llvm dev - Jul 2015 - [LLVMdev] How to fix RegisterPressureTracker after inserting a new Machine Instruction during MachineScheduler Pass

Hi

I'm doing some MI scheduling optimization in which involves creating a new
machine instruction and insert it into the machine basic block that is
being scheduled. The new MI created is using a new virtual register which
means at least I have to make registerPressureTracker be aware of it.

After a little digging, I found that in RegisterPressure.h

LiveRegSet LiveRegs   is responsible for keeping track of all the live
virtual/physical registers, but it's using two sparseSets that only allow
to resize before any element is inserted. But I have to insert the new
instruction after calling buildDAGWithRegPressure() in which it calls
 initRegPressure()

So I'm wondering what's the right approach of doing this, and also is
there
any other book keeping structure that I have to update.

Thanks,
Patrick
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