Re: [PATCH] fold-const: Don't consider NaN non-negative [PR97965]

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Nov 26, 2020 at 09:16:29AM +0000, Richard Biener wrote:
> > So, I really don't know if we want this or not, posting it for discussions.
> 
> Is copysign (x, NaN) supposed to be well-defined?  We'd stop folding
> this then, no?

Yes, we'd stop folding several cases with NaNs.

>  I think the ABS_EXPR<x> < 0 to false folding is
> simply incomplete and should first check whether the operands are
> ordered?  That said, NaN is nonnegative but NaN < 0 isn't false(?)
> 
> So I don't think the patch is good.

Another possibility (if we have this optimization already in match.pd too)
would be to only optimize the < 0 case in GENERIC if !HONOR_NANS like
the >= 0 case is and only optimize it in GIMPLE.  Though with the
default -ftrapping-math I think even optimizing qNaN < 0 to 0 is incorrect,
even that should raise invalid exception, shouldn't it?
So perhaps add a defaulted argument to the *nonnegative* APIs that would
say whether unordered is ok or not?

	Jakub