diff --git a/utils/TableGen/CodeGenRegisters.cpp b/utils/TableGen/CodeGenRegisters.cpp
index 37f890531ad..3284ba46a44 100644
--- a/utils/TableGen/CodeGenRegisters.cpp
+++ b/utils/TableGen/CodeGenRegisters.cpp
@@ -258,6 +258,8 @@ CodeGenRegister::computeSubRegs(CodeGenRegBank &RegBank) {
 
     // Look at the possible compositions of Idx.
     // They may not all be supported by SR.
+    // NOTE: Iteration order does not matter here because the EnumValue's
+    // of subreg indices are unique.
     for (CodeGenSubRegIndex::CompMap::const_iterator I = Comps.begin(),
            E = Comps.end(); I != E; ++I) {
       SubRegMap::const_iterator SRI = Map.find(I->first);
diff --git a/utils/TableGen/CodeGenRegisters.h b/utils/TableGen/CodeGenRegisters.h
index 7c723d93a23..dad0f6cec7e 100644
--- a/utils/TableGen/CodeGenRegisters.h
+++ b/utils/TableGen/CodeGenRegisters.h
@@ -74,8 +74,7 @@ namespace llvm {
     std::string getQualifiedName() const;
 
     // Map of composite subreg indices.
-    typedef std::map<CodeGenSubRegIndex *, CodeGenSubRegIndex *,
-                     deref<llvm::less>> CompMap;
+    typedef DenseMap<CodeGenSubRegIndex *, CodeGenSubRegIndex *> CompMap;
 
     // Returns the subreg index that results from composing this with Idx.
     // Returns NULL if this and Idx don't compose.
diff --git a/utils/TableGen/RegisterInfoEmitter.cpp b/utils/TableGen/RegisterInfoEmitter.cpp
index 5a6694ea7f1..0e9cbeb439f 100644
--- a/utils/TableGen/RegisterInfoEmitter.cpp
+++ b/utils/TableGen/RegisterInfoEmitter.cpp
@@ -610,17 +610,19 @@ static void printMask(raw_ostream &OS, unsigned Val) {
 static bool combine(const CodeGenSubRegIndex *Idx,
                     SmallVectorImpl<CodeGenSubRegIndex*> &Vec) {
   const CodeGenSubRegIndex::CompMap &Map = Idx->getComposites();
-  for (CodeGenSubRegIndex::CompMap::const_iterator
-       I = Map.begin(), E = Map.end(); I != E; ++I) {
-    CodeGenSubRegIndex *&Entry = Vec[I->first->EnumValue - 1];
-    if (Entry && Entry != I->second)
+  for (const auto &I : Map) {
+    CodeGenSubRegIndex *&Entry = Vec[I.first->EnumValue - 1];
+    if (Entry && Entry != I.second)
       return false;
   }
 
   // All entries are compatible. Make it so.
-  for (CodeGenSubRegIndex::CompMap::const_iterator
-       I = Map.begin(), E = Map.end(); I != E; ++I)
-    Vec[I->first->EnumValue - 1] = I->second;
+  for (const auto &I : Map) {
+    auto *&Entry = Vec[I.first->EnumValue - 1];
+    assert((!Entry || Entry == I.second) &&
+           "Expected EnumValue to be unique");
+    Entry = I.second;
+  }
   return true;
 }