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

[Jakob Stoklund Olesen]

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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