[llvm] [RISCV] Share ArgGPRs array between SelectionDAG and GISel. (PR #74152)

[Wang Pengcheng via llvm-commits]

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@@ -986,6 +986,9 @@ bool CC_RISCV_FastCC(const DataLayout &DL, RISCVABI::ABI ABI, unsigned ValNo,
 bool CC_RISCV_GHC(unsigned ValNo, MVT ValVT, MVT LocVT,
                   CCValAssign::LocInfo LocInfo, ISD::ArgFlagsTy ArgFlags,
                   CCState &State);
+
+ArrayRef<MCPhysReg> getArgGPRs();
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wangpc-pp wrote:

> GISel doesn't need the other lists like ArgFPR16s, , ArgFPR32s, ArgFPR64s, ArgVRs, etc. If we move ArgGPRs I think we should move those to, but I don't see any good reason to do that since GISel doesn't need them. We already exported `CC_RISCV`, `CC_RISCV_FastCC`, `CC_RISCV_GHC` functions from SelectionDAG so GISel could use them. Those are the primary users of these arrays.

I didn't follow the process of GISel porting, is it because we haven't support FP CC so we don't need these lists? Anyway, I won't block this PR if you think it's better to not move. :-)

> 
> Or alternatively, maybe I could add something like this that we could share with SelectionDAG and GISel?
> 
> ```
> unsigned RISCV::getVarArgsRegistersToSave(CCState &State) {
>   unsigned Idx = State.getFirstUnallocated(ArgRegs);
>   return (ArgRegs.size() - Idx);
> }
> ```
This could be done. And, we can still see a lot of duplications in SelectionDAG and GISel, can these be shared too?


https://github.com/llvm/llvm-project/pull/74152