https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54192
Drea Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |pinskia at gcc dot gnu.org
--- Comment #13 from Drea Pinski <pinskia at gcc dot gnu.org> ---
(In reply to [email protected] from comment #9)
> On Tue, 21 Sep 2021, joseph at codesourcery dot com wrote:
>
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54192
> >
> > --- Comment #8 from joseph at codesourcery dot com <joseph at codesourcery
> > dot com> ---
> > On Tue, 21 Sep 2021, rguenther at suse dot de via Gcc-bugs wrote:
> >
> > > Yes, as said in other contexts GCC happily _removes_ traps if trapping
> > > is the only side-effect. _Unless_ you also have -fnon-call-exceptions
> > > enabled which is the only way to observe traps. So we consider
> > > a trap invoking undefined behavior unless you make them well-defined
> > > via -fnon-call-exceptions.
> >
> > That might be relevant to traps in the sense of changing control flow when
> > a floating-point exception is signaled. -fnon-call-exceptions doesn't
> > seem very relevant to the -ftrapping-math effects on transformations that
> > might affect the set of floating-point exception flags raised by some
> > code. As per my previous comment, -ftrapping-math currently affects (or
> > might affect if fully implemented) several different things:
> >
> > * Disallowing code transformations that cause some code to raise more
> > exception flags than it would have before.
> >
> > * Disallowing code transformations that cause some code to raise fewer
> > exception flags than it would have before.
>
> I might add that this particular point isn't followed thoroughly.
yes a good example is PR 53805:
```
int f(double a,double b){
if(a>b) if(a<b) return 1;
return 0;
}
```
Which since GCC 13 gets optimized (again) to return 0 at -O2. Because evrp
figures out that `a<b` will be zero and then cleanup cfg comes along and
removes the `a>b` comparison too.
So Maybe it is time to declare -fno-trapping-math as the default and go through
the testsuite and add -ftrapping-math as needed for the testcases that are
testing the bits.
