llvm dev - May 2016 - Default Implementation for Pseudo Instructions

Hi LLVM-dev,

I'm working on XRay, and one of the parts of it is the introduction of a
couple of pseudo instructions -- one of which is PATCHABLE_RET, which wraps
a return instruction (similar to how PATCHABLE_OP works).

In http://reviews.llvm.org/D19904, I'm trying to find a way of making
PATCHABLE_RET have a default implementation in platforms where they're not
supported yet (i.e., not X86).

In my very limited research into this, I think I've narrowed it down to two
places:

- SelectionDAGTargetInfo has some hooks for specific instructions to be
lowered/handled. A default implementation could be to just unpack the
operands from the PATCHABLE_* instructions. In particular I'm looking at
potentially adding some more functions to
https://github.com/llvm-mirror/llvm/blob/master/include/llvm/CodeGen/SelectionDAGTargetInfo.h
specifically
for the PATCHABLE_* instructions, and finding where the SelectionDAG stuff
is invoked to special-case the PATCHABLE_* instructions.

- TargetLowering is another place where this might work. We can have
special dispatch for the PATCHABLE_* instructions there, and also where we
use a custom inserter. We can mark the instructions to use a custom
inserter, have some overridable hooks in TargetLowering which by default
just unpacks the operands into new MachineInstr instances, then override
those in X86's specific lowering. Requisite link:
http://llvm.org/docs/doxygen/html/classllvm_1_1TargetLowering.html

Some questions:

1) Is this even something I need to worry about? Since PATCHABLE_RET is
being inserted by a late MachineFunctionPass, it could just do this for a
white-listed set of platforms. This seems not ideal.

2) Machine IR (mir) seems to be where PATCHABLE_RET might show up, and may
need to be hand-crafted. Are the above two places the right places where
these instructions could be intercepted, or am I missing something more
fundamental?

Thanks in advance!
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