[llvm-dev] COPYs between register classes

[Jonas Paulsson via llvm-dev]

Hi,

On SystemZ there are a set of "access registers" that can be copied in 
and out of 32-bit GPRs with special instructions. These instructions can 
only perform the copy using low 32-bit parts of the 64-bit GPRs. As 
reported and discussed at https://bugs.llvm.org/show_bug.cgi?id=44254, 
this is currently broken due to the fact that the default register class 
for 32-bit integers is GRX32, which also contains the high 32-bit part 
registers.

I have tried to find a simple way to constrain the register class of 
such copies (also at -O0), but this turned out to not be quite simple. 
Just selecting a pseudo instruction with a custom inserter does not seem 
to work since CopyToReg/CopyFromReg have special handlings in InstrEmitter.

I then tried in SystemZDAGToDAG.cpp to select a CopyToReg to (CopyToReg 
COPY_TO_REGCLASS), which worked fine. But I could not get the same 
results with CopyFromReg. (COPY_TO_REGCLASS CopyFromReg) only resulted 
in a later COPY into GR32, but the COPY from the Access register was 
still first made to GRX32.

One alternative might be to let InstrEmitter deduce the needed register 
class for a CopyFromReg if there is only one user which is a 
COPY_TO_REGCLASS. I thought this seemed a bit messy, so I instead tried 
adding a new TargetRegisterInfo hook that is used in InstrEmitter when 
emitting CopyToReg/CopyFromReg nodes (https://reviews.llvm.org/D75014).

Does this make sense, or is there a better way?

A related question is if the target is required to be able to implement 
*any* move between register classes?

Thanks,

Jonas