Index: clang/lib/CodeGen/CGBuiltin.cpp
===================================================================
--- clang/lib/CodeGen/CGBuiltin.cpp
+++ clang/lib/CodeGen/CGBuiltin.cpp
@@ -1235,7 +1235,7 @@
                                        WidthAndSignedness Op2Info,
                                        WidthAndSignedness ResultInfo) {
   return BuiltinID == Builtin::BI__builtin_mul_overflow &&
-         Op1Info.Width == Op2Info.Width && Op1Info.Width >= ResultInfo.Width &&
+         std::max(Op1Info.Width, Op2Info.Width) >= ResultInfo.Width &&
          Op1Info.Signed != Op2Info.Signed;
 }
 
@@ -1257,10 +1257,26 @@
   llvm::Value *Signed = CGF.EmitScalarExpr(SignedOp);
   llvm::Value *Unsigned = CGF.EmitScalarExpr(UnsignedOp);
 
+  // One of the operands may be smaller than the other. If so, [s|z]ext it.
+  bool NeedSExt = false;
+  bool NeedZExt = false;
+  if (Op1Info.Width < Op2Info.Width) {
+    NeedSExt = Op1Info.Signed;
+    NeedZExt = !Op1Info.Signed;
+  } else if (Op1Info.Width > Op2Info.Width) {
+    NeedSExt = Op2Info.Signed;
+    NeedZExt = !Op2Info.Signed;
+  }
+  if (NeedSExt)
+    Signed = CGF.Builder.CreateSExt(Signed, Unsigned->getType(), "op.sext");
+  if (NeedZExt)
+    Unsigned = CGF.Builder.CreateZExt(Unsigned, Signed->getType(), "op.zext");
+
   llvm::Type *OpTy = Signed->getType();
   llvm::Value *Zero = llvm::Constant::getNullValue(OpTy);
   Address ResultPtr = CGF.EmitPointerWithAlignment(ResultArg);
   llvm::Type *ResTy = ResultPtr.getElementType();
+  unsigned OpWidth = std::max(Op1Info.Width, Op2Info.Width);
 
   // Take the absolute value of the signed operand.
   llvm::Value *IsNegative = CGF.Builder.CreateICmpSLT(Signed, Zero);
@@ -1278,8 +1294,8 @@
   if (ResultInfo.Signed) {
     // Signed overflow occurs if the result is greater than INT_MAX or lesser
     // than INT_MIN, i.e when |Result| > (INT_MAX + IsNegative).
-    auto IntMax = llvm::APInt::getSignedMaxValue(ResultInfo.Width)
-                      .zextOrSelf(Op1Info.Width);
+    auto IntMax =
+        llvm::APInt::getSignedMaxValue(ResultInfo.Width).zextOrSelf(OpWidth);
     llvm::Value *MaxResult =
         CGF.Builder.CreateAdd(llvm::ConstantInt::get(OpTy, IntMax),
                               CGF.Builder.CreateZExt(IsNegative, OpTy));
@@ -1297,9 +1313,9 @@
     llvm::Value *Underflow = CGF.Builder.CreateAnd(
         IsNegative, CGF.Builder.CreateIsNotNull(UnsignedResult));
     Overflow = CGF.Builder.CreateOr(UnsignedOverflow, Underflow);
-    if (ResultInfo.Width < Op1Info.Width) {
+    if (ResultInfo.Width < OpWidth) {
       auto IntMax =
-          llvm::APInt::getMaxValue(ResultInfo.Width).zext(Op1Info.Width);
+          llvm::APInt::getMaxValue(ResultInfo.Width).zext(OpWidth);
       llvm::Value *TruncOverflow = CGF.Builder.CreateICmpUGT(
           UnsignedResult, llvm::ConstantInt::get(OpTy, IntMax));
       Overflow = CGF.Builder.CreateOr(Overflow, TruncOverflow);
Index: clang/test/CodeGen/builtins-overflow.c
===================================================================
--- clang/test/CodeGen/builtins-overflow.c
+++ clang/test/CodeGen/builtins-overflow.c
@@ -339,6 +339,27 @@
   return result;
 }
 
+int test_mixed_sign_mul_overflow_sext_signed_op(int x, unsigned long long y) {
+// CHECK: @test_mixed_sign_mul_overflow_sext_signed_op
+// CHECK: [[SignedOp:%.*]] = sext i32 %0 to i64
+// CHECK: [[IsNeg:%.*]] = icmp slt i64 [[SignedOp]], 0
+  int result;
+  if (__builtin_mul_overflow(x, y, &result))
+    return LongErrorCode;
+  return result;
+}
+
+int test_mixed_sign_mul_overflow_zext_unsigned_op(long long x, unsigned y) {
+// CHECK: @test_mixed_sign_mul_overflow_zext_unsigned_op
+// CHECK: [[UnsignedOp:%.*]] = zext i32 %0 to i64
+// CHECK: [[IsNeg:%.*]] = icmp slt i64 %0, 0
+// CHECK: @llvm.umul.with.overflow.i64({{.*}}, i64 [[UnsignedOp]])
+  int result;
+  if (__builtin_mul_overflow(x, y, &result))
+    return LongErrorCode;
+  return result;
+}
+
 int test_mixed_sign_mull_overflow(int x, unsigned y) {
 // CHECK: @test_mixed_sign_mull_overflow
 // CHECK:       [[IsNeg:%.*]] = icmp slt i32 [[Op1:%.*]], 0