Index: include/llvm/MC/MCRegisterInfo.h
===================================================================
--- include/llvm/MC/MCRegisterInfo.h
+++ include/llvm/MC/MCRegisterInfo.h
@@ -381,7 +381,17 @@
   int getDwarfRegNum(unsigned RegNum, bool isEH) const;
 
   /// \brief Map a dwarf register back to a target register.
-  int getLLVMRegNum(unsigned RegNum, bool isEH) const;
+  /// This method might assert if no mapping exists.
+  unsigned getLLVMRegNum(unsigned RegNum, bool isEH) const {
+    int LLVMRegNum = tryGetLLVMRegNum(RegNum, isEH);
+    assert(LLVMRegNum != -1 && "Invalid RegNum");
+    return LLVMRegNum;
+  }
+
+  /// \brief Map a Dwarf register back to a target register.
+  /// \returns the corresponding LLVM register number or -1 if the
+  /// passed Dwarf register is invalid.
+  int tryGetLLVMRegNum(unsigned RegNum, bool isEH) const;
 
   /// \brief Map a target register to an equivalent SEH register
   /// number.  Returns LLVM register number if there is no equivalent value.
Index: lib/MC/MCRegisterInfo.cpp
===================================================================
--- lib/MC/MCRegisterInfo.cpp
+++ lib/MC/MCRegisterInfo.cpp
@@ -69,13 +69,14 @@
   return I->ToReg;
 }
 
-int MCRegisterInfo::getLLVMRegNum(unsigned RegNum, bool isEH) const {
+int MCRegisterInfo::tryGetLLVMRegNum(unsigned RegNum, bool isEH) const {
   const DwarfLLVMRegPair *M = isEH ? EHDwarf2LRegs : Dwarf2LRegs;
   unsigned Size = isEH ? EHDwarf2LRegsSize : Dwarf2LRegsSize;
 
   DwarfLLVMRegPair Key = { RegNum, 0 };
   const DwarfLLVMRegPair *I = std::lower_bound(M, M+Size, Key);
-  assert(I != M+Size && I->FromReg == RegNum && "Invalid RegNum");
+  if (I == M+Size || I->FromReg != RegNum)
+    return -1;
   return I->ToReg;
 }