On 2017-07-25 18:04 +0200, Astor Piaz wrote:
> Hi Alexander,
>
> Thank you, your answer is pretty clear and your example is much better.
>
> Could I ask you though, why wouldn't this be allowed once the user has
> requested -fassociative-math ?
Alexander said this is not implemented, but it should be allowed. For
integer operands, this is allowed by ISO C/C++ standards. For floating-
point operands, this should be allowed by -fassociative-math.
> Is there a fundamental problem to provide this optimization or it is
> just too difficult?
I don't know, but it seems no existing compilers has implemented it yet.
> Thanks a lot.
>
> Best,
> --
> Astor
>
> On Tue, Jul 25, 2017 at 5:11 PM, Alexander Monakov <amonakov@xxxxxxxxx> wrote:
> > Hi,
> >
> > I think what you're asking is not the "usual" loop invariant motion, but rather
> > applying distributive law to reductions. In your example you want the optimizer
> > to replace
> >
> > R = 0;
> > for ( ... )
> > R += X * C;
> >
> > by
> >
> > R = 0;
> > for ( ... )
> > R += X;
> > R *= C;
> >
> > We don't do that even for integer operands, the following isn't optimized either:
> > (neither do Clang and ICC according to my experiments on gcc.godbolt.org)
> >
> > int f(int *a)
> > {
> > int r = 0;
> > for (int i = 0; i < 1024; i++)
> > r += a[i] * 5;
> > return r;
> > }
> >
> > Alexander
--
Xi Ruoyao <ryxi@xxxxxxxxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
