[Bug middle-end/84997] Optimize integer operations on floating point constants without -ffast-math

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84997

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-03-21
     Ever confirmed|0                           |1

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to joseph@codesourcery.com from comment #1)
> On Tue, 20 Mar 2018, antoshkka at gmail dot com wrote:
> 
> > For example
> > 
> > int test2(int lhs) {
> >     lhs += 2.0;
> >     return lhs;
> > }
> 
> That would need -fno-trapping-math, because if the addition results in a 
> double value larger than INT_MAX, under Annex F the conversion back to int 
> is defined to raise FE_INVALID along with producing an unspecified value.

If it produces an unspecified value it doesn't raise undefined behavior, right?
So replacing it with lhs += 2; is incorrect as if that overflows that's
undefined behavior.  So we could only replace it with lhs = (int)((unsigned)lhs
+ 2) and produce implementation defined behavior from "unspecified" which I
suppose is OK.