Re: Move some flag_unsafe_math_optimizations using simplify and match

[Marc Glisse <marc dot glisse at inria dot fr>]

On Thu, 20 Aug 2015, Richard Biener wrote:

> On Thu, Aug 20, 2015 at 7:38 AM, Marc Glisse <marc.glisse@inria.fr> wrote:
> > On Thu, 20 Aug 2015, Hurugalawadi, Naveen wrote:
> >
> > > The following testcase does not generate "x" as needed.
> > > ====================
> > > double t (double x)
> > > {
> > > x = sqrt (x) * sqrt (x);
> > > return x;
> > > }
> > > ====================
> >
> >
> > With -fno-math-errno, we CSE the calls to sqrt, so I would expect this to
> > match:
> >
> >   (mult (SQRT@1 @0) @1)
> >
> > Without the flag, I expect that one will apply
> >
> >  (simplify
> >   (mult (SQRT:s @0) (SQRT:s @1))
> >   (SQRT (mult @0 @1)))
> >
> > and then maybe we have something converting sqrt(x*x) to abs(x) or maybe
> > not.
>
> ICK.  I'd rather have CSE still CSE the two calls by adding some tricks 
> regarding to errno ...
>
> > I wonder if all the unsafe math optimizations are really ok without
> > -fno-math-errno...
>
> Well, on GIMPLE they will preserve the original calls because of their 
> side-effects setting errno...  on GENERIC probably not.

But we are also introducing new math calls, and I am afraid those might 
set errno at an unexpected place in the code...

I don't know if anyone interested in errno would ever use 
-funsafe-math-optimizations though.

--
Marc Glisse