GCC Bugzilla – Bug 29383
Fortran 2003/F95[TR15580:1999]: Floating point exception (IEEE) support
Last modified: 2013-11-25 01:15:27 UTC
See chapter 14 of the Fortran 2003 standard.
use,intrinsic :: ieee_arithmetic
if(.not. IEEE_support_standard(1.0d0) &
.or. .not.IEEE_support_halting(IEEE_INVALID) &
.or. .not.IEEE_support_halting(IEEE_DIVIDE_BY_ZERO)) &
stop 'No IEEE support available!'
call ieee_set_halting_mode([IEEE_INVALID, IEEE_DIVIDE_BY_ZERO],&
double precision, intent(in) :: x
double precision :: y
y = log(x)
if(.not. ieee_is_finite(y)) then
write(*,*) 'Ignoring calculation for x = ', x,'; result is: ',y
print *, 'Result is: log(',x,') = ',y
end subroutine printLog
end program ieee
Expected result (NAG f95 gives the following):
Result is: log( 1.0000000000000000 ) = 0.0000000000000000
Ignoring calculation for x = 0.0000000000000000 ; result is:
Ignoring calculation for x = -1.0000000000000000 ; result is: NaN
Result is: log( 2.0000000000000000 ) = 0.6931471805599453
Warning: Floating invalid operand occurred
Warning: Floating divide by zero occurred
This one will probably be implemented after ISO_C_BINDING.
Add links to some discussions to make sure it won't get lost:
The thread which is here and maybe also some other fortran@gcc emails by Tim and Nick around this time:
Some underflow module for Fortran:
Malcolm Cohen wrote in "Implementing the Standards...",
http://www.fortran.bcs.org/2007/jubilee/f50.pdf, the following:
IEEE module implementation
• Only if IEEE_GET_FLAG is directly called in a routine: save then clear the flags on entry, merge the flags on exit.
• Only in a routine that uses a mode setting procedure: save mode on entry,
restore mode on exit.
• Parallelism and other optimisations are little impeded by the use of IEEE facilities (all IEEE semantics being local).
Even a partial, incomplete support of module IEEE_ARITHMETIC would be very useful and much appreciated:
a) To set an IEEE value (NaN, INF, etc.)
b) Check whether a value is NaN, etc.
and IEEE_VALUE(X, IEEE_QUIET_NAN)
So we wouldn't have to resort to non-standard functions like ISNAN() (http://gcc.gnu.org/onlinedocs/gfortran/ISNAN.html) anymore.
Since most other compilers support ieee_arithmetic now, the lack of support in gfortran is becoming a portability issue. Simple test case with 4.6.1:
> cat t1.f90
use,intrinsic :: ieee_arithmetic
real :: x
read *, x
if (ieee_is_nan(x)) then
print *, "Nan"
print *, "Not NaN"
end program test
> gfortran t1.f90
use,intrinsic :: ieee_arithmetic
Fatal Error: Can't find an intrinsic module named 'ieee_arithmetic' at (1)
*** Bug 54840 has been marked as a duplicate of this bug. ***
Interesting. Didn't see this dup originally in my search.
In response to your other email (that it is very very hard), can you explain
a bit why if you've got the time/inclination? My first thought was that it
would be a fairly simple pass through/reimplementation of the fpclassify
functionality of c.
On Oct 6, 2012, at 6:08 PM, kargl at gcc dot gnu.org wrote:
> kargl at gcc dot gnu.org changed:
> What |Removed |Added
> CC| |andy.nelson at lanl dot gov
> --- Comment #7 from kargl at gcc dot gnu.org 2012-10-07 00:08:49 UTC ---
> *** Bug 54840 has been marked as a duplicate of this bug. ***
> Configure bugmail: http://gcc.gnu.org/bugzilla/userprefs.cgi?tab=email
> ------- You are receiving this mail because: -------
> You are on the CC list for the bug.
(In reply to comment #8)
> In response to your other email (that it is very very hard), can you explain
> a bit why if you've got the time/inclination? My first thought was that it
> would be a fairly simple pass through/reimplementation of the fpclassify
> functionality of c.
There are a few reason (and this is just my opinion).
1) gcc runs on numerous cpu architectures (i386, x86_64,
arm, sparc, powerpc, etc). Some (many? all?) do not
have hardware support for IEEE754. An implementation
needs to be able to work on most (all?) of these cpus.
2) gcc runs on numerous operating systems. The operating
systems may or may not have sufficient support to allow
an efficient IEEE754 implementation (a software
implementation of IEEE754 will be slow).
3) Using C interoperability to access the fenv.h facility
seems appealing, but gcc would need to deal with systems
that do not have fenv.h.
4) gcc performs constant folding with mpfr, and in gfortran
this constant folding is performed in round-to-nearest mode.
gfortran would need to be update to delay constant folding
until the rounding mode has been established.
x = 0.3 + 0.1999 ! Should be down in round-down?
print '(Z0)', x
end program foo
5) Finally, for some intrinsic functions, it is not possible
to map to a libm routine without now putting a wrapper around
the libm routine. For example, consider FRACTION(). The standard
Result Value. The result has the value X*b**(-e), where b
and e are as defined in 13.4 for the model representation of
X. If X has the value zero, the result has the value zero.
If X is an IEEE infinity, the result is that infinity. If X
is an IEEE NaN, the result is that NaN.
The word "that" in "the result is that NaN" is problematic.
There are something like 2**p bit patterns that much NaN.
Unfortunately, mapping FRACTION to libm's frexp gives the
wrong NaN. See PR 48979 for a longer explanation.
One can side step 1), 2), and 3) by unilaterally returning
false for ieee_support_standard(), which of course indicates
that the IEEE754 isn't standard. 4) and 5) are much harder
Of course, I could be wrong.
Comment 9 seems a bit too pessimistic to me.
For one, returning ".false." for ieee_supports_* doesn't seem like a sidestep, but more like the correct behavior in an environment that lacks IEEE support (maybe even if the only reason is because the compiler has not implemented it).
For another, as noted above, one important use of the ieee_arithmetic module is to test for NaN/Inf in a standard-conforming way, rather than using compiler-specific extensions. (AFAIK, gfortran is the only F2003 compiler that forces you to use a non-portable extension to do this.)
For that functionality, the only likely problem I can see is in deciding when e.g. ieee_support_nan should return ".true.".
Of course this is only a fraction of what needs to be done (presumably the easiest part), but it's much better than nothing, particularly to address what I would expect to be one of the most common uses of this module.
I'm doing a bit of research because I'm considering trying to tackle part of this soon.
I believe that the Fortran standard does not require the rounding mode to apply to operations performed during constant folding, so point 4 above is moot.
Please see the collected interpretations of F2003:
Particularly see F03/0040 and F03/0078. In combination they guarantee that the compiler is allowed to transform expressions at compile time according to the "mathematical equivalence" rule regardless of the rounding mode. In any case, the rounding mode cannot always be established at compile time; if the committee had not decided upon this behavior, then constant folding would often be impossible, period.
The Fortran 2003 and 2008 standards also give examples where constant folding and propagation are allowed to influence whether or not an IEEE exception is signaled.