Index: cfe/trunk/include/clang/Basic/Attr.td
===================================================================
--- cfe/trunk/include/clang/Basic/Attr.td
+++ cfe/trunk/include/clang/Basic/Attr.td
@@ -668,6 +668,12 @@
   let Documentation = [Undocumented];
 }
 
+def Flatten : InheritableAttr {
+  let Spellings = [GCC<"flatten">];
+  let Subjects = SubjectList<[Function], ErrorDiag>;
+  let Documentation = [FlattenDocs];
+}
+
 def Format : InheritableAttr {
   let Spellings = [GCC<"format">];
   let Args = [IdentifierArgument<"Type">, IntArgument<"FormatIdx">,
Index: cfe/trunk/include/clang/Basic/AttrDocs.td
===================================================================
--- cfe/trunk/include/clang/Basic/AttrDocs.td
+++ cfe/trunk/include/clang/Basic/AttrDocs.td
@@ -877,6 +877,14 @@
   }];
 }
 
+def FlattenDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``flatten`` attribute causes calls within the attributed function to be
+inlined if possible.
+  }];
+}
+
 def FormatDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{
Index: cfe/trunk/lib/CodeGen/CGCall.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/CGCall.cpp
+++ cfe/trunk/lib/CodeGen/CGCall.cpp
@@ -2935,6 +2935,12 @@
   if (callOrInvoke)
     *callOrInvoke = CS.getInstruction();
 
+  if (CurCodeDecl && CurCodeDecl->hasAttr<FlattenAttr>() &&
+      !CS.hasFnAttr(llvm::Attribute::NoInline))
+    Attrs =
+        Attrs.addAttribute(getLLVMContext(), llvm::AttributeSet::FunctionIndex,
+                           llvm::Attribute::AlwaysInline);
+
   CS.setAttributes(Attrs);
   CS.setCallingConv(static_cast<llvm::CallingConv::ID>(CallingConv));
 
Index: cfe/trunk/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaDeclAttr.cpp
+++ cfe/trunk/lib/Sema/SemaDeclAttr.cpp
@@ -4147,6 +4147,9 @@
   case AttributeList::AT_OptimizeNone:
     handleOptimizeNoneAttr(S, D, Attr);
     break;
+  case AttributeList::AT_Flatten:
+    handleSimpleAttribute<FlattenAttr>(S, D, Attr);
+    break;
   case AttributeList::AT_Format:
     handleFormatAttr(S, D, Attr);
     break;
Index: cfe/trunk/test/CodeGen/flatten.c
===================================================================
--- cfe/trunk/test/CodeGen/flatten.c
+++ cfe/trunk/test/CodeGen/flatten.c
@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -triple=x86_64-linux-gnu %s -emit-llvm -o - | FileCheck %s
+
+void f(void) {}
+
+__attribute__((noinline)) void ni(void) {}
+
+__attribute__((flatten))
+// CHECK: define void @g()
+void g(void) {
+  // CHECK-NOT: call {{.*}} @f
+  f();
+  // CHECK: call {{.*}} @ni
+  ni();
+}
+
+void h(void) {
+  // CHECK: call {{.*}} @f
+  f();
+}
Index: cfe/trunk/test/CodeGenCXX/flatten.cpp
===================================================================
--- cfe/trunk/test/CodeGenCXX/flatten.cpp
+++ cfe/trunk/test/CodeGenCXX/flatten.cpp
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -triple=x86_64-linux-gnu -std=c++11 %s -emit-llvm -o - | FileCheck %s
+
+void f(void) {}
+
+[[gnu::flatten]]
+// CHECK: define void @_Z1gv()
+void g(void) {
+  // CHECK-NOT: call {{.*}} @_Z1fv
+  f();
+}
Index: cfe/trunk/test/SemaCXX/attr-flatten.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/attr-flatten.cpp
+++ cfe/trunk/test/SemaCXX/attr-flatten.cpp
@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
+
+int i __attribute__((flatten)); // expected-error {{'flatten' attribute only applies to functions}}
+
+void f1() __attribute__((flatten));
+void f2() __attribute__((flatten(1))); // expected-error {{'flatten' attribute takes no arguments}}
+
+template <typename T>
+void tf1() __attribute__((flatten));
+
+int f3(int __attribute__((flatten)), int); // expected-error{{'flatten' attribute only applies to functions}}
+
+struct A {
+  int f __attribute__((flatten));  // expected-error{{'flatten' attribute only applies to functions}}
+  void mf1() __attribute__((flatten));
+  static void mf2() __attribute__((flatten));
+};
+
+int ci [[gnu::flatten]]; // expected-error {{'flatten' attribute only applies to functions}}
+
+[[gnu::flatten]] void cf1();
+[[gnu::flatten(1)]] void cf2(); // expected-error {{'flatten' attribute takes no arguments}}
+
+template <typename T>
+[[gnu::flatten]]
+void ctf1();
+
+int cf3(int c[[gnu::flatten]], int); // expected-error{{'flatten' attribute only applies to functions}}
+
+struct CA {
+  int f [[gnu::flatten]];  // expected-error{{'flatten' attribute only applies to functions}}
+  [[gnu::flatten]] void mf1();
+  [[gnu::flatten]] static void mf2();
+};