[Bug libfortran/58020] Code for handling IEEE exceptions

["burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58020

Tobias Burnus <burnus at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |burnus at gcc dot gnu.org

--- Comment #16 from Tobias Burnus <burnus at gcc dot gnu.org> ---
(In reply to Francois-Xavier Coudert from comment #13)
> I may have a bit of time in November and December to look at this. I
> actually had started implementing the IEEE modules in gfortran back in
> 2008-2009, but never got too far (I was unclear, back then, on the exact
> requirements for things like constant folding… some interps have made clear
> that our job is actually easier than I thought back then).

One problem is that behaviour changes when a module is loaded. Related:

- Similar issue in C/C++, cf. "#pragma STDC FENV_ACCESS ON" and also
http://gcc.gnu.org/ml/gcc-patches/2013-10/msg01131.html

- Changes required for the new ISO/IEC/IEEE 60559:2011 standard
(http://www.iso.org/iso/catalogue_detail.htm?csnumber=57469):
http://j3-fortran.org/doc/year/13/13-356.txt


>  2. Although you don't plan to support other architectures than i386/x86_64,
> we have to think about it.

I want to mention that there is now some additional support for IEEE in
libgfortran/config. None of those functions is exported, yet - that permits to
tweak their ABI. All work on x86 hardware, glibc, sysv (= Solaris, FreeBSD),
AIX - which should cover most systems.

a) Enable trapping: set_fpu (invalid, denormal, zero, overflow, underflow,
inexact)  [Used by -ffpe-trap=...]

b) get_fpu_except_flags - returns which exception have been raised by the CPU.
[New in 4.9; used to print those with STOP/ERROR STOP; -ffpe-summary=...]

c) set_fpu_rounding_mode/get_fpu_rounding_mode [New in 4.9; used for I/O
rounding on *input*, only works with "atoi"s which honour the rounding mode,
such as very recent GLIBCs.] - Note that Fortran's required rounding mode
"compatible" is not supported ("nearest" comes close but has different results
for a tie).
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From: "fxcoudert at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libfortran/58020] Code for handling IEEE exceptions
Date: Tue, 05 Nov 2013 16:02:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idX020

--- Comment #17 from Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> ---
(In reply to Tobias Burnus from comment #16)
> I want to mention that there is now some additional support for IEEE in
> libgfortran/config.

Yes, I see that this has grown since I first introduced it (in 2005! it's a
while back). That's good, and we can use this (possibly modified) interface to
the hardware/OS-specific function calls.

Another issue I wanted to get your opinion on, Tobias: we can't implement the
IEEE modules fully on the library side, because of three things:

  1. Changes in behavior when the modules are loaded: I'm not sure I understand
the full extent of these changes. Comment #11 in PR29383 seems to imply that
there are no such changes.

  2. Some IEEE functions are allowed in constant or specification expressions,
thus requiring front-end cooperation: IEEE_SELECTED_REAL_KIND is allowed in
constant expressions; IEEE_SUPPORT_* functions are allowed in specification
expressions.

  3. Library-side implementation of some functions (IEEE_IS_NAN, to give only
one example) will require a function call, while emitting code from the
front-end (like we do for our own ISNAN) will not incur that penalty.

However, for the rest of it, library-side implementation is easier. So, do you
think we can implement the module in libgfortran, and add only minimal
front-end tweaking to "catch" the necessary cases?