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Re: [PATCH] Fix PR71762
On Thu, 10 Nov 2016, Richard Biener wrote:
> On November 10, 2016 7:39:57 PM GMT+01:00, Marc Glisse <marc.glisse@inria.fr> wrote:
> >On Thu, 10 Nov 2016, Richard Biener wrote:
> >
> >> The following fixes PR71762 via reverting the transforms of
> >> ~X & Y to X < Y and similar because when the bools they apply to
> >> are expanded to RTL undefined values are not reliably zero-extended
> >> and thus the transform is invalid. Ensuring the zero-extension
> >> is too costly IMHO and the proper fix is to move the transform
> >> to RTL where we can check known-zero-bits to validate validity
> >> or to fix GIMPLE not not have operations on types not matching their
> >mode
> >> in precision.
> >
> >Can you explain why this particular transformation is special? We have
> >a
> >number of other optimizations that take advantage of the fact that bool
> >is
> >in [0, 1], even without looking at VRP, say for instance x != 0 -> x.
> >Are
> >we supposed to remove all of them?
>
> No. But UNDEFINED & 0 is well-defined
> Even if precision ends up being bigger. Undefined != 0 is never well-defined.
Now applied to trunk.
Richard.