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
@@ -1451,6 +1451,16 @@
   let Documentation = [Undocumented];
 }
 
+def OptimizeSize: InheritableAttr {
+  // This attribute has no spellings as it is only ever created implicitly.
+  let Spellings = [];
+  let Subjects = SubjectList<[Function]>;
+  let Args = [EnumArgument<"Kind", "Kind",
+                ["off", "on"],
+                ["Off", "On"]>];
+  let Documentation = [Undocumented];
+}
+
 def FlagEnum : InheritableAttr {
   let Spellings = [Clang<"flag_enum">];
   let Subjects = SubjectList<[Enum]>;
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -1653,6 +1653,19 @@
   if (!GD.getDecl())
     return;
 
+  bool HasOptnone = GD.getDecl()->hasAttr<OptimizeNoneAttr>();
+  bool HasOptsizeOn = false;
+  bool HasOptsizeOff = false;
+  if (GD.getDecl()->hasAttr<OptimizeSizeAttr>()) {
+    OptimizeSizeAttr *Attr = GD.getDecl()->getAttr<OptimizeSizeAttr>();
+    HasOptsizeOn = Attr->getKind() == OptimizeSizeAttr::On;
+    HasOptsizeOff = Attr->getKind() == OptimizeSizeAttr::Off;
+  }
+  if (HasOptsizeOn && !HasOptnone)
+    F->addFnAttr(llvm::Attribute::OptimizeForSize);
+  if (HasOptsizeOff)
+    F->removeFnAttr(llvm::Attribute::OptimizeForSize);
+
   if (GD.getDecl()->hasAttr<FramePointerAttr>()) {
     F->removeFnAttr("frame-pointer");
     FramePointerAttr *Attr = GD.getDecl()->getAttr<FramePointerAttr>();
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
@@ -6763,6 +6763,19 @@
     checkSwiftAsyncErrorBlock(S, D, ErrorAttr, AsyncAttr);
 }
 
+static void handleOptimizeSizeAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  if (!AL.isArgIdent(0)) {
+    S.Diag(AL.getLoc(), diag::err_attribute_argument_n_type)
+        << AL << 1 << AANT_ArgumentIdentifier;
+    return;
+  }
+
+  OptimizeSizeAttr::Kind Kind;
+  IdentifierInfo *II = AL.getArgAsIdent(0)->Ident;
+  OptimizeSizeAttr::ConvertStrToKind(II->getName(), Kind);
+  D->addAttr(::new (S.Context) OptimizeSizeAttr(S.Context, AL, Kind));
+}
+
 static void handleFramePointer(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (!AL.isArgIdent(0)) {
     S.Diag(AL.getLoc(), diag::err_attribute_argument_n_type)
@@ -8534,6 +8547,9 @@
   case ParsedAttr::AT_MinSize:
     handleMinSizeAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_OptimizeSize:
+    handleOptimizeSizeAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_OptimizeNone:
     handleOptimizeNoneAttr(S, D, AL);
     break;