Index: include/llvm/IR/CallingConv.h
===================================================================
--- include/llvm/IR/CallingConv.h
+++ include/llvm/IR/CallingConv.h
@@ -20,10 +20,13 @@
 /// the well-known calling conventions.
 ///
 namespace CallingConv {
+  /// LLVM IR allows to use arbitrary numbers as calling convention identifiers.
+  typedef unsigned ID;
+
   /// A set of enums which specify the assigned numeric values for known llvm
   /// calling conventions.
   /// @brief LLVM Calling Convention Representation
-  enum ID {
+  enum {
     /// C - The default llvm calling convention, compatible with C.  This
     /// convention is the only calling convention that supports varargs calls.
     /// As with typical C calling conventions, the callee/caller have to
Index: lib/AsmParser/LLParser.h
===================================================================
--- lib/AsmParser/LLParser.h
+++ lib/AsmParser/LLParser.h
@@ -226,7 +226,7 @@
     }
     bool ParseOptionalVisibility(unsigned &Visibility);
     bool ParseOptionalDLLStorageClass(unsigned &DLLStorageClass);
-    bool ParseOptionalCallingConv(CallingConv::ID &CC);
+    bool ParseOptionalCallingConv(unsigned &CC);
     bool ParseOptionalAlignment(unsigned &Alignment);
     bool ParseOptionalDereferenceableBytes(uint64_t &Bytes);
     bool ParseScopeAndOrdering(bool isAtomic, SynchronizationScope &Scope,
Index: lib/AsmParser/LLParser.cpp
===================================================================
--- lib/AsmParser/LLParser.cpp
+++ lib/AsmParser/LLParser.cpp
@@ -1464,7 +1464,7 @@
 ///   ::= 'preserve_allcc'
 ///   ::= 'cc' UINT
 ///
-bool LLParser::ParseOptionalCallingConv(CallingConv::ID &CC) {
+bool LLParser::ParseOptionalCallingConv(unsigned &CC) {
   switch (Lex.getKind()) {
   default:                       CC = CallingConv::C; return false;
   case lltok::kw_ccc:            CC = CallingConv::C; break;
@@ -1489,12 +1489,8 @@
   case lltok::kw_preserve_mostcc:CC = CallingConv::PreserveMost; break;
   case lltok::kw_preserve_allcc: CC = CallingConv::PreserveAll; break;
   case lltok::kw_cc: {
-      unsigned ArbitraryCC;
       Lex.Lex();
-      if (ParseUInt32(ArbitraryCC))
-        return true;
-      CC = static_cast<CallingConv::ID>(ArbitraryCC);
-      return false;
+      return ParseUInt32(CC);
     }
   }
 
@@ -3131,7 +3127,7 @@
   unsigned Visibility;
   unsigned DLLStorageClass;
   AttrBuilder RetAttrs;
-  CallingConv::ID CC;
+  unsigned CC;
   Type *RetType = nullptr;
   LocTy RetTypeLoc = Lex.getLoc();
   if (ParseOptionalLinkage(Linkage) ||
@@ -3803,7 +3799,7 @@
   AttrBuilder RetAttrs, FnAttrs;
   std::vector<unsigned> FwdRefAttrGrps;
   LocTy NoBuiltinLoc;
-  CallingConv::ID CC;
+  unsigned CC;
   Type *RetType = nullptr;
   LocTy RetTypeLoc;
   ValID CalleeID;
@@ -4217,7 +4213,7 @@
   AttrBuilder RetAttrs, FnAttrs;
   std::vector<unsigned> FwdRefAttrGrps;
   LocTy BuiltinLoc;
-  CallingConv::ID CC;
+  unsigned CC;
   Type *RetType = nullptr;
   LocTy RetTypeLoc;
   ValID CalleeID;