llvm dev - Jan 2009 - [LLVMdev] Question about VNInfo updates by LiveIntervals::addIntervalsForSpills

Hi,

It looks like LiveIntervals::addIntervalsForSpills does not update all
of the LiveIntervals infos quite correctly.

In particular, if a live interval L is defined by Reg<-Reg copy
instructions whose srcReg is later spilled by the
addIntervalsForSpills() function, its VNInfo information is not
updated in a proper way. It still points to the same MachineInstr as
before, even though the MachineInstr was rewritten by
addIntervalsForSpills(). This may lead to some problems during
coalescing attempts after spilling (I'm trying to do coalescing in an
iterative way), because the assertion in the
LiveIntervals::getVNInfoSourceReg() fails due to the fact that the
copy instruction defining the VNInfo of the live interval L is not a
Reg<-Reg move any more. It is a Reg<-Mem move after spilling function
invocation.

How should such a situation with VNInfos be handled in a proper way?
Any suggestions?

Thanks,
  -Roman

On Jan 30, 2009, at 10:18 AM, Roman Levenstein wrote:
> Hi,
>
> It looks like LiveIntervals::addIntervalsForSpills does not update all
> of the LiveIntervals infos quite correctly.
Correct.
>
>
> In particular, if a live interval L is defined by Reg<-Reg copy
> instructions whose srcReg is later spilled by the
> addIntervalsForSpills() function, its VNInfo information is not
> updated in a proper way. It still points to the same MachineInstr as
> before, even though the MachineInstr was rewritten by
> addIntervalsForSpills(). This may lead to some problems during
> coalescing attempts after spilling (I'm trying to do coalescing in an
> iterative way), because the assertion in the
> LiveIntervals::getVNInfoSourceReg() fails due to the fact that the
> copy instruction defining the VNInfo of the live interval L is not a
> Reg<-Reg move any more. It is a Reg<-Mem move after spilling function
> invocation.
The current implementation simply does not support this. If you want  
to do iterative coalescing like this, I would recommend you write a  
different addIntervalsForSpills. The current version is too  
complicated as it is.

Evan
>
>
> How should such a situation with VNInfos be handled in a proper way?
> Any suggestions?
>
> Thanks,
>  -Roman
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