Avoid accessing LLVM/DWARF register mappings if undefined

[ddcc]

Summary

If the backend does not define LLVM/DWARF register mappings, the associated
variables are undefined since the map initializer is called by auto-generated
TableGen routines. This patch initializes the pointers and sizes to nullptr
and zero, respectively, and checks that they are valid before searching
for a mapping.

include/llvm/MC/MCRegisterInfo.h

void InitMCRegisterInfo(const MCRegisterDesc *D, unsigned NR, unsigned RA,
     RegClassStrings = ClassStrings;
     NumClasses = NC;
     RegUnitRoots = RURoots;
     NumRegUnits = NRU;
     SubRegIndices = SubIndices;
     NumSubRegIndices = NumIndices;
     SubRegIdxRanges = SubIdxRanges;
     RegEncodingTable = RET;
+
+    // Initialize DWARF register mapping variables
+    EHL2DwarfRegs = nullptr;
+    EHL2DwarfRegsSize = 0;
+    L2DwarfRegs = nullptr;
+    L2DwarfRegsSize = 0;
+    EHDwarf2LRegs = nullptr;
+    EHDwarf2LRegsSize = 0;
+    Dwarf2LRegs = nullptr;
+    Dwarf2LRegsSize = 0;
   }
 
   /// \brief Used to initialize LLVM register to Dwarf
   /// register number mapping. Called by TableGen auto-generated routines.
   /// *DO NOT USE*.
   void mapLLVMRegsToDwarfRegs(const DwarfLLVMRegPair *Map, unsigned Size,
                               bool isEH) {
     if (isEH) {

lib/MC/MCRegisterInfo.cpp

assert(Idx && Idx < getNumSubRegIndices() &&
          "This is not a subregister index");
   return SubRegIdxRanges[Idx].Offset;
 }
 
 int MCRegisterInfo::getDwarfRegNum(unsigned RegNum, bool isEH) const {
   const DwarfLLVMRegPair *M = isEH ? EHL2DwarfRegs : L2DwarfRegs;
   unsigned Size = isEH ? EHL2DwarfRegsSize : L2DwarfRegsSize;
 
+  if (!M)
+    return -1;
   DwarfLLVMRegPair Key = { RegNum, 0 };
   const DwarfLLVMRegPair *I = std::lower_bound(M, M+Size, Key);
   if (I == M+Size || I->FromReg != RegNum)
     return -1;
   return I->ToReg;
 }
 
 int MCRegisterInfo::getLLVMRegNum(unsigned RegNum, bool isEH) const {
   const DwarfLLVMRegPair *M = isEH ? EHDwarf2LRegs : Dwarf2LRegs;
   unsigned Size = isEH ? EHDwarf2LRegsSize : Dwarf2LRegsSize;
 
+  if (!M)
+    return -1;
   DwarfLLVMRegPair Key = { RegNum, 0 };
   const DwarfLLVMRegPair *I = std::lower_bound(M, M+Size, Key);
   assert(I != M+Size && I->FromReg == RegNum && "Invalid RegNum");
   return I->ToReg;
 }
 
 int MCRegisterInfo::getSEHRegNum(unsigned RegNum) const {
   const DenseMap<unsigned, int>::const_iterator I = L2SEHRegs.find(RegNum);