[SystemZ][z/OS] Add vararg support to z/OS (#68834)

This PR adds vararg support to z/OS and updates the call-zos-vararg.ll
lit test.

Co-authored-by: Yusra Syeda <yusra.syeda@ibm.com>

llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp

@@ -1275,6 +1275,30 @@ void SystemZXPLINKFrameLowering::emitPrologue(MachineFunction &MF,
     for (MachineBasicBlock &B : llvm::drop_begin(MF))
       B.addLiveIn(Regs.getFramePointerRegister());
   }
+
+  // Save GPRs used for varargs, if any.
+  const TargetInstrInfo *TII = Subtarget.getInstrInfo();
+  bool IsVarArg = MF.getFunction().isVarArg();
+
+  if (IsVarArg) {
+    // FixedRegs is the number of used registers, accounting for shadow
+    // registers.
+    unsigned FixedRegs = ZFI->getVarArgsFirstGPR() + ZFI->getVarArgsFirstFPR();
+    auto &GPRs = SystemZ::XPLINK64ArgGPRs;
+    for (unsigned I = FixedRegs; I < SystemZ::XPLINK64NumArgGPRs; I++) {
+      uint64_t StartOffset = MFFrame.getOffsetAdjustment() +
+                             MFFrame.getStackSize() + Regs.getCallFrameSize() +
+                             getOffsetOfLocalArea() + I * 8;
+      unsigned Reg = GPRs[I];
+      BuildMI(MBB, MBBI, DL, TII->get(SystemZ::STG))
+          .addReg(Reg)
+          .addReg(Regs.getStackPointerRegister())
+          .addImm(StartOffset)
+          .addReg(0);
+      if (!MBB.isLiveIn(Reg))
+        MBB.addLiveIn(Reg);
+    }
+  }
 }
 
 void SystemZXPLINKFrameLowering::emitEpilogue(MachineFunction &MF,

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

@@ -1613,7 +1613,23 @@ SDValue SystemZTargetLowering::LowerFormalArguments(
       InVals.push_back(convertLocVTToValVT(DAG, DL, VA, Chain, ArgValue));
   }
 
-  // FIXME: Add support for lowering varargs for XPLINK64 in a later patch.
+  if (IsVarArg && Subtarget.isTargetXPLINK64()) {
+    // Save the number of non-varargs registers for later use by va_start, etc.
+    FuncInfo->setVarArgsFirstGPR(NumFixedGPRs);
+    FuncInfo->setVarArgsFirstFPR(NumFixedFPRs);
+
+    auto *Regs = static_cast<SystemZXPLINK64Registers *>(
+        Subtarget.getSpecialRegisters());
+
+    // Likewise the address (in the form of a frame index) of where the
+    // first stack vararg would be.  The 1-byte size here is arbitrary.
+    // FIXME: Pre-include call frame size in the offset, should not
+    // need to manually add it here.
+    int64_t VarArgOffset = CCInfo.getStackSize() + Regs->getCallFrameSize();
+    int FI = MFI.CreateFixedObject(1, VarArgOffset, true);
+    FuncInfo->setVarArgsFrameIndex(FI);
+  }
+
   if (IsVarArg && Subtarget.isTargetELF()) {
     // Save the number of non-varargs registers for later use by va_start, etc.
     FuncInfo->setVarArgsFirstGPR(NumFixedGPRs);