How to implement lowerReturn for poring GlobalISel to RISCV?

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1

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!

[1] http://llvm.org/devmtg/2017-10/#tutorial2

[2] riscv-isa-manual/machine.tex at master · riscv/riscv-isa-manual · GitHub

[3] ⚙ D40383 Add RISCV privileged instructions

[4] llvm/ARMCallLowering.cpp at master · llvm-mirror/llvm · GitHub

2

lowerReturn corresponds to the LLVM IR "ret" instruction, which is always a normal function return (JALR), not a return from an interrupt.

-Eli

3

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!

lowerReturn corresponds to the LLVM IR "ret" instruction, which is always a normal function return (JALR), not a return from an interrupt.

Already fixed https://github.com/xiangzhai/llvm/commit/0adf9aa558dd452f8e5bffa3f5b127c58f9d3a43 but thank you to point out my fault!