[Patch, fortran] PR 49010/24518 MOD/MODULO fixes

[Janne Blomqvist]

Hi,

the attached patch implements a few fixes and cleanups for the MOD and
MODULO intrinsics.

- When the arguments are constant, use mpfr_fmod instead of the naive
algorithms which are numerically unstable for large arguments. This
extends the PR 24518 fix to constant arguments as well, and makes the
compile-time evaluation match the runtime implementation which also
uses fmod in the same manner.

- Remove the old fallback path for the case builtin_fmod is not
available, as the builtin is AFAICS always available.

The patch does not per se fix the corner-case bug as reported in PR
49010, in fact it makes it worse in a way as with the patch the result
if the arguments are parameters is the same as the runtime result
(previously, the compile-time result was correct). But, I think we
should leave it as it is. Due to the reasons above, we're not using
the naive algorithms anyway, and IMHO -0.0 is quite a good
approximation for +0.0 anyway. One might even argue that due to the
numerical instability, specifying the naive algorithms is a bug in the
standard.

The patch adds notes to the documentation about the usage of fmod, so
users interested in corner-case behavior can look up how that function
is supposed to behave on their target. FWIW, AFAICS MPFR and glibc
fmod conform to the behavior specified in C99 Annex F.

Regtested on x86_64-unknown-linux-gnu, Ok for trunk?

2012-03-14  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/49010
	PR fortran/24518
	* intrinsic.texi (MOD,MODULO): Mention usage of fmod instead of
	naive algorithm.
	* simplify.c (gfc_simplify_mod): Use mpfr_fmod.
	(gfc_simplify_modulo): Likewise.
	* trans-intrinsic.c (gfc_conv_intrinsic_mod): Remove fallback as
	builtin_fmod is always available.


-- 
Janne Blomqvist
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