Index: cfe/trunk/lib/AST/ExprConstant.cpp
===================================================================
--- cfe/trunk/lib/AST/ExprConstant.cpp
+++ cfe/trunk/lib/AST/ExprConstant.cpp
@@ -8155,6 +8155,124 @@
   case Builtin::BIomp_is_initial_device:
     // We can decide statically which value the runtime would return if called.
     return Success(Info.getLangOpts().OpenMPIsDevice ? 0 : 1, E);
+  case Builtin::BI__builtin_add_overflow:
+  case Builtin::BI__builtin_sub_overflow:
+  case Builtin::BI__builtin_mul_overflow:
+  case Builtin::BI__builtin_sadd_overflow:
+  case Builtin::BI__builtin_uadd_overflow:
+  case Builtin::BI__builtin_uaddl_overflow:
+  case Builtin::BI__builtin_uaddll_overflow:
+  case Builtin::BI__builtin_usub_overflow:
+  case Builtin::BI__builtin_usubl_overflow:
+  case Builtin::BI__builtin_usubll_overflow:
+  case Builtin::BI__builtin_umul_overflow:
+  case Builtin::BI__builtin_umull_overflow:
+  case Builtin::BI__builtin_umulll_overflow:
+  case Builtin::BI__builtin_saddl_overflow:
+  case Builtin::BI__builtin_saddll_overflow:
+  case Builtin::BI__builtin_ssub_overflow:
+  case Builtin::BI__builtin_ssubl_overflow:
+  case Builtin::BI__builtin_ssubll_overflow:
+  case Builtin::BI__builtin_smul_overflow:
+  case Builtin::BI__builtin_smull_overflow:
+  case Builtin::BI__builtin_smulll_overflow: {
+    LValue ResultLValue;
+    APSInt LHS, RHS;
+
+    QualType ResultType = E->getArg(2)->getType()->getPointeeType();
+    if (!EvaluateInteger(E->getArg(0), LHS, Info) ||
+        !EvaluateInteger(E->getArg(1), RHS, Info) ||
+        !EvaluatePointer(E->getArg(2), ResultLValue, Info))
+      return false;
+
+    APSInt Result;
+    bool DidOverflow = false;
+
+    // If the types don't have to match, enlarge all 3 to the largest of them.
+    if (BuiltinOp == Builtin::BI__builtin_add_overflow ||
+        BuiltinOp == Builtin::BI__builtin_sub_overflow ||
+        BuiltinOp == Builtin::BI__builtin_mul_overflow) {
+      bool IsSigned = LHS.isSigned() || RHS.isSigned() ||
+                      ResultType->isSignedIntegerOrEnumerationType();
+      bool AllSigned = LHS.isSigned() && RHS.isSigned() &&
+                      ResultType->isSignedIntegerOrEnumerationType();
+      uint64_t LHSSize = LHS.getBitWidth();
+      uint64_t RHSSize = RHS.getBitWidth();
+      uint64_t ResultSize = Info.Ctx.getTypeSize(ResultType);
+      uint64_t MaxBits = std::max(std::max(LHSSize, RHSSize), ResultSize);
+
+      // Add an additional bit if the signedness isn't uniformly agreed to. We
+      // could do this ONLY if there is a signed and an unsigned that both have
+      // MaxBits, but the code to check that is pretty nasty.  The issue will be
+      // caught in the shrink-to-result later anyway.
+      if (IsSigned && !AllSigned)
+        ++MaxBits;
+
+      LHS = APSInt(IsSigned ? LHS.sextOrSelf(MaxBits) : LHS.zextOrSelf(MaxBits),
+                   !IsSigned);
+      RHS = APSInt(IsSigned ? RHS.sextOrSelf(MaxBits) : RHS.zextOrSelf(MaxBits),
+                   !IsSigned);
+      Result = APSInt(MaxBits, !IsSigned);
+    }
+
+    // Find largest int.
+    switch (BuiltinOp) {
+    default:
+      llvm_unreachable("Invalid value for BuiltinOp");
+    case Builtin::BI__builtin_add_overflow:
+    case Builtin::BI__builtin_sadd_overflow:
+    case Builtin::BI__builtin_saddl_overflow:
+    case Builtin::BI__builtin_saddll_overflow:
+    case Builtin::BI__builtin_uadd_overflow:
+    case Builtin::BI__builtin_uaddl_overflow:
+    case Builtin::BI__builtin_uaddll_overflow:
+      Result = LHS.isSigned() ? LHS.sadd_ov(RHS, DidOverflow)
+                              : LHS.uadd_ov(RHS, DidOverflow);
+      break;
+    case Builtin::BI__builtin_sub_overflow:
+    case Builtin::BI__builtin_ssub_overflow:
+    case Builtin::BI__builtin_ssubl_overflow:
+    case Builtin::BI__builtin_ssubll_overflow:
+    case Builtin::BI__builtin_usub_overflow:
+    case Builtin::BI__builtin_usubl_overflow:
+    case Builtin::BI__builtin_usubll_overflow:
+      Result = LHS.isSigned() ? LHS.ssub_ov(RHS, DidOverflow)
+                              : LHS.usub_ov(RHS, DidOverflow);
+      break;
+    case Builtin::BI__builtin_mul_overflow:
+    case Builtin::BI__builtin_smul_overflow:
+    case Builtin::BI__builtin_smull_overflow:
+    case Builtin::BI__builtin_smulll_overflow:
+    case Builtin::BI__builtin_umul_overflow:
+    case Builtin::BI__builtin_umull_overflow:
+    case Builtin::BI__builtin_umulll_overflow:
+      Result = LHS.isSigned() ? LHS.smul_ov(RHS, DidOverflow)
+                              : LHS.umul_ov(RHS, DidOverflow);
+      break;
+    }
+
+    // In the case where multiple sizes are allowed, truncate and see if
+    // the values are the same.
+    if (BuiltinOp == Builtin::BI__builtin_add_overflow ||
+        BuiltinOp == Builtin::BI__builtin_sub_overflow ||
+        BuiltinOp == Builtin::BI__builtin_mul_overflow) {
+      // APSInt doesn't have a TruncOrSelf, so we use extOrTrunc instead,
+      // since it will give us the behavior of a TruncOrSelf in the case where
+      // its parameter <= its size.  We previously set Result to be at least the
+      // type-size of the result, so getTypeSize(ResultType) <= Result.BitWidth
+      // will work exactly like TruncOrSelf.
+      APSInt Temp = Result.extOrTrunc(Info.Ctx.getTypeSize(ResultType));
+      Temp.setIsSigned(ResultType->isSignedIntegerOrEnumerationType());
+
+      if (!APSInt::isSameValue(Temp, Result))
+        DidOverflow = true;
+      Result = Temp;
+    }
+
+    APValue APV{Result};
+    handleAssignment(Info, E, ResultLValue, ResultType, APV);
+    return Success(DidOverflow, E);
+  }
   }
 }
 
Index: cfe/trunk/test/SemaCXX/builtins-overflow.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/builtins-overflow.cpp
+++ cfe/trunk/test/SemaCXX/builtins-overflow.cpp
@@ -1,6 +1,8 @@
 // RUN: %clang_cc1 -fsyntax-only -std=c++17 -verify %s
 // expected-no-diagnostics
 
+#include <limits.h>
+
 int a() {
   const int x = 3;
   static int z;
@@ -13,3 +15,85 @@
   constexpr int *y = &z;
   return []() { return __builtin_sub_overflow(x, x, y); }();
 }
+
+template<typename T>
+struct Result {
+  bool B;
+  T Value;
+  constexpr bool operator==(const Result<T> &Other) {
+    return B == Other.B && Value == Other.Value;
+  }
+};
+
+
+template <typename RET, typename LHS, typename RHS>
+constexpr Result<RET> add(LHS &&lhs, RHS &&rhs) {
+  RET sum{};
+  bool b = __builtin_add_overflow(lhs, rhs, &sum);
+  return {b, sum};
+}
+
+static_assert(add<short>(static_cast<char>(120), static_cast<char>(10)) == Result<short>{false, 130});
+static_assert(add<char>(static_cast<short>(120), static_cast<short>(10)) == Result<char>{true, -126});
+static_assert(add<unsigned int>(INT_MAX, INT_MAX) == Result<unsigned int>{false, static_cast<unsigned int>(INT_MAX) * 2u});
+static_assert(add<int>(static_cast<unsigned int>(INT_MAX), 1u) == Result<int>{true, INT_MIN});
+
+static_assert(add<int>(17, 22) == Result<int>{false, 39});
+static_assert(add<int>(INT_MAX - 22, 24) == Result<int>{true, INT_MIN + 1});
+static_assert(add<int>(INT_MIN + 22, -23) == Result<int>{true, INT_MAX});
+
+template <typename RET, typename LHS, typename RHS>
+constexpr Result<RET> sub(LHS &&lhs, RHS &&rhs) {
+  RET sum{};
+  bool b = __builtin_sub_overflow(lhs, rhs, &sum);
+  return {b, sum};
+}
+
+static_assert(sub<unsigned char>(static_cast<char>(0),static_cast<char>(1)) == Result<unsigned char>{true, UCHAR_MAX});
+static_assert(sub<char>(static_cast<unsigned char>(0),static_cast<unsigned char>(1)) == Result<char>{false, -1});
+static_assert(sub<unsigned short>(static_cast<short>(0),static_cast<short>(1)) == Result<unsigned short>{true, USHRT_MAX});
+
+static_assert(sub<int>(17,22) == Result<int>{false, -5});
+static_assert(sub<int>(INT_MAX - 22, -23) == Result<int>{true, INT_MIN});
+static_assert(sub<int>(INT_MIN + 22, 23) == Result<int>{true, INT_MAX});
+
+template <typename RET, typename LHS, typename RHS>
+constexpr Result<RET> mul(LHS &&lhs, RHS &&rhs) {
+  RET sum{};
+  bool b  = __builtin_mul_overflow(lhs, rhs, &sum);
+  return {b, sum};
+}
+
+static_assert(mul<int>(17,22) == Result<int>{false, 374});
+static_assert(mul<int>(INT_MAX / 22, 23) == Result<int>{true, -2049870757});
+static_assert(mul<int>(INT_MIN / 22, -23) == Result<int>{true, -2049870757});
+
+constexpr Result<int> sadd(int lhs, int rhs) {
+  int sum{};
+  bool b = __builtin_sadd_overflow(lhs, rhs, &sum);
+  return {b, sum};
+}
+
+static_assert(sadd(17,22) == Result<int>{false, 39});
+static_assert(sadd(INT_MAX - 22, 23) == Result<int>{true, INT_MIN});
+static_assert(sadd(INT_MIN + 22, -23) == Result<int>{true, INT_MAX});
+
+constexpr Result<int> ssub(int lhs, int rhs) {
+  int sum{};
+  bool b = __builtin_ssub_overflow(lhs, rhs, &sum);
+  return {b, sum};
+}
+
+static_assert(ssub(17,22) == Result<int>{false, -5});
+static_assert(ssub(INT_MAX - 22, -23) == Result<int>{true, INT_MIN});
+static_assert(ssub(INT_MIN + 22, 23) == Result<int>{true, INT_MAX});
+
+constexpr Result<int> smul(int lhs, int rhs) {
+  int sum{};
+  bool b = __builtin_smul_overflow(lhs, rhs, &sum);
+  return {b, sum};
+}
+
+static_assert(smul(17,22) == Result<int>{false, 374});
+static_assert(smul(INT_MAX / 22, 23) == Result<int>{true, -2049870757});
+static_assert(smul(INT_MIN / 22, -23) == Result<int>{true, -2049870757});