Index: clang/lib/CodeGen/CGBuiltin.cpp
===================================================================
--- clang/lib/CodeGen/CGBuiltin.cpp
+++ clang/lib/CodeGen/CGBuiltin.cpp
@@ -586,7 +586,9 @@
 getIntegerWidthAndSignedness(const clang::ASTContext &context,
                              const clang::QualType Type) {
   assert(Type->isIntegerType() && "Given type is not an integer.");
-  unsigned Width = Type->isBooleanType() ? 1 : context.getTypeInfo(Type).Width;
+  unsigned Width = Type->isBooleanType()  ? 1
+                   : Type->isExtIntType() ? context.getIntWidth(Type)
+                                          : context.getTypeInfo(Type).Width;
   bool Signed = Type->isSignedIntegerType();
   return {Width, Signed};
 }
Index: clang/lib/Sema/SemaChecking.cpp
===================================================================
--- clang/lib/Sema/SemaChecking.cpp
+++ clang/lib/Sema/SemaChecking.cpp
@@ -283,7 +283,8 @@
   return false;
 }
 
-static bool SemaBuiltinOverflow(Sema &S, CallExpr *TheCall) {
+static bool SemaBuiltinOverflow(Sema &S, CallExpr *TheCall,
+                                unsigned BuiltinID) {
   if (checkArgCount(S, TheCall, 3))
     return true;
 
@@ -325,6 +326,25 @@
       return true;
     TheCall->setArg(2, Arg.get());
   }
+
+  // Disallow ExtIntType args larger than 128 bits to mul function until we
+  // improve backend support.
+  if (BuiltinID == Builtin::BI__builtin_mul_overflow) {
+    for (unsigned I = 0; I < 3; ++I) {
+      ExprResult Arg = TheCall->getArg(I);
+      // Third argument will be a pointer
+      QualType Ty =
+          I < 2 ? Arg.get()->getType()
+                : Arg.get()->getType()->getAs<PointerType>()->getPointeeType();
+      if (Ty->isExtIntType() && Ty->isSignedIntegerType() &&
+          S.getASTContext().getIntWidth(Ty) > 128) {
+        S.Diag(Arg.get()->getBeginLoc(), diag::err_ext_int_max_size)
+            << /*signed*/ 0 << 128;
+        return true;
+      }
+    }
+  }
+
   return false;
 }
 
@@ -1728,7 +1748,7 @@
   case Builtin::BI__builtin_add_overflow:
   case Builtin::BI__builtin_sub_overflow:
   case Builtin::BI__builtin_mul_overflow:
-    if (SemaBuiltinOverflow(*this, TheCall))
+    if (SemaBuiltinOverflow(*this, TheCall, BuiltinID))
       return ExprError();
     break;
   case Builtin::BI__builtin_operator_new:
Index: clang/test/CodeGen/builtins-overflow.c
===================================================================
--- clang/test/CodeGen/builtins-overflow.c
+++ clang/test/CodeGen/builtins-overflow.c
@@ -41,6 +41,20 @@
   return r;
 }
 
+int test_add_overflow_xint31_xint31_xint31(_ExtInt(31) x, _ExtInt(31) y) {
+  // CHECK-LABEL: define {{(dso_local )?}}i32 @test_add_overflow_xint31_xint31_xint31({{.+}})
+  // CHECK-NOT: ext
+  // CHECK: [[S:%.+]] = call { i31, i1 } @llvm.sadd.with.overflow.i31(i31 %{{.+}}, i31 %{{.+}})
+  // CHECK-DAG: [[C:%.+]] = extractvalue { i31, i1 } [[S]], 1
+  // CHECK-DAG: [[Q:%.+]] = extractvalue { i31, i1 } [[S]], 0
+  // CHECK: store i31 [[Q]], i31*
+  // CHECK: br i1 [[C]]
+  _ExtInt(31) r;
+  if (__builtin_add_overflow(x, y, &r))
+    overflowed();
+  return r;
+}
+
 unsigned test_sub_overflow_uint_uint_uint(unsigned x, unsigned y) {
   // CHECK-LABEL: define {{(dso_local )?}}i32 @test_sub_overflow_uint_uint_uint
   // CHECK-NOT: ext
@@ -69,6 +83,20 @@
   return r;
 }
 
+int test_sub_overflow_xint31_xint31_xint31(_ExtInt(31) x, _ExtInt(31) y) {
+  // CHECK-LABEL: define {{(dso_local )?}}i32 @test_sub_overflow_xint31_xint31_xint31({{.+}})
+  // CHECK-NOT: ext
+  // CHECK: [[S:%.+]] = call { i31, i1 } @llvm.ssub.with.overflow.i31(i31 %{{.+}}, i31 %{{.+}})
+  // CHECK-DAG: [[C:%.+]] = extractvalue { i31, i1 } [[S]], 1
+  // CHECK-DAG: [[Q:%.+]] = extractvalue { i31, i1 } [[S]], 0
+  // CHECK: store i31 [[Q]], i31*
+  // CHECK: br i1 [[C]]
+  _ExtInt(31) r;
+  if (__builtin_sub_overflow(x, y, &r))
+    overflowed();
+  return r;
+}
+
 unsigned test_mul_overflow_uint_uint_uint(unsigned x, unsigned y) {
   // CHECK-LABEL: define {{(dso_local )?}}i32 @test_mul_overflow_uint_uint_uint
   // CHECK-NOT: ext
@@ -97,6 +125,48 @@
   return r;
 }
 
+int test_mul_overflow_xint31_xint31_xint31(_ExtInt(31) x, _ExtInt(31) y) {
+  // CHECK-LABEL: define {{(dso_local )?}}i32 @test_mul_overflow_xint31_xint31_xint31({{.+}})
+  // CHECK-NOT: ext
+  // CHECK: [[S:%.+]] = call { i31, i1 } @llvm.smul.with.overflow.i31(i31 %{{.+}}, i31 %{{.+}})
+  // CHECK-DAG: [[C:%.+]] = extractvalue { i31, i1 } [[S]], 1
+  // CHECK-DAG: [[Q:%.+]] = extractvalue { i31, i1 } [[S]], 0
+  // CHECK: store i31 [[Q]], i31*
+  // CHECK: br i1 [[C]]
+  _ExtInt(31) r;
+  if (__builtin_mul_overflow(x, y, &r))
+    overflowed();
+  return r;
+}
+
+int test_mul_overflow_xint127_xint127_xint127(_ExtInt(127) x, _ExtInt(127) y) {
+  // CHECK-LABEL: define {{(dso_local )?}}i32 @test_mul_overflow_xint127_xint127_xint127({{.+}})
+  // CHECK-NOT: ext
+  // CHECK: [[S:%.+]] = call { i127, i1 } @llvm.smul.with.overflow.i127(i127 %{{.+}}, i127 %{{.+}})
+  // CHECK-DAG: [[C:%.+]] = extractvalue { i127, i1 } [[S]], 1
+  // CHECK-DAG: [[Q:%.+]] = extractvalue { i127, i1 } [[S]], 0
+  // CHECK: store i127 [[Q]], i127*
+  // CHECK: br i1 [[C]]
+  _ExtInt(127) r;
+  if (__builtin_mul_overflow(x, y, &r))
+    overflowed();
+  return r;
+}
+
+int test_mul_overflow_xint128_xint128_xint128(_ExtInt(128) x, _ExtInt(128) y) {
+  // CHECK-LABEL: define {{(dso_local )?}}i32 @test_mul_overflow_xint128_xint128_xint128({{.+}})
+  // CHECK-NOT: ext
+  // CHECK: [[S:%.+]] = call { i128, i1 } @llvm.smul.with.overflow.i128(i128 %{{.+}}, i128 %{{.+}})
+  // CHECK-DAG: [[C:%.+]] = extractvalue { i128, i1 } [[S]], 1
+  // CHECK-DAG: [[Q:%.+]] = extractvalue { i128, i1 } [[S]], 0
+  // CHECK: store i128 [[Q]], i128*
+  // CHECK: br i1 [[C]]
+  _ExtInt(128) r;
+  if (__builtin_mul_overflow(x, y, &r))
+    overflowed();
+  return r;
+}
+
 int test_add_overflow_uint_int_int(unsigned x, int y) {
   // CHECK-LABEL: define {{(dso_local )?}}i32 @test_add_overflow_uint_int_int
   // CHECK: [[XE:%.+]] = zext i32 %{{.+}} to i33
Index: clang/test/Sema/builtins-overflow.c
===================================================================
--- clang/test/Sema/builtins-overflow.c
+++ clang/test/Sema/builtins-overflow.c
@@ -19,4 +19,23 @@
   __builtin_add_overflow(1, 1, 3);  // expected-error {{result argument to overflow builtin must be a pointer to a non-const integer ('int' invalid)}}
   __builtin_add_overflow(1, 1, &f);  // expected-error {{result argument to overflow builtin must be a pointer to a non-const integer ('float *' invalid)}}
   __builtin_add_overflow(1, 1, &q);  // expected-error {{result argument to overflow builtin must be a pointer to a non-const integer ('const unsigned int *' invalid)}}
+
+  {
+    _ExtInt(128) x = 1;
+    _ExtInt(128) y = 1;
+    _ExtInt(128) result;
+    _Bool status = __builtin_mul_overflow(x, y, &result); // expect ok
+  }
+  {
+    unsigned _ExtInt(129) x = 1;
+    unsigned _ExtInt(129) y = 1;
+    unsigned _ExtInt(129) result;
+    _Bool status = __builtin_mul_overflow(x, y, &result); // expect ok
+  }
+  {
+    _ExtInt(129) x = 1;
+    _ExtInt(129) y = 1;
+    _ExtInt(129) result;
+    _Bool status = __builtin_mul_overflow(x, y, &result); // expected-error {{signed _ExtInt of bit sizes greater than 128 not supported}}
+  }
 }