++/64497] New: std::scalbln does not round correctly for long doubles walter.mascarenhas at gmail dot com
2015-01-05 12:29 ` [Bug c++/64497] " redi at gcc dot gnu.org
2015-01-05 13:11 ` walter.mascarenhas at gmail dot com
@ 2015-01-05 14:07 ` redi at gcc dot gnu.org
2015-01-05 14:13 ` redi at gcc dot gnu.org
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: redi at gcc dot gnu.org @ 2015-01-05 14:07 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64497
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Walter Mascarenhas from comment #2)
> What if there is a difference in the expected behavior
> for this function in C and C++11?
There isn't any difference, so it doesn't matter.
> 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.)
Yes.
> 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."
The C++ standard doesn't say that.
> I looked at the standard (N3797.pdf) but did not find anything
> specific about std::scalbln.
It says it is the same as C, so cppreference.com is wrong.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/64497] std::scalbln does not round correctly for long doubles
2015-01-05 10:40 [Bug c++/64497] New: std::scalbln does not round correctly for long doubles walter.mascarenhas at gmail dot com
` (2 preceding siblings ...)
2015-01-05 14:07 ` redi at gcc dot gnu.org
@ 2015-01-05 14:13 ` redi at gcc dot gnu.org
2015-01-06 4:33 ` cubbi at cubbi dot org
2015-01-06 9:07 ` jakub at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: redi at gcc dot gnu.org @ 2015-01-05 14:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64497
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Looking at the C standard, it seems that the result is implementation-defined
on underflow, and zero is a valid result. C++ doesn't change that.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/64497] std::scalbln does not round correctly for long doubles
2015-01-05 10:40 [Bug c++/64497] New: std::scalbln does not round correctly for long doubles walter.mascarenhas at gmail dot com
` (3 preceding siblings ...)
2015-01-05 14:13 ` redi at gcc dot gnu.org
@ 2015-01-06 4:33 ` cubbi at cubbi dot org
2015-01-06 9:07 ` jakub at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: cubbi at cubbi dot org @ 2015-01-06 4:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64497
Sergey Zubkov <cubbi at cubbi dot org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |cubbi at cubbi dot org
--- Comment #5 from Sergey Zubkov <cubbi at cubbi dot org> ---
FYI, cppreference got that phrasing from POSIX's "If the correct value would
cause underflow, and is representable, a range error may occur and the correct
value shall be returned.", which is a part of its optional IEC 60559
Floating-Point extension to the C standard:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/scalbln.html
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/64497] std::scalbln does not round correctly for long doubles
2015-01-05 10:40 [Bug c++/64497] New: std::scalbln does not round correctly for long doubles walter.mascarenhas at gmail dot com
` (4 preceding siblings ...)
2015-01-06 4:33 ` cubbi at cubbi dot org
@ 2015-01-06 9:07 ` jakub at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: jakub at gcc dot gnu.org @ 2015-01-06 9:07 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64497
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
CC| |jakub at gcc dot gnu.org
Resolution|--- |INVALID
--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Closing as this is definitely not a GCC issue.
Please file it against glibc instead. Note, on x86_64, scalbnl is implemented
in assembly, while scalblnl is not.
^ permalink raw reply [flat|nested] 7+ messages in thread
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2015-01-05 10:40 [Bug c++/64497] New: std::scalbln does not round correctly for long doubles walter.mascarenhas at gmail dot com
2015-01-05 12:29 ` [Bug c++/64497] " redi at gcc dot gnu.org
2015-01-05 13:11 ` walter.mascarenhas at gmail dot com
2015-01-05 14:07 ` redi at gcc dot gnu.org
2015-01-05 14:13 ` redi at gcc dot gnu.org
2015-01-06 4:33 ` cubbi at cubbi dot org
2015-01-06 9:07 ` jakub at gcc dot gnu.org
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