diff --git a/llvm/test/TableGen/RegisterInfoEmitter-BaseClassOrder.td b/llvm/test/TableGen/RegisterInfoEmitter-BaseClassOrder.td
--- a/llvm/test/TableGen/RegisterInfoEmitter-BaseClassOrder.td
+++ b/llvm/test/TableGen/RegisterInfoEmitter-BaseClassOrder.td
@@ -28,11 +28,13 @@
 
 def MyTarget : Target;
 
-// CHECK: static const TargetRegisterClass *BaseClasses[4] = {
-// CHECK-NEXT:   nullptr,
-// CHECK-NEXT:   &MyTarget::BaseCRegClass,
-// CHECK-NEXT:   &MyTarget::BaseARegClass,
-// CHECK-NEXT:   &MyTarget::BaseBRegClass,
-// CHECK-NEXT: }
-// CHECK-NEXT: static const uint8_t Mapping[8] = {
-// CHECK-NEXT:   0,2,2,1,1,3,3,0, };
+// CHECK:      static const uint16_t Mapping[8] = {
+// CHECK-NEXT:   InvalidRegClassID,  // NoRegister
+// CHECK-NEXT:   MyTarget::BaseARegClassID,  // R0
+// CHECK-NEXT:   MyTarget::BaseARegClassID,  // R1
+// CHECK-NEXT:   MyTarget::BaseCRegClassID,  // R2
+// CHECK-NEXT:   MyTarget::BaseCRegClassID,  // R3
+// CHECK-NEXT:   MyTarget::BaseBRegClassID,  // R4
+// CHECK-NEXT:   MyTarget::BaseBRegClassID,  // R5
+// CHECK-NEXT:   InvalidRegClassID,  // R6
+// CHECK-NEXT: };
diff --git a/llvm/utils/TableGen/RegisterInfoEmitter.cpp b/llvm/utils/TableGen/RegisterInfoEmitter.cpp
--- a/llvm/utils/TableGen/RegisterInfoEmitter.cpp
+++ b/llvm/utils/TableGen/RegisterInfoEmitter.cpp
@@ -1591,8 +1591,8 @@
         BaseClasses.push_back(&RC);
     }
     if (!BaseClasses.empty()) {
-      // Represent class indexes with uint8_t and allocate one index for nullptr
-      assert(BaseClasses.size() <= UINT8_MAX && "Too many base register classes");
+      assert(BaseClasses.size() < UINT16_MAX &&
+             "Too many base register classes");
 
       // Apply order
       struct BaseClassOrdering {
@@ -1603,30 +1603,35 @@
       };
       llvm::stable_sort(BaseClasses, BaseClassOrdering());
 
-      // Build mapping for Regs (+1 for NoRegister)
-      std::vector<uint8_t> Mapping(Regs.size() + 1, 0);
-      for (int RCIdx = BaseClasses.size() - 1; RCIdx >= 0; --RCIdx) {
-        for (const auto Reg : BaseClasses[RCIdx]->getMembers())
-          Mapping[Reg->EnumValue] = RCIdx + 1;
-      }
-
       OS << "\n// Register to base register class mapping\n\n";
       OS << "\n";
       OS << "const TargetRegisterClass *" << ClassName
          << "::getPhysRegBaseClass(MCRegister Reg)"
          << " const {\n";
-      OS << "  static const TargetRegisterClass *BaseClasses[" << (BaseClasses.size() + 1) << "] = {\n";
-      OS << "    nullptr,\n";
-      for (const auto RC : BaseClasses)
-        OS << "    &" << RC->getQualifiedName() << "RegClass,\n";
-      OS << "  };\n";
-      OS << "  static const uint8_t Mapping[" << Mapping.size() << "] = {\n    ";
-      for (const uint8_t Value : Mapping)
-        OS << (unsigned)Value << ",";
-      OS << "  };\n\n";
-      OS << "  assert(Reg < sizeof(Mapping));\n";
-      OS << "  return BaseClasses[Mapping[Reg]];\n";
-      OS << "}\n";
+      OS << "  static const uint16_t InvalidRegClassID = UINT16_MAX;\n\n";
+      OS << "  static const uint16_t Mapping[" << Regs.size() + 1 << "] = {\n";
+      OS << "    InvalidRegClassID,  // NoRegister\n";
+      for (const CodeGenRegister &Reg : Regs) {
+        const CodeGenRegisterClass *BaseRC = nullptr;
+        for (const CodeGenRegisterClass *RC : BaseClasses) {
+          if (is_contained(RC->getMembers(), &Reg)) {
+            BaseRC = RC;
+            break;
+          }
+        }
+
+        OS << "    "
+           << (BaseRC ? BaseRC->getQualifiedName() + "RegClassID"
+                      : "InvalidRegClassID")
+           << ",  // " << Reg.getName() << "\n";
+      }
+      OS << "  };\n\n"
+            "  assert(Reg < ArrayRef(Mapping).size());\n"
+            "  unsigned RCID = Mapping[Reg];\n"
+            "  if (RCID == InvalidRegClassID)\n"
+            "    return nullptr;\n"
+            "  return RegisterClasses[RCID];\n"
+            "}\n";
     }
   }