llvm dev - Feb 2009 - [LLVMdev] undefs in phis

On Monday 02 February 2009 23:55, Evan Cheng wrote:
> >> 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.
> >
> > Ok, right. The trick to fixing is to make sure the valno of the def of
> > v1177 hasPHIKill to true and make sure the coalescer checks it.
What does hasPHIKill mean, what are the consequences of using it and how do I 
know when I can set it?  I assume this would have been set by someone.  Or is 
that part of the bug?
> Actually liveintervals can construct a v1177 live range starting from  
> the beginning mbb with a val# of unknown def.
What's "the beginning mbb?"  The start of the bb containing the
phi (bb74 in
this case)?  Again, how would I instruct liveintervals to do this?  The 
IMPLICIT_DEF is already gone by that point.

                                                   -Dave

On Feb 5, 2009, at 5:30 PM, David Greene wrote:
> On Monday 02 February 2009 23:55, Evan Cheng wrote:
>
>>>> 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.
>>>
>>> Ok, right. The trick to fixing is to make sure the valno of the  
>>> def of
>>> v1177 hasPHIKill to true and make sure the coalescer checks it.
>
> What does hasPHIKill mean, what are the consequences of using it and  
> how do I
> know when I can set it?  I assume this would have been set by  
> someone.  Or is
> that part of the bug?
hasPHIKill just means it has a phi use so it's not possible to  
determine where the value is killed. Look for LiveIntervalAnalysis.cpp.
>
>
>> Actually liveintervals can construct a v1177 live range starting from
>> the beginning mbb with a val# of unknown def.
>
> What's "the beginning mbb?"  The start of the bb containing
the phi
> (bb74 in
> this case)?  Again, how would I instruct liveintervals to do this?   
> The
> IMPLICIT_DEF is already gone by that point.
%v1645 =  ...
loop:
%v1176 = %v1645
...
= %v1176
= %v1177
%v1645 = op ...
%v1177 = %v1176
jmp loop

When the live interval for v1177 is created, the code should recognize  
v1177 is live-in to the MBB. So even though it's not clear where it's  
defined, it's still clear it's live-in to the MBB.

Evan
>
>
>                                                   -Dave
>
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On Thursday 05 February 2009 19:55, Evan Cheng wrote:
> hasPHIKill just means it has a phi use so it's not possible to
> determine where the value is killed. Look for LiveIntervalAnalysis.cpp.
Ok.

> %v1645 =  ...
> loop:
> %v1176 = %v1645
> ...
> = %v1176
> = %v1177
> %v1645 = op ...
> %v1177 = %v1176
> jmp loop
>
> When the live interval for v1177 is created, the code should recognize
> v1177 is live-in to the MBB. So even though it's not clear where
it's
> defined, it's still clear it's live-in to the MBB.
Yeah, ok, that should work.  Thanks!

                                         -Dave