Spare Register at one Machine Instruction

Hi All,

Given a machine instruction, is it possible to tell which register(s) is still not in use?

For example, given one instruction A, if the one follows it (say B) defines register rax, then I can tell rax should spare at instruction A.

The purpose is to use the spare register to replace registers used by A, for instrumentation purpose.

I believe what you’re after is the register scavenger.

It’s in: include/llvm/CodeGen/RegisterScavenging.h
Implementation: lib/CodeGen/RegisterScavenging.cpp

There is also the LivePhysReg facility that I would recomment if you just want to query for a free register and do not need the full feature set of the RegisterScavenger.

  • Matthias

Hi Nemanja and Matthias,

Thanks for the reply.

I checked both Register Scavenger and LivePhysReg. It seems that both work on the BasicBlock level, right?

Is it possible to perform such analysis in a whole function? For example, considering the calling convention (where the rsi, rdi will be in use from the beginning) and the return convention (where rax will be in use in the last bb).

I’m not sure exactly what you’re after.

I was under the impression that you want to know which register is live at a specific point (an instruction). If that’s the case, how do one of the two suggested solutions not suffice?
If a register is live-in to a block and not killed before your instruction or it has a def and no kill within the block, it is live. Otherwise it is dead and available. Isn’t it?

Or are you interested in whether a physical register is unused in the entire function?

I just realized that I forgot to mention where you can get the info about a physical register unused in the function (if that’s what you’re after):
MachineRegisterInfo::isConstantPhysReg()

“I was under the impression that you want to know which register is live at a specific point (an instruction).” This is exactly what I’m looking for.

Sorry for the misunderstanding. After double checking I found that LivePhysReg does consider the arguments and the return value of a function. So both of them work for my purpose.

Thanks again.

Sounds like Hu found a solution, so this is just for the record:

I just realized that I forgot to mention where you can get the info about a physical register unused in the function (if that’s what you’re after):
MachineRegisterInfo::isConstantPhysReg()

isConstantPhysReg() is not really a test for checking whether a register is completely unused in a function. That is done with MachineRegisterInfo::isPhysRegUsed().

Thanks for clarifying Matthias.