On Sat, Jul 07, 2018 at 11:55:17AM +0200, Marc Glisse wrote:
> On Sat, 7 Jul 2018, Jakub Jelinek wrote:
>
> > 2018-07-07 Jakub Jelinek <ja...@redhat.com>
> >
> > PR c/86420
> > * real.c (real_nextafter): Return true if result is denormal.
>
> I have a question on the side: would it be hard / useful, in cases where
> nextafter may set errno or some exception flag, to fold the result to a
> constant while keeping the function call (ignoring the value it returns)? To
> clarify, I mean replace
>
> _2 = nextafter(DBL_DENORM_MIN, 0);
>
> with
>
> nextafter(DBL_DENORM_MIN, 0);
> _2 = 0;
>
> I think we already do that for some other calls, although I can't remember
> where. The point would be that we have the value of _2 and can keep folding
> its uses.
For errno purposes alone that would be possible, but the function is marked
#define ATTR_MATHFN_ERRNO (flag_errno_math ? \
ATTR_NOTHROW_LEAF_LIST : ATTR_CONST_NOTHROW_LEAF_LIST)
and thus with -ftrapping-math -fno-math-errno I'm afraid we'd immediately
DCE the call in the second form (without lhs).
Jakub
