Implement getIPRACSRegs hook for riscv

This patch implements the getIPRACSRegs hook for riscv, similar to how
it's done for x86 in 14b567d.

Differently from the x86 hook, in this patch we only save ra (x1) and
not the frame pointer. I'm still working on a test case that shows the
need to save the fp.

The fixed test case was extracted from the gcc torture suite, but the
same issue is found in SPEC's xz.

Author: mikhailramalho

llvm/lib/Target/RISCV/RISCVCallingConv.td

@@ -42,6 +42,8 @@ def CSR_ILP32D_LP64D_V
 // Needed for implementation of RISCVRegisterInfo::getNoPreservedMask()
 def CSR_NoRegs : CalleeSavedRegs<(add)>;
 
+def CSR_IPRA : CalleeSavedRegs<(add X1)>;
+
 // Interrupt handler needs to save/restore all registers that are used,
 // both Caller and Callee saved registers.
 def CSR_Interrupt : CalleeSavedRegs<(add X1, (sequence "X%u", 5, 31))>;

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -55,6 +55,11 @@ RISCVRegisterInfo::RISCVRegisterInfo(unsigned HwMode)
     : RISCVGenRegisterInfo(RISCV::X1, /*DwarfFlavour*/0, /*EHFlavor*/0,
                            /*PC*/0, HwMode) {}
 
+const MCPhysReg *
+RISCVRegisterInfo::getIPRACSRegs(const MachineFunction *MF) const {
+  return CSR_IPRA_SaveList;
+}
+
 const MCPhysReg *
 RISCVRegisterInfo::getCalleeSavedRegs(const MachineFunction *MF) const {
   auto &Subtarget = MF->getSubtarget<RISCVSubtarget>();

llvm/lib/Target/RISCV/RISCVRegisterInfo.h

@@ -62,6 +62,8 @@ struct RISCVRegisterInfo : public RISCVGenRegisterInfo {
 
   const MCPhysReg *getCalleeSavedRegs(const MachineFunction *MF) const override;
 
+  const MCPhysReg *getIPRACSRegs(const MachineFunction *MF) const override;
+
   BitVector getReservedRegs(const MachineFunction &MF) const override;
   bool isAsmClobberable(const MachineFunction &MF,
                         MCRegister PhysReg) const override;