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

[Gábor Spaits via llvm-commits]

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@@ -790,6 +806,68 @@ 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.
+      // NOTE: In the case, when the the pointer pointing to the value is passed
+      // in a register there is an exception to this, that is detailed bellow.
+      if (VA.getLocInfo() == CCValAssign::Indirect && Flags.isSplit() &&
+          !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;
+
+        Align FlagAlignment{};
+        if (Flags.isByVal()) {
+          FlagAlignment = Flags.getNonZeroByValAlign();
+        } else {
+          FlagAlignment = Flags.getNonZeroOrigAlign();
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spaits wrote:

I will use `DataLayout` `getStackAlignment` function, so the stack alignment is taken into account.

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