Re: [PATCH] Don't fold nextafter/nexttoward if -ftrapping-math or -fmath-errno if they produce denormal results (PR c/86420)

[Richard Biener <rguenther@suse.de>]

On Sat, 7 Jul 2018, Marc Glisse wrote:

> On Sat, 7 Jul 2018, Jakub Jelinek wrote:
> 
> > 2018-07-07  Jakub Jelinek  <jakub@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.

There's tree-call-dce.c which is doing a related (more complex) transform
but nothing doing constant propagation through calls.  I think it would
be more useful for the target C library to expose sth like
set_errno (...) so we can constant fold the errno setting as well.
Maybe there's some cheap non-DCEable math function we could abuse...

Richard.