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Hi Matthias, Thanks for doing this. Each time we talk about it, it takes us 10 min to rebuild those rules form our recollection, so definitely useful to write them down. I am in agreement with what you wrote down. I just think we need additional rules for the constant registers like Jakob mentioned: - Their value is constant (i.e., copy propagation is fine, unlike regular reserved registers). -

Hi Matthias, This pretty much matches my memory. I think that the rules are a bit ad hoc and not followed to the letter everywhere. It would be good to codify something concrete. I thought that I added some way of distinguishing between constant registers and other reserved registers but I can’t find it now. We do some register coalescing that is not consistent with your rules here: If a virtual

There is MachineRegisterInfo::isConstantPhysReg(), in the current implementation this just returns true if it cannot find any def operand for the register (or on of its aliases). I think we also write to zero registers at times and then this function would return false... For this to work reliably targets would need to provide the constant information explicitely. For the "writing to them

Lately I have had a few discussions of what it means for a register to be unallocatable or reserved. As this comes up every now and again and I often struggled answering such questions I decided to write down some definite rules and codify the current usage and assumptions. I plan to put the rules below into the doxygen comments of MachineRegisterInfo etc. And I also hope that people will correct