I have ported LLC to a risc cpu. It can pass benchmark that I have at current.
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.
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.).
1. Does <kill> attribute of R4 at MBB0 is a unimportant and redundancy messages, Or a little bug???
2. Is it unreliable to get a physical register's liverange by Kill, Dead messages from MachineBasicBlock??
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.
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@2pi.dk> 写道：
