[llvm] [GISel][RISCV]Implement indirect parameter passing (PR #95429)

[Michael Maitland via llvm-commits]

================
@@ -790,6 +805,41 @@ bool CallLowering::handleAssignments(ValueHandler &Handler,
       CCValAssign &VA = ArgLocs[j + Idx];
       const ISD::ArgFlagsTy Flags = Args[i].Flags[Part];
 
+      // We found an indirect parameter passing, and we have an
+      // OutgoingValueHandler as our handler (so we are at the call site or the
+      // return value). In this case, start the construction of the following
+      // GMIR, that is responsible for the preparation of indirect parameter
+      // passing:
+      //
+      // %1(indirectly passed type) = The value to pass
+      // %3(pointer) = G_FRAME_INDEX %stack.0
+      // G_STORE %1, %3 :: (store (s128), align 8)
+      //
+      // After this GMIR, the remaining part of the loop body will decide how
+      // to get the value to the caller and we break out of the loop.
+      if (VA.getLocInfo() == CCValAssign::Indirect &&
+          !Handler.isIncomingArgumentHandler()) {
+        Align StackAlign = DL.getPrefTypeAlign(Args[i].Ty);
+        MachineFrameInfo &MFI = MF.getFrameInfo();
+        int FrameIdx = MFI.CreateStackObject(OrigTy.getScalarSizeInBits(),
+                                             StackAlign, false);
+
+        Register PointerToStackReg =
+            MIRBuilder.buildFrameIndex(PointerTy, FrameIdx).getReg(0);
+
+        MachinePointerInfo DstMPO =
+            MachinePointerInfo::getFixedStack(MF, FrameIdx);
+
+        Align DstAlign =
+            std::max(DL.getStackAlignment(), inferAlignFromPtrInfo(MF, DstMPO));
+
+        MIRBuilder.buildStore(Args[i].OrigRegs[Part], PointerToStackReg, DstMPO,
+                              DstAlign);
+
+        ArgReg = PointerToStackReg;
+        IndirectParameterPassingHandled = true;
+      }
+
       if (VA.isMemLoc() && !Flags.isByVal()) {
----------------
michaelmaitland wrote:

Is it true that `!VA.isMemLoc` when we are handling an indirect parameter passing? I recall that you said it may have been the case [here](https://github.com/llvm/llvm-project/pull/95429#discussion_r1639481063).

If that is the case, can we do 
```
Handler.assignValueToAddress(ArgReg, StackAddr, PointerTy, MPO, VA);
break;
```

on line 840 (inside the indirect block above) (and remove IndirectParameterPassingHandled)?


https://github.com/llvm/llvm-project/pull/95429