Add support for getRegisterByName.

[sidneym]

Summary

Support required to build the Hexagon Linux kernel.
Code like this:
register int r19 asm("r19");
int foo() {

return r19;

}

[sidneym]

Hexagon only supports named register, r19.

llvm/trunk/lib/Target/Hexagon/HexagonISelLowering.h

public:
 
     SDValue LowerReturn(SDValue Chain, CallingConv::ID CallConv, bool isVarArg,
                         const SmallVectorImpl<ISD::OutputArg> &Outs,
                         const SmallVectorImpl<SDValue> &OutVals,
                         const SDLoc &dl, SelectionDAG &DAG) const override;
 
     bool mayBeEmittedAsTailCall(const CallInst *CI) const override;
 
+    unsigned getRegisterByName(const char* RegName, EVT VT,
+                               SelectionDAG &DAG) const override;
+
     /// If a physical register, this returns the register that receives the
     /// exception address on entry to an EH pad.
     unsigned
     getExceptionPointerRegister(const Constant *PersonalityFn) const override {
       return Hexagon::R0;
     }
 
     /// If a physical register, this returns the register that receives the

llvm/trunk/lib/Target/Hexagon/HexagonISelLowering.cpp

 #include "HexagonMachineFunctionInfo.h"
 #include "HexagonRegisterInfo.h"
 #include "HexagonSubtarget.h"
 #include "HexagonTargetMachine.h"
 #include "HexagonTargetObjectFile.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringSwitch.h"
 #include "llvm/CodeGen/CallingConvLower.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineMemOperand.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/RuntimeLibcalls.h"
 #include "llvm/CodeGen/SelectionDAG.h"
 #include "llvm/CodeGen/TargetCallingConv.h"
bool HexagonTargetLowering::mayBeEmittedAsTailCall(const CallInst *CI) const {
   auto Attr =
       CI->getParent()->getParent()->getFnAttribute("disable-tail-calls");
   if (!CI->isTailCall() || Attr.getValueAsString() == "true")
     return false;
 
   return true;
 }
 
+unsigned  HexagonTargetLowering::getRegisterByName(const char* RegName, EVT VT,
+                                              SelectionDAG &DAG) const {
+  // Just support r19, the linux kernel uses it.
+  unsigned Reg = StringSwitch<unsigned>(RegName)
+                     .Case("r19", Hexagon::R19)
+                     .Default(0);
+  if (Reg)
+    return Reg;
+
+  report_fatal_error("Invalid register name global variable");
+}
+
 /// LowerCallResult - Lower the result values of an ISD::CALL into the
 /// appropriate copies out of appropriate physical registers.  This assumes that
 /// Chain/Glue are the input chain/glue to use, and that TheCall is the call
 /// being lowered. Returns a SDNode with the same number of values as the
 /// ISD::CALL.
 SDValue HexagonTargetLowering::LowerCallResult(
     SDValue Chain, SDValue Glue, CallingConv::ID CallConv, bool IsVarArg,
     const SmallVectorImpl<ISD::InputArg> &Ins, const SDLoc &dl,

llvm/trunk/test/CodeGen/Hexagon/namedreg.ll

+; RUN: llc -mattr=+reserved-r19 -march=hexagon < %s | FileCheck %s
+define dso_local i32 @r19f() #0 {
+entry:
+  %0 = call i32 @llvm.read_register.i32(metadata !0)
+  ret i32 %0
+}
+
+declare i32 @llvm.read_register.i32(metadata) #1
+
+!llvm.named.register.r19 = !{!0}
+
+!0 = !{!"r19"}
+; CHECK: r0 = r19