* [Bug libfortran/49336] ATAN2 values differ from those specified in documentation
2011-06-09 0:19 [Bug libfortran/49336] New: ATAN2 values differ from those specified in documentation ahn567 at gmail dot com
@ 2011-06-09 2:45 ` kargl at gcc dot gnu.org
2011-06-09 6:05 ` burnus at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: kargl at gcc dot gnu.org @ 2011-06-09 2:45 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49336
kargl at gcc dot gnu.org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |kargl at gcc dot gnu.org
--- Comment #1 from kargl at gcc dot gnu.org 2011-06-09 02:44:07 UTC ---
The documentation is correct for a Fortran 95 compiler. Try
adding -std=f95 to your command line.
gfortran implements most of the Fortran 2003 and some Fortran
2008 features. With F2003, one finds in Sec. 1.6.1.
If the processor can distinguish between positive and negative
real zero, this standard requires different returned values for
ATAN2(Y,X) when X < 0 and Y is negative real zero and for LOG(X)
and SQRT(X) when X is complex with REAL(X) < 0 and negative zero
imaginary part.
If you want to be hero, you are more than welcomed to submit a
patch. F2003 says the results are
If Y = 0 and X > 0, the result is Y. If Y = 0 and X < 0, then
the result is pi if Y is positive real zero or the processor
cannot distinguish between positive and negative real zero, and
−pi if Y is negative real zero.
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug libfortran/49336] ATAN2 values differ from those specified in documentation
2011-06-09 0:19 [Bug libfortran/49336] New: ATAN2 values differ from those specified in documentation ahn567 at gmail dot com
2011-06-09 2:45 ` [Bug libfortran/49336] " kargl at gcc dot gnu.org
@ 2011-06-09 6:05 ` burnus at gcc dot gnu.org
2011-06-10 0:34 ` ahn567 at gmail dot com
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: burnus at gcc dot gnu.org @ 2011-06-09 6:05 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49336
Tobias Burnus <burnus at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |documentation
CC| |burnus at gcc dot gnu.org
--- Comment #2 from Tobias Burnus <burnus at gcc dot gnu.org> 2011-06-09 06:04:41 UTC ---
(In reply to comment #0)
> the value returned by ATAN2(-0d0, -1d0) is -pi (not +pi).
The result matches POSIX:
If y is ±0 and x is < 0, ±pi shall be returned.
If y is ±0 and x is > 0, ±0 shall be returned.
Thus, from POSIX I would expect the result one obtains with gfortran.
Looking at IEEE 754:2008, one finds (9.2.1 Special values):
"atan2 (y, x) [...] atan2(±0, x) is ±π for x < 0"
>From the Steve's quote of the Fortran 2003 standard (comment 1) one sees that
gfortran's result is also in line with Fortran 2003.
> The documentation for ATAN2(Y,X) says:
> "If Y is zero, then the return value is zero if X is positive and pi if X is
> negative."
We should indeed update http://gcc.gnu.org/onlinedocs/gfortran/ATAN2.html -
thus, I have marked it now as documentation bug.
ahn567: Do you rely on atan (+/-0, x< 0) returning PI instead of -PI? If it is
really needed, on could add a special handling for -fno-signed-zeros. However,
that would be slower - and other compilers also follow IEEE/POSIX/F2003 - even
if they do not print signed zeros by default. Hence, I would prefer to just
update the gfortran documentation.
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug libfortran/49336] ATAN2 values differ from those specified in documentation
2011-06-09 0:19 [Bug libfortran/49336] New: ATAN2 values differ from those specified in documentation ahn567 at gmail dot com
2011-06-09 2:45 ` [Bug libfortran/49336] " kargl at gcc dot gnu.org
2011-06-09 6:05 ` burnus at gcc dot gnu.org
@ 2011-06-10 0:34 ` ahn567 at gmail dot com
2011-11-07 22:26 ` fxcoudert at gcc dot gnu.org
2011-11-07 22:26 ` fxcoudert at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: ahn567 at gmail dot com @ 2011-06-10 0:34 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49336
--- Comment #3 from ahn567 at gmail dot com 2011-06-10 00:33:41 UTC ---
(In reply to comment #2)
> ahn567: Do you rely on atan (+/-0, x< 0) returning PI instead of -PI? If it is
> really needed, on could add a special handling for -fno-signed-zeros. However,
> that would be slower - and other compilers also follow IEEE/POSIX/F2003 - even
> if they do not print signed zeros by default. Hence, I would prefer to just
> update the gfortran documentation.
No, I do not rely on this. I was just surprised that the following program
would ever output "T F"
program main
double precision :: y1, y2
y1 = +0d0
y2 = -0d0
write(*,*) y1==y2, atan2(y1,-1d0)==atan2(y2,-1d0)
end program
The output is "T F" for both of these compilation commands,
gfortran prog.f90
gfortran -std=f95 prog.f90
Apparently this is correct standard Fortran behaviour, eg. according to
ISO/IEC JTC 1/SC 22/WG 5/N1826.
I had not come across signed zero before. For anyone interested...
http://en.wikipedia.org/wiki/Signed_zero
Updating the gfortran documentation would seem to be the proper thing to do
eg., as per page 331 of the above standard. (The docs for SQRT, SIGN, LOG,...
might also need checking.)
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libfortran/49336] ATAN2 values differ from those specified in documentation
2011-06-09 0:19 [Bug libfortran/49336] New: ATAN2 values differ from those specified in documentation ahn567 at gmail dot com
` (2 preceding siblings ...)
2011-06-10 0:34 ` ahn567 at gmail dot com
@ 2011-11-07 22:26 ` fxcoudert at gcc dot gnu.org
2011-11-07 22:26 ` fxcoudert at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: fxcoudert at gcc dot gnu.org @ 2011-11-07 22:26 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49336
Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
CC| |fxcoudert at gcc dot
| |gnu.org
Resolution| |FIXED
Target Milestone|--- |4.7.0
--- Comment #5 from Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> 2011-11-07 22:24:25 UTC ---
Fixed on trunk.
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug libfortran/49336] ATAN2 values differ from those specified in documentation
2011-06-09 0:19 [Bug libfortran/49336] New: ATAN2 values differ from those specified in documentation ahn567 at gmail dot com
` (3 preceding siblings ...)
2011-11-07 22:26 ` fxcoudert at gcc dot gnu.org
@ 2011-11-07 22:26 ` fxcoudert at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: fxcoudert at gcc dot gnu.org @ 2011-11-07 22:26 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49336
--- Comment #4 from Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> 2011-11-07 22:22:02 UTC ---
Author: fxcoudert
Date: Mon Nov 7 22:21:54 2011
New Revision: 181127
URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=181127
Log:
PR libfortran/49188
PR libfortran/49336
* invoke.texi: Fix documentation of fsign-zero option. Remove
contractions.
* intrinsic.texi: Fix ATAN2 documentation for signed zeros.
Remove contractions.
* gfortran.texi: Remove contractions.
Modified:
trunk/gcc/fortran/ChangeLog
trunk/gcc/fortran/gfortran.texi
trunk/gcc/fortran/intrinsic.texi
trunk/gcc/fortran/invoke.texi
^ permalink raw reply [flat|nested] 6+ messages in thread