diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -3865,6 +3865,14 @@
   let Documentation = [ReleaseHandleDocs];
 }
 
+def DiagnoseAsBuiltin : InheritableAttr {
+  let Spellings = [Clang<"diagnose_as_builtin">];
+  let Args = [DeclArgument<Function, "Function">,
+              VariadicUnsignedArgument<"ArgIndices">];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [DiagnoseAsBuiltinDocs];
+}
+
 def Builtin : InheritableAttr {
   let Spellings = [];
   let Args = [UnsignedArgument<"ID">];
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -5984,6 +5984,50 @@
   }];
 }
 
+def DiagnoseAsBuiltinDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``diagnose_as_builtin` attribute indicates that Fortify diagnostics are to
+be applied to the declared function as if it were the function specified by the
+attribute. The builtin function whose diagnostics are to be mimicked should be
+given. In addition, the order in which arguments should be applied must also
+be given.
+
+For example, the attribute can be used as follows.
+
+  .. code-block:: c
+
+    __attribute__((diagnose_as_builtin(__builtin_memset, 3, 2, 1)))
+    void *mymemset(int n, int c, void *s) {
+      // ...
+    }
+
+This indicates that calls to ``mymemset`` should be diagnosed as if they were
+calls to ``__builtin_memset``. The arguments ``3, 2, 1`` indicate by index the
+order in which arguments of ``mymemset`` should be applied to
+``__builtin_memset``. The third argument should be applied first, then the
+second, and then the first. Thus (when Fortify warnings are enabled) the call
+``mymemset(n, c, s)`` will diagnose overflows as if it were the call
+``__builtin_memset(s, c, n)``.
+
+For variadic functions, the variadic arguments must come in the same order as
+they would to the builtin function, after all normal arguments. For instance,
+to diagnose a new function as if it were `sscanf`, we can use the attribute as
+follows.
+
+  .. code-block:: c
+  __attribute__((diagnose_as_builtin(sscanf, 1, 2)))
+  int mysscanf(const char *str, const char *format, ...)  {
+    // ...
+  }
+
+Then the call `mysscanf("abc def", "%4s %4s", buf1, buf2)` will be diagnosed as
+if it were the call `sscanf("abc def", "%4s %4s", buf1, buf2)`.
+
+This attribute cannot be applied to non-static member functions.
+}];
+}
+
 def ArmSveVectorBitsDocs : Documentation {
   let Category = DocCatType;
   let Content = [{
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2956,6 +2956,17 @@
 def err_attribute_wrong_number_arguments : Error<
   "%0 attribute %plural{0:takes no arguments|1:takes one argument|"
   ":requires exactly %1 arguments}1">;
+def err_attribute_wrong_number_arguments_for : Error <
+  "%0 attribute references function %1, which %plural{0:takes no arguments|1:takes one argument|"
+  ":takes exactly %2 arguments}2">;
+def err_attribute_bounds_for_function : Error<
+  "%0 attribute references parameter %1, but the function %2 has only %3 parameters">;
+def err_attribute_no_member_function : Error<
+  "%0 attribute cannot be applied to non-static member functions">;
+def err_attribute_parameter_types : Error<
+  "%0 attribute parameter types do not match: parameter %1 of function %2 has type %3, "
+  "but parameter %4 of function %5 has type %6">;
+
 def err_attribute_too_many_arguments : Error<
   "%0 attribute takes no more than %1 argument%s1">;
 def err_attribute_too_few_arguments : Error<
@@ -3013,7 +3024,7 @@
   "%0 attribute cannot be applied to sizeless type %1">;
 def err_attribute_argument_n_type : Error<
   "%0 attribute requires parameter %1 to be %select{int or bool|an integer "
-  "constant|a string|an identifier|a constant expression}2">;
+  "constant|a string|an identifier|a constant expression|a builtin function}2">;
 def err_attribute_argument_type : Error<
   "%0 attribute requires %select{int or bool|an integer "
   "constant|a string|an identifier}1">;
diff --git a/clang/include/clang/Sema/ParsedAttr.h b/clang/include/clang/Sema/ParsedAttr.h
--- a/clang/include/clang/Sema/ParsedAttr.h
+++ b/clang/include/clang/Sema/ParsedAttr.h
@@ -1097,6 +1097,7 @@
   AANT_ArgumentString,
   AANT_ArgumentIdentifier,
   AANT_ArgumentConstantExpr,
+  AANT_ArgumentBuiltinFunction,
 };
 
 /// These constants match the enumerated choices of
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -446,14 +446,14 @@
       break;
     }
 
-    auto OptionalFW = FS.getFieldWidth();
-    if (OptionalFW.getHowSpecified() !=
+    analyze_format_string::OptionalAmount FW = FS.getFieldWidth();
+    if (FW.getHowSpecified() !=
         analyze_format_string::OptionalAmount::HowSpecified::Constant)
       return true;
 
-    unsigned SourceSize = OptionalFW.getConstantAmount() + NulByte;
+    unsigned SourceSize = FW.getConstantAmount() + NulByte;
 
-    auto DestSizeAPS = ComputeSizeArgument(FS.getArgIndex());
+    Optional<llvm::APSInt> DestSizeAPS = ComputeSizeArgument(FS.getArgIndex());
     if (!DestSizeAPS)
       return true;
 
@@ -652,20 +652,53 @@
       isConstantEvaluated())
     return;
 
-  unsigned BuiltinID = FD->getBuiltinID(/*ConsiderWrappers=*/true);
+  bool UseDABAttr = false;
+  const FunctionDecl *UseDecl = FD;
+
+  const auto *DABAttr = FD->getAttr<DiagnoseAsBuiltinAttr>();
+  if (DABAttr) {
+    UseDecl = DABAttr->getFunction();
+    assert(UseDecl && "Missing FunctionDecl in DiagnoseAsBuiltin attribute!");
+    UseDABAttr = true;
+  }
+
+  unsigned BuiltinID = UseDecl->getBuiltinID(/*ConsiderWrappers=*/true);
+
   if (!BuiltinID)
     return;
 
   const TargetInfo &TI = getASTContext().getTargetInfo();
   unsigned SizeTypeWidth = TI.getTypeWidth(TI.getSizeType());
 
+  auto TranslateIndex = [&](unsigned Index) -> Optional<unsigned> {
+    // If we refer to a diagnose_as_builtin attribute, we need to change the
+    // argument index to refer to the arguments of the called function. Unless
+    // the index is out of bounds, which presumably means it's a variadic
+    // function.
+    if (!UseDABAttr)
+      return Index;
+    unsigned DABIndices = DABAttr->argIndices_size();
+    unsigned NewIndex = Index < DABIndices
+                            ? DABAttr->argIndices_begin()[Index]
+                            : Index - DABIndices + FD->getNumParams();
+    if (NewIndex >= TheCall->getNumArgs())
+      return llvm::None;
+    return NewIndex;
+  };
+
   auto ComputeExplicitObjectSizeArgument =
       [&](unsigned Index) -> Optional<llvm::APSInt> {
+    Optional<unsigned> IndexOptional = TranslateIndex(Index);
+    if (!IndexOptional)
+      return llvm::None;
+    unsigned NewIndex = IndexOptional.getValue();
     Expr::EvalResult Result;
-    Expr *SizeArg = TheCall->getArg(Index);
+    Expr *SizeArg = TheCall->getArg(NewIndex);
     if (!SizeArg->EvaluateAsInt(Result, getASTContext()))
       return llvm::None;
-    return Result.Val.getInt();
+    llvm::APSInt Integer = Result.Val.getInt();
+    Integer.setIsUnsigned(true);
+    return Integer;
   };
 
   auto ComputeSizeArgument = [&](unsigned Index) -> Optional<llvm::APSInt> {
@@ -680,7 +713,12 @@
         BOSType = POS->getType();
     }
 
-    const Expr *ObjArg = TheCall->getArg(Index);
+    Optional<unsigned> IndexOptional = TranslateIndex(Index);
+    if (!IndexOptional)
+      return llvm::None;
+    unsigned NewIndex = IndexOptional.getValue();
+
+    const Expr *ObjArg = TheCall->getArg(NewIndex);
     uint64_t Result;
     if (!ObjArg->tryEvaluateObjectSize(Result, getASTContext(), BOSType))
       return llvm::None;
@@ -690,7 +728,12 @@
   };
 
   auto ComputeStrLenArgument = [&](unsigned Index) -> Optional<llvm::APSInt> {
-    Expr *ObjArg = TheCall->getArg(Index);
+    Optional<unsigned> IndexOptional = TranslateIndex(Index);
+    if (!IndexOptional)
+      return llvm::None;
+    unsigned NewIndex = IndexOptional.getValue();
+
+    const Expr *ObjArg = TheCall->getArg(NewIndex);
     uint64_t Result;
     if (!ObjArg->tryEvaluateStrLen(Result, getASTContext()))
       return llvm::None;
@@ -898,7 +941,8 @@
   }
 
   if (!SourceSize || !DestinationSize ||
-      SourceSize.getValue().ule(DestinationSize.getValue()))
+      llvm::APSInt::compareValues(SourceSize.getValue(),
+                                  DestinationSize.getValue()) <= 0)
     return;
 
   StringRef FunctionName = GetFunctionName();
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -1001,6 +1001,84 @@
 };
 }
 
+static void handleDiagnoseAsBuiltinAttr(Sema &S, Decl *D,
+                                        const ParsedAttr &AL) {
+  const auto *DeclFD = cast<FunctionDecl>(D);
+
+  if (const auto *MethodDecl = dyn_cast<CXXMethodDecl>(DeclFD))
+    if (!MethodDecl->isStatic()) {
+      S.Diag(AL.getLoc(), diag::err_attribute_no_member_function) << AL;
+      return;
+    }
+
+  auto DiagnoseType = [&](unsigned Index, AttributeArgumentNType T) {
+    SourceLocation Loc = [&]() {
+      auto Union = AL.getArg(Index - 1);
+      if (Union.is<Expr *>())
+        return Union.get<Expr *>()->getBeginLoc();
+      return Union.get<IdentifierLoc *>()->Loc;
+    }();
+
+    S.Diag(Loc, diag::err_attribute_argument_n_type) << AL << Index << T;
+  };
+
+  FunctionDecl *AttrFD = [&]() -> FunctionDecl * {
+    if (!AL.isArgExpr(0))
+      return nullptr;
+    auto *F = dyn_cast_or_null<DeclRefExpr>(AL.getArgAsExpr(0));
+    if (!F)
+      return nullptr;
+    return dyn_cast_or_null<FunctionDecl>(F->getFoundDecl());
+  }();
+
+  if (!AttrFD || !AttrFD->getBuiltinID(true)) {
+    DiagnoseType(1, AANT_ArgumentBuiltinFunction);
+    return;
+  }
+
+  if (AttrFD->getNumParams() != AL.getNumArgs() - 1) {
+    S.Diag(AL.getLoc(), diag::err_attribute_wrong_number_arguments_for)
+        << AL << AttrFD << AttrFD->getNumParams();
+    return;
+  }
+
+  SmallVector<unsigned, 8> Indices;
+
+  for (unsigned I = 1; I < AL.getNumArgs(); ++I) {
+    if (!AL.isArgExpr(I)) {
+      DiagnoseType(I + 1, AANT_ArgumentIntegerConstant);
+      return;
+    }
+
+    const Expr *IndexExpr = AL.getArgAsExpr(I);
+    uint32_t Index;
+
+    if (!checkUInt32Argument(S, AL, IndexExpr, Index, I + 1, false))
+      return;
+
+    if (Index > DeclFD->getNumParams()) {
+      S.Diag(AL.getLoc(), diag::err_attribute_bounds_for_function)
+          << AL << Index << DeclFD << DeclFD->getNumParams();
+      return;
+    }
+
+    QualType T1 = AttrFD->getParamDecl(I - 1)->getType();
+    QualType T2 = DeclFD->getParamDecl(Index - 1)->getType();
+
+    if (T1.getCanonicalType().getUnqualifiedType() !=
+        T2.getCanonicalType().getUnqualifiedType()) {
+      S.Diag(IndexExpr->getBeginLoc(), diag::err_attribute_parameter_types)
+          << AL << Index << DeclFD << T2 << I << AttrFD << T1;
+      return;
+    }
+
+    Indices.push_back(Index - 1);
+  }
+
+  D->addAttr(::new (S.Context) DiagnoseAsBuiltinAttr(
+      S.Context, AL, AttrFD, Indices.data(), Indices.size()));
+}
+
 static void handleDiagnoseIfAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   S.Diag(AL.getLoc(), diag::ext_clang_diagnose_if);
 
@@ -8159,6 +8237,9 @@
   case ParsedAttr::AT_DiagnoseIf:
     handleDiagnoseIfAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_DiagnoseAsBuiltin:
+    handleDiagnoseAsBuiltinAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_NoBuiltin:
     handleNoBuiltinAttr(S, D, AL);
     break;
diff --git a/clang/test/Misc/pragma-attribute-supported-attributes-list.test b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
--- a/clang/test/Misc/pragma-attribute-supported-attributes-list.test
+++ b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
@@ -57,6 +57,7 @@
 // CHECK-NEXT: DLLExport (SubjectMatchRule_function, SubjectMatchRule_variable, SubjectMatchRule_record, SubjectMatchRule_objc_interface)
 // CHECK-NEXT: DLLImport (SubjectMatchRule_function, SubjectMatchRule_variable, SubjectMatchRule_record, SubjectMatchRule_objc_interface)
 // CHECK-NEXT: Destructor (SubjectMatchRule_function)
+// CHECK-NEXT: DiagnoseAsBuiltin (SubjectMatchRule_function)
 // CHECK-NEXT: DisableSanitizerInstrumentation (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_variable_is_global)
 // CHECK-NEXT: DisableTailCalls (SubjectMatchRule_function, SubjectMatchRule_objc_method)
 // CHECK-NEXT: EnableIf (SubjectMatchRule_function)
diff --git a/clang/test/Sema/attr-diagnose-as-builtin.c b/clang/test/Sema/attr-diagnose-as-builtin.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/attr-diagnose-as-builtin.c
@@ -0,0 +1,118 @@
+// RUN: %clang_cc1 -Wfortify-source -triple x86_64-apple-macosx10.14.0 %s -verify
+// RUN: %clang_cc1 -Wfortify-source -xc++ -triple x86_64-apple-macosx10.14.0 %s -verify
+
+typedef unsigned long size_t;
+
+__attribute__((diagnose_as_builtin(__builtin_memcpy, 3, 1, 2))) int x; // expected-warning {{'diagnose_as_builtin' attribute only applies to functions}}
+
+void *test_memcpy(const void *src, size_t c, void *dst) __attribute__((diagnose_as_builtin(__builtin_memcpy, 3, 1, 2))) {
+  return __builtin_memcpy(dst, src, c);
+}
+
+void call_test_memcpy() {
+  char bufferA[10];
+  char bufferB[11];
+  test_memcpy(bufferB, 10, bufferA);
+  test_memcpy(bufferB, 11, bufferA); // expected-warning {{'memcpy' will always overflow; destination buffer has size 10, but size argument is 11}}
+}
+
+void failure_function_doesnt_exist() __attribute__((diagnose_as_builtin(__function_that_doesnt_exist))) {} // expected-error {{use of undeclared identifier '__function_that_doesnt_exist'}}
+
+void not_a_builtin() {}
+
+void failure_not_a_builtin() __attribute__((diagnose_as_builtin(not_a_builtin))) {} // expected-error {{'diagnose_as_builtin' attribute requires parameter 1 to be a builtin function}}
+
+void failure_too_many_parameters(void *dst, const void *src, size_t count, size_t nothing) __attribute__((diagnose_as_builtin(__builtin_memcpy, 1, 2, 3, 4))) {} // expected-error {{'diagnose_as_builtin' attribute references function '__builtin_memcpy', which takes exactly 3 arguments}}
+
+void failure_too_few_parameters(void *dst, const void *src) __attribute__((diagnose_as_builtin(__builtin_memcpy, 1, 2))) {} // expected-error{{'diagnose_as_builtin' attribute references function '__builtin_memcpy', which takes exactly 3 arguments}}
+
+void failure_not_an_integer(void *dst, const void *src, size_t count) __attribute__((diagnose_as_builtin(__builtin_memcpy, "abc", 2, 3))) {} // expected-error{{'diagnose_as_builtin' attribute requires parameter 2 to be an integer constant}}
+
+void failure_not_a_builtin2() __attribute__((diagnose_as_builtin("abc"))) {} // expected-error{{'diagnose_as_builtin' attribute requires parameter 1 to be a builtin function}}
+
+void failure_parameter_index_bounds(void *dst, const void *src) __attribute__((diagnose_as_builtin(__builtin_memcpy, 1, 2, 3))) {} // expected-error{{'diagnose_as_builtin' attribute references parameter 3, but the function 'failure_parameter_index_bounds' has only 2 parameters}}
+
+void failure_parameter_types(double dst, const void *src, size_t count) __attribute__((diagnose_as_builtin(__builtin_memcpy, 1, 2, 3))) {} // expected-error{{'diagnose_as_builtin' attribute parameter types do not match: parameter 1 of function 'failure_parameter_types' has type 'double', but parameter 1 of function '__builtin_memcpy' has type 'void *'}}
+
+void to_redeclare(void *dst, const void *src, size_t count);
+
+void use_to_redeclare() {
+  char src[10];
+  char dst[9];
+  // We shouldn't get an error yet.
+  to_redeclare(dst, src, 10);
+}
+
+void to_redeclare(void *dst, const void *src, size_t count) __attribute((diagnose_as_builtin(__builtin_memcpy, 1, 2, 3)));
+
+void error_to_redeclare() {
+  char src[10];
+  char dst[9];
+  // Now we get an error.
+  to_redeclare(dst, src, 10); // expected-warning {{'memcpy' will always overflow; destination buffer has size 9, but size argument is 10}}
+}
+
+// Make sure this works even with extra qualifiers and the pass_object_size attribute.
+void *memcpy2(void *const dst __attribute__((pass_object_size(0))), const void *src, size_t copy_amount) __attribute((diagnose_as_builtin(__builtin_memcpy, 1, 2, 3)));
+
+void call_memcpy2() {
+  char buf1[10];
+  char buf2[11];
+  memcpy2(buf1, buf2, 11); // expected-warning {{'memcpy' will always overflow; destination buffer has size 10, but size argument is 11}}
+}
+
+// We require the types to be identical, modulo canonicalization and qualifiers.
+// Maybe this could be relaxed if it proves too restrictive.
+void failure_type(void *dest, char val, size_t n) __attribute__((diagnose_as_builtin(__builtin_memset, 1, 2, 3))) {} // expected-error {{'diagnose_as_builtin' attribute parameter types do not match: parameter 2 of function 'failure_type' has type 'char', but parameter 2 of function '__builtin_memset' has type 'int'}}
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern int sscanf(const char *input, const char *format, ...);
+
+#ifdef __cplusplus
+}
+#endif
+
+// Variadics should work.
+int mysscanf(const char *str, const char *format, ...) __attribute__((diagnose_as_builtin(sscanf, 1, 2)));
+
+void warn_mysccanf() {
+  char buf[10];
+  mysscanf("", "%10s", buf); // expected-warning{{'sscanf' may overflow; destination buffer in argument 3 has size 10, but the corresponding specifier may require size 11}}
+}
+
+#ifdef __cplusplus
+
+template <class T>
+void some_templated_function(int x) {
+  return;
+}
+
+void failure_template(int x) __attribute__((diagnose_as_builtin(some_templated_function, 1))) {} // expected-error{{'diagnose_as_builtin' attribute requires parameter 1 to be a builtin function}}
+
+void failure_template_instantiation(int x) __attribute__((diagnose_as_builtin(some_templated_function<int>, 1))) {} // expected-error{{'diagnose_as_builtin' attribute requires parameter 1 to be a builtin function}}
+
+void overloaded_function(unsigned);
+
+void overloaded_function(int);
+
+void failure_overloaded_function(int) __attribute__((diagnose_as_builtin(overloaded_function, 1))) {} // expected-error{{'diagnose_as_builtin' attribute requires parameter 1 to be a builtin function}}
+
+struct S {
+  __attribute__((diagnose_as_builtin(__builtin_memset))) void f(); // expected-error{{'diagnose_as_builtin' attribute cannot be applied to non-static member functions}}
+
+  __attribute__((diagnose_as_builtin(__builtin_memcpy, 3, 1, 2))) static void *test_memcpy(const void *src, size_t c, void *dst) {
+    return __builtin_memcpy(dst, src, c);
+  }
+};
+
+void call_static_test_memcpy() {
+  char bufferA[10];
+  char bufferB[11];
+  S::test_memcpy(bufferB, 10, bufferA);
+  S::test_memcpy(bufferB, 11, bufferA); // expected-warning {{'memcpy' will always overflow; destination buffer has size 10, but size argument is 11}}
+}
+
+#endif