[Systemz/z/OS] Add llvm.read_register() intrinsic support for zOS

[yusra.syeda]

Summary

This patch adds the llvm.read_register() intrinsic for zOS by adding support for the getRegisterByName() function.

[Kai]

LGTM.

[uweigand]

LGTM.

[abhina.sreeskantharajan]

LGTM

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

if (Constraint[1] == 'v') {
                                  SystemZMC::VR128Regs, 32);
     }
   }
   return TargetLowering::getRegForInlineAsmConstraint(TRI, Constraint, VT);
 }
 
 // FIXME? Maybe this could be a TableGen attribute on some registers and
 // this table could be generated automatically from RegInfo.
-Register SystemZTargetLowering::getRegisterByName(const char *RegName, LLT VT,
-                                                  const MachineFunction &MF) const {
+Register
+SystemZTargetLowering::getRegisterByName(const char *RegName, LLT VT,
+  const SystemZSubtarget *Subtarget = &MF.getSubtarget<SystemZSubtarget>();
 
-  Register Reg = StringSwitch<Register>(RegName)
-                   .Case("r15", SystemZ::R15D)
-                   .Default(0);
+  Register Reg =
+      StringSwitch<Register>(RegName)
+          .Case("r4", Subtarget->isTargetXPLINK64() ? SystemZ::R4D : 0)
+          .Case("r15", Subtarget->isTargetELF() ? SystemZ::R15D : 0)
+
   if (Reg)
     return Reg;
   report_fatal_error("Invalid register name global variable");
 }
 
 void SystemZTargetLowering::
 LowerAsmOperandForConstraint(SDValue Op, std::string &Constraint,
                              std::vector<SDValue> &Ops,

llvm/test/CodeGen/SystemZ/zos-stackpointer.ll

+; RUN: llc < %s -mtriple=s390x-ibm-zos | FileCheck %s
+
+; CHECK-LABEL: get_stack:
+; CHECK: lgr   3, 4
+; CHECK: b 2(7)
+
+define i8* @get_stack() nounwind {
+entry:
+        %0 = call i64 @llvm.read_register.i64(metadata !0)
+        %1 = inttoptr i64 %0 to i8*
+  ret i8* %1
+}
+
+declare i64 @llvm.read_register.i64(metadata) nounwind
+
+!0 = !{!"r4"}