[Bug tree-optimization/57994] Constant folding of infinity

["vincent-gcc at vinc17 dot net" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57994

--- Comment #21 from Vincent LefÃvre <vincent-gcc at vinc17 dot net> ---
(In reply to Paolo Carlini from comment #19)
> If I change fold_builtin_logarithm to pass a true as last argument to
> do_mpfr_arg1 (thus 0 is accepted) and do_mpfr_ckconv to accept a folded
> result which is infinity, things finally work. Patchlet below. Note however,
> that I also need -O1 otherwise, at -O0, we don't try to propagate the
> constant num and mpfr isn't used, we again have a library call which returns
> -nan.

I don't know much about GCC internals, but did you think about exceptions, e.g.
FE_DIVBYZERO for log(0)?

Note that MPFR's divide-by-zero exception (flag) and associated functions are
new in MPFR 3.1.0.

> @@ -13527,7 +13527,7 @@ do_mpfr_ckconv (mpfr_srcptr m, tree type, int inex
>    /* Proceed iff we get a normal number, i.e. not NaN or Inf and no
>       overflow/underflow occurred.  If -frounding-math, proceed iff the
>       result of calling FUNC was exact.  */
> -  if (mpfr_number_p (m) && !mpfr_overflow_p () && !mpfr_underflow_p ()
> +  if (!mpfr_nan_p (m) && !mpfr_overflow_p () && !mpfr_underflow_p ()
>        && (!flag_rounding_math || !inexact))
[...]

If you do this, don't forget to update the comment. Ditto for the other
changes.