[llvm-dev] How to implement lowerReturn for poring GlobalISel to RISCV?

[Alex Bradbury via llvm-dev]

On 21 December 2017 at 05:03, Leslie Zhai <lesliezhai at llvm.org.cn> wrote:
> Hi LLVM developers,
>
> Thank Daniel Sanders, Aditya Nandakumar and Justin Bogner's Tutorial[1]:
> Head First into GlobalISel about how to port, and Aditya took BPF target as
> a simple instance:
>
>
> bool BPFCallLowering::lowerReturn(MachineIRBuilder &MIRBuilder,
>                                   const Value *Val, unsigned VReg) const {
>   assert(!Val == !VReg && "Return value without a vreg");
>   MIRBuilder.buildInstr(BPF::RET);
>   return true;
> }
>
>
> But how to implement it for RISCV target?
> https://github.com/xiangzhai/llvm/commit/c49146edbbf655e97727e22e4a87a020fb8da6e5
>
> Because there are separate trap return instructions[2] per privilege level:
> MRET, SRET, and URET. MRET is always provided, while SRET must be provided
> if supervisor mode is supported. URET is only provided if user-mode traps
> are supported. and David added RISCV privileged instructionsit[3], then
> merged into upstream, it might be more complex than ARM[4] please give me
> some hint, thanks a lot!

Hi Leslie. Although it's useful to think ahead, I'd really recommend
to try to incrementally build up support as was done in my RISC-V
SelectionDAGISel implementation and the GlobalISel tutorials. An
initial milestone might be generating a return from a void function,
then a function with a simple addition in its body and so on.

The code generator doesn't need to generate MRET/SRET/URET
instructions. Typically these will just be used in hand-written
assembly (you can find examples of usage in
https://github.com/riscv/riscv-pk for instance). It might make sense
in the future to define a function attribute for interrupt handlers
much like Mips (https://gcc.gnu.org/onlinedocs/gcc/MIPS-Function-Attributes.html),
but that would require co-ordination with the GCC developers.

So the good news is that return lowering should be much simpler than
you're currently thinking.

Best,

Alex