Cleanup and document MachineLocation.

Clarify documentation and API to make the difference between register and
register-indirect addressed locations more explicit. Put in a comment
to point out that with the current implementation we cannot specify
a register-indirect location with offset 0 (a breg 0 in DWARF).
No functionality change intended.

rdar://problem/13658587

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180641 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MachineLocation.h

@@ -9,7 +9,7 @@
 // The MachineLocation class is used to represent a simple location in a machine
 // frame.  Locations will be one of two forms; a register or an address formed
 // from a base address plus an offset.  Register indirection can be specified by
-// using an offset of zero.
+// explicitly passing an offset to the constructor.
 //
 // The MachineMove class is used to represent abstract move operations in the
 // prolog/epilog of a compiled function.  A collection of these objects can be
@@ -37,8 +37,10 @@ public:
   };
   MachineLocation()
     : IsRegister(false), Register(0), Offset(0) {}
+  /// Create a direct register location.
   explicit MachineLocation(unsigned R)
     : IsRegister(true), Register(R), Offset(0) {}
+  /// Create a register-indirect location with an offset.
   MachineLocation(unsigned R, int O)
     : IsRegister(false), Register(R), Offset(O) {}
 
@@ -48,17 +50,20 @@ public:
   }
 
   // Accessors
+  bool isIndirect()      const { return !IsRegister; }
   bool isReg()           const { return IsRegister; }
   unsigned getReg()      const { return Register; }
   int getOffset()        const { return Offset; }
   void setIsRegister(bool Is)  { IsRegister = Is; }
   void setRegister(unsigned R) { Register = R; }
   void setOffset(int O)        { Offset = O; }
+  /// Make this location a direct register location.
   void set(unsigned R) {
     IsRegister = true;
     Register = R;
     Offset = 0;
   }
+  /// Make this location a register-indirect+offset location.
   void set(unsigned R, int O) {
     IsRegister = false;
     Register = R;

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -813,7 +813,7 @@ void AsmPrinter::EmitDwarfRegOp(const MachineLocation &MLoc) const {
   // caller might be in the middle of an dwarf expression. We should
   // probably assert that Reg >= 0 once debug info generation is more mature.
 
-  if (int Offset =  MLoc.getOffset()) {
+  if (MLoc.isIndirect()) {
     if (Reg < 32) {
       OutStreamer.AddComment(
         dwarf::OperationEncodingString(dwarf::DW_OP_breg0 + Reg));
@@ -824,7 +824,7 @@ void AsmPrinter::EmitDwarfRegOp(const MachineLocation &MLoc) const {
       OutStreamer.AddComment(Twine(Reg));
       EmitULEB128(Reg);
     }
-    EmitSLEB128(Offset);
+    EmitSLEB128(MLoc.getOffset());
   } else {
     if (Reg < 32) {
       OutStreamer.AddComment(

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -1131,6 +1131,12 @@ static DotDebugLocEntry getDebugLocEntry(AsmPrinter *Asm,
   }
   if (MI->getOperand(0).isReg() && MI->getOperand(1).isImm()) {
     MachineLocation MLoc;
+    // TODO: Currently an offset of 0 in a DBG_VALUE means
+    // we need to generate a direct register value.
+    // There is no way to specify an indirect value with offset 0.
+    if (MI->getOperand(1).getImm() == 0)
+      MLoc.set(MI->getOperand(0).getReg());
+    else
       MLoc.set(MI->getOperand(0).getReg(), MI->getOperand(1).getImm());
     return DotDebugLocEntry(FLabel, SLabel, MLoc, Var);
   }