Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -19568,9 +19568,6 @@
     return;
   }
 
-  // TODO: If the result value doesn't fit in an int, it must be a long or long
-  // long value.  ISO C does not support this, but GCC does as an extension,
-  // emit a warning.
   unsigned IntWidth = Context.getTargetInfo().getIntWidth();
   unsigned CharWidth = Context.getTargetInfo().getCharWidth();
   unsigned ShortWidth = Context.getTargetInfo().getShortWidth();
@@ -19690,14 +19687,24 @@
       BestPromotionType
         = (NumPositiveBits == BestWidth || !getLangOpts().CPlusPlus)
                            ? Context.UnsignedLongTy : Context.LongTy;
-    } else {
+    } else if (NumPositiveBits <= Context.getTargetInfo().getLongLongWidth()) {
       BestWidth = Context.getTargetInfo().getLongLongWidth();
-      assert(NumPositiveBits <= BestWidth &&
-             "How could an initializer get larger than ULL?");
       BestType = Context.UnsignedLongLongTy;
       BestPromotionType
         = (NumPositiveBits == BestWidth || !getLangOpts().CPlusPlus)
                            ? Context.UnsignedLongLongTy : Context.LongLongTy;
+    } else {
+      BestWidth = 128;
+      assert(NumPositiveBits <= BestWidth &&
+             "How could an initializer get larger than 128-bit?");
+      assert(Context.getTargetInfo().hasInt128Type() &&
+             "How could an initializer get larger than ULL in target without "
+             "128-bit interger type support?");
+      BestType = Context.UnsignedInt128Ty;
+      BestPromotionType =
+          (NumPositiveBits == BestWidth || !getLangOpts().CPlusPlus)
+              ? Context.UnsignedInt128Ty
+              : Context.Int128Ty;
     }
   }
 
Index: clang/test/CodeGen/enum3.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/enum3.c
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -triple x86_64-linux-gnu %s -debug-info-kind=limited -emit-llvm -o - | FileCheck %s
+
+enum b {
+   b0 = (__uint128_t)0x123456789abcdef0ULL << 64|0x0fedcba987654321ULL,
+};
+__uint128_t v;
+
+void foo (void)
+{
+  v = b0;
+}
+// CHECK: !DICompositeType(tag: DW_TAG_enumeration_type,
+// CHECK-SAME: baseType: ![[BTYPE:[0-9]+]]
+// CHECK-SAME: elements: ![[ELTS:[0-9]+]]
+// CHECK: ![[BTYPE]] = !DIBasicType(name: "unsigned __int128", size: 128, encoding: DW_ATE_unsigned)
+// CHECK: ![[ELTS]] = !{![[E:[0-9]+]]}
+// CHECK: ![[E]] = !DIEnumerator(name: "b0", value: 24197857203266734864629346612071973665, isUnsigned: true)
Index: clang/test/Sema/enum.c
===================================================================
--- clang/test/Sema/enum.c
+++ clang/test/Sema/enum.c
@@ -1,4 +1,9 @@
 // RUN: %clang_cc1 -triple %itanium_abi_triple %s -fsyntax-only -verify -pedantic
+enum b {
+   b0 = (__uint128_t)-1, // expected-warning {{ISO C restricts enumerator values to range of 'int'}}
+   b1 = (__uint128_t)0x123456789abcdef0ULL << 64|0x0fedcba987654321ULL, // expected-warning {{ISO C restricts enumerator values to range of 'int'}}
+};
+
 enum e {A,
         B = 42LL << 32,        // expected-warning {{ISO C restricts enumerator values to range of 'int'}}
       C = -4, D = 12456 };