[LLVMdev] ComputeMaskedBits Bug

[David Greene]

On Friday 18 July 2008 00:36, Nick Lewycky wrote:
> David Greene wrote:
> > Is my analysis correct?  If so, is the PHI code the culprit (for not
> > returning the min of the KnownZero bits) or is the Shl code the culprit
> > (for not paying attention to the Mask passed in (it right shifts it)?
>
> I think your analysis is correct, and that Shl -- and many of the other
> operations (AShr, LShr, SExt, Add?, Call?) -- should be modified to
> always respect Mask.

After thinking about this some more, I'm not so sure.  The Shl really DOES
result in six low zero bits, so it should return that.  That PHI node has
semantics that it should take care of itself (taking the min of the known
zero and one bits).

It seems a little brittle to me to have the PHI pass its semantics down
to children via the Mask.  It should just do what it knows is right in the
first place.

That said, there are many places that don't respect the Mask.  Closer
reading of the comment leads me to believe the Mask is simply a 
time-saving device, not a correctness-enforcing mechanism.

I've fixed the PHI analysis to do the min in our code and it fixes the
testcase I was working on.  Doing a min like this would also allow us
to have PHI nodes compute known zero and one bits even when there
isn't a recurrence.

                                                    -Dave