On Tue, May 10, 2016 at 11:14:35AM +0200, Richard Biener wrote:
> On Mon, May 9, 2016 at 3:36 PM, Marc Glisse <marc.gli...@inria.fr> wrote:
> > On Mon, 9 May 2016, Dominik Vogt wrote:
> >
> >> This turns out to be quite difficult. A small test function
> >> effectively just returns the argument:
> >>
> >> unsigned long bar (unsigned long in)
> >> {
> >> if ((in & 1) == 0)
> >> in = (in & ~(unsigned long)1);
> >>
> >> return in;
> >> }
> >>
> >> However, Gcc does not notice that the AND is a no-op. As far as I
> >> understand, zero bit tracking is only done in "combine", so when
> >> folding the assignment statement the information that the lowest
> >> bit is zero is not available and therefore the no-op is not
> >> detected?
> >
> >
> > VRP is also supposed to track bits that may/must be non-zero. It may be
> > possible to enhance it to handle this case.
>
> Actually VRP only tracks value-ranges, CCP tracks may/must be non-zero bits
> but does not have a conditional lattice.
Does it track known *one* bits too? Because the AND doesn't get
eliminated here either:
in = in | 3;
in = in ^ 1;
in = (in & ~(unsigned long)1);
Ciao
Dominik ^_^ ^_^
--
Dominik Vogt
IBM Germany
