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.

lib/Target/Hexagon/HexagonISelLowering.h

 
     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

lib/Target/Hexagon/HexagonISelLowering.cpp

 #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"

   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

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