[LLVMdev] <IsKill> getting from MachineOperand is just <Used> attribute from logic.
Jakob Stoklund Olesen
stoklund at 2pi.dk
Thu Jan 14 19:44:39 PST 2010
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.
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