[llvm-dev] Running GlobaISel passes after SelectionDAG instruction selection

[Tom Stellard via llvm-dev]

Hi,

I've been experimenting with global isel over the last few weeks and it
is such a vast improvement over the SelectionDAG for the AMDGPU target
that I would really like to begin using it as soon as possible.

Given the lack of a replacement for SelectionDAG's legalizer / combiner,
and how much work this will be to implement, I think the fastest path to
doing this would be to run some of the GlobalISel passes after SelectionDAG
instruction selection.

What I would like to do is modify the AMDGPU target to select SelectionDAG
nodes to generic opcodes, and then run the InstructionSelector pass on
the resulting MachineFunction.

I would start by doing this with loads/stores and then move on to other
opcodes one at a time.

These changes will be a big improvement to AMDGPU, because it will allow
us to replace our SIFixSGPRCopies pass which is essentially just another
instruction selector that re-selects instructions based on their register
classes and also let us do a much better job of matching addressing modes,
which is very important for the AMDGPU target.

I already have a working prototype where I run SelectionDAG ISel and
then the InstructionSelector pass right afterwards, but I think to
make this approach work long-term, we will need to agree that this is a
supported use case of GlobalISel, and we will also need to start
building the GlobalISel code by default.

What do people think about this?

Thanks,
Tom