I wish someone could help me to clarify a register allocation question. I will use X86 in this example, but it really does not matter too much.
Imagine that I have a virtual v stored in register AL, and I am about to cross a function call, that effectively overwrites AL. Well, I have, say, register ESI free, but as it happens, ESI is bigger than AL. It should be technically possible to move the contents of AL into ESI, to avoid spilling 'v', but I don't know if that is possible in LLVM. Basically, ESI is not in the class of 'v', and it has no 8 bit alias. Is
this a problem (the move can't be done), or am I missing something?
all the best,
Very interesting idea. Yes, this is doable. On X86 you would copy EAX to ESI, which implicitly saves AL because it's a superclass of AL. MRegisterInfo has information about super and subclass registers, Evan can tell you more.
The short answer is this is not currently done. LLVM register allocator does not currently use a move to a different register class in lieu of spills. This is definitely something we should do in the future. Can you file a bugzilla enhancement request?
Does this depend on vreg subreg support? It sounds as if it could be vaguely related, but I’m not sure it’s necessary.
I consider this an orthogonal issue. While this particular example make use of sub-register info, that’s not always the case. You can conceivably see a target where it’s cheap(er) to move from an integer register to a floating point one, etc.