[LLVMdev] undefs in phis

[David Greene]

On Monday 02 February 2009 14:29, Evan Cheng wrote:
> On Feb 2, 2009, at 12:12 PM, David Greene wrote:
> > On Monday 02 February 2009 13:14, Evan Cheng wrote:
> >> I am sorry I don't really follow it. Is this what you are describing?
> >>
> >> %v1177 = undef
> >> ...
> >> loop:
> >> ...
> >> %v1176 = op ...
> >>                 = %v1177
> >> %v1177 = %v1176
> >> jmp loop
> >>
> >> Why is not safe to coalesce the 2 registers?
> >
> > Not quite.  The original code is:
> >
> > %v1177 = undef
> > %v1645 =  ...
> > loop:
> > %v1176 = %v1645
> > ...
> >  = %v1176
> >  = %v1177
> > %v1645 = op ...
> > %v1177 = %v1176
> > jmp loop
> >
> > We can't coalesce %v1177 and %v1176 legally.  But we do.
>
> Seriously, why not? In the first iteration, it's totally legal for
> v1177 has the same value as v1176. It's defined by an undef, it's
> allowed to have contain any value.

Think about what will happen the 2nd iteration.  %v1177 will have the value of 
%v1645 which is wrong.  This is because %v1176 in bb74 will be replaced with 
%v1177.  That's incorrect.

                                              -Dave