[RISCV GlobalISel] Adding initial GlobalISel infrastructure

Summary:
Add an initial GlobalISel skeleton for RISCV. It can only run ir translator for `ret void`.

Patch by Andrew Wei

Reviewers: asb, sabuasal, apazos, lenary, simoncook, lewis-revill, edward-jones, rogfer01, xiangzhai, rovka, Petar.Avramovic, mgorny, dsanders

Reviewed By: dsanders

Subscribers: pzheng, s.egerton, dsanders, hiraditya, rbar, johnrusso, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, MartinMosbeck, brucehoult, the_o, PkmX, jocewei, psnobl, benna, Jim, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65219

llvm-svn: 369467
diff --git a/llvm/lib/Target/RISCV/RISCVCallLowering.h b/llvm/lib/Target/RISCV/RISCVCallLowering.h
new file mode 100644
index 0000000..7ce074a
--- /dev/null
+++ b/llvm/lib/Target/RISCV/RISCVCallLowering.h
@@ -0,0 +1,42 @@
+//===-- RISCVCallLowering.h - Call lowering ---------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// This file describes how to lower LLVM calls to machine code calls.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIB_TARGET_RISCV_RISCVCALLLOWERING_H
+#define LLVM_LIB_TARGET_RISCV_RISCVCALLLOWERING_H
+
+#include "llvm/CodeGen/CallingConvLower.h"
+#include "llvm/CodeGen/GlobalISel/CallLowering.h"
+#include "llvm/CodeGen/ValueTypes.h"
+
+namespace llvm {
+
+class RISCVTargetLowering;
+
+class RISCVCallLowering : public CallLowering {
+
+public:
+  RISCVCallLowering(const RISCVTargetLowering &TLI);
+
+  bool lowerReturn(MachineIRBuilder &MIRBuiler, const Value *Val,
+                   ArrayRef<Register> VRegs) const override;
+
+  bool lowerFormalArguments(MachineIRBuilder &MIRBuilder, const Function &F,
+                            ArrayRef<ArrayRef<Register>> VRegs) const override;
+
+  bool lowerCall(MachineIRBuilder &MIRBuilder,
+                 CallLoweringInfo &Info) const override;
+};
+
+} // end namespace llvm
+
+#endif // LLVM_LIB_TARGET_RISCV_RISCVCALLLOWERING_H