[LLVMdev] <IsKill> getting from MachineOperand is just <Used> attribute from logic.

[任坤]

Hi, Jakob:

  Thanks for your answer.
  I hope to trace all physical register liverange in MachineBasicBlock.
In my test, I find LiveIn message of MBB can not give all livein 
physical register. So I write a pass to recollect livein message
by scan MBB. 
  
  Current case tell me that just to scan MachineOperand's isDef, isKill, IsDead attribute to rebuild physical register's livein will have bug.

  If I use add missing live-in into <live-in list for each MBB>, 
Could I can know which physical register is live at any time? 

  If yes, it is easy for my pass. 

  If not, I need to treat isKill and isDead as isUse, then implement a pass to anaylze CFG to delete unvalid livein message. 


--- 10年1月15日，周五, Jakob Stoklund Olesen <stoklund at 2pi.dk> 写道：

> 发件人: Jakob Stoklund Olesen <stoklund at 2pi.dk>
> 主题: Re: [LLVMdev] <IsKill> getting from MachineOperand is just <Used> attribute from logic.
> 收件人: "任坤" <hbrenkun at yahoo.cn>
> 抄送: "llvm" <llvmdev at cs.uiuc.edu>
> 日期: 2010年1月15日,周五,上午11:44
> 
> On Jan 14, 2010, at 6:39 PM, 任坤 wrote:
> 
> > But I want do some optimization after register
> alloction by adjusting 
> > register using. I scan MachineBasicBlock to analyze
> operand's IsKill, IsDead , IsDef attribute to get a physical
> register's liverange. But I get a strange case at MBB.jpg.
> 
> You can also look at RegisterScavenging.cpp and
> MachineVerifier.cpp. They are doing the same thing.
> 
> >  R4 is marked <kill> at MBB0.  If I
> scan R4's liverange by [MBB0->MBB1->MBB2]. I will find
> R4 first is killed, then is used. It can not unlogisch.
> Attually R4 just is <Used>. It will cause my
> optimization pass crash(Actually, I ingore Live In message
> of MBB. I recollect live in messges at my pass.).
> 
> A register should not be used after it is killed, and if it
> is needed by a successor block, it should be live out.
> 
> Note that a register in the live-in list of an MBB is not
> always live-out from all predecessors. A register defined by
> IMPLICIT_DEF can be optimized away entirely.
> 
> >  1. Does <kill> attribute of R4 at MBB0 is
> a unimportant  and redundancy messages, Or a little
> bug???
> 
> You have probably found a bug. Can you reproduce it with
> one of the normal back ends?
> 
> >  2. Is it unreliable to get a physical register's
> liverange by Kill, Dead messages from MachineBasicBlock?? 
> 
> You also need to use the live-in list for each MBB, but
> otherwise it should be reliable. Look at how
> RegisterScavenger is doing it.
> 
> /jakob
> 
> 


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