Index: clang/include/clang/Basic/Attr.td
===================================================================
--- clang/include/clang/Basic/Attr.td
+++ clang/include/clang/Basic/Attr.td
@@ -1147,6 +1147,12 @@
   let Documentation = [FallthroughDocs];
 }
 
+def Likelihood : StmtAttr {
+  let Spellings = [CXX11<"", "likely", 201803>, Clang<"likely">, CXX11<"", "unlikely", 201803>, Clang<"unlikely">];
+// let Subjects = [Stmt, LabelStmt];
+  let Documentation = [LikelihoodDocs];
+}
+
 def FastCall : DeclOrTypeAttr {
   let Spellings = [GCC<"fastcall">, Keyword<"__fastcall">,
                    Keyword<"_fastcall">];
Index: clang/include/clang/Basic/AttrDocs.td
===================================================================
--- clang/include/clang/Basic/AttrDocs.td
+++ clang/include/clang/Basic/AttrDocs.td
@@ -1492,6 +1492,39 @@
   }];
 }
 
+def LikelihoodDocs : Documentation {
+  let Category = DocCatStmt;
+  let Heading = "likely / unlikely";
+  let Content = [{
+
+The ``likely`` or ``unlikely`` attribute is used to annotate that a statement or label is likely or unlikely to executed
+
+Here is an example:
+
+.. code-block:: c++
+
+  void g(int);
+  int f(int n) {
+    if (n > 5) [[unlikely]] {     // n > 5 is considered to be arbitrarily unlikely
+      g(0);
+      return n * 2 + 1;
+    }
+
+    switch (n) {
+    case 1:
+      g(1);
+      [[fallthrough]];
+
+    [[likely]] case 2:            // n == 2 is considered to be arbitrarily more
+      g(2);                       // likely than any other value of n
+      break;
+    }
+    return 3;
+  }
+
+  }];
+}
+
 def ARMInterruptDocs : Documentation {
   let Category = DocCatFunction;
   let Heading = "interrupt (ARM)";
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -7339,6 +7339,8 @@
   "use of the %0 attribute is a C++14 extension">, InGroup<CXX14>;
 def ext_cxx17_attr : Extension<
   "use of the %0 attribute is a C++17 extension">, InGroup<CXX17>;
+def ext_cxx2a_attr : Extension<
+  "use of the %0 attribute is a C++2a extension">, InGroup<CXX2a>;
 
 def warn_unused_comparison : Warning<
   "%select{equality|inequality|relational|three-way}0 comparison result unused">,
@@ -8158,6 +8160,15 @@
   "fallthrough annotation in unreachable code">,
   InGroup<ImplicitFallthrough>, DefaultIgnore;
 
+def err_likelihood_outside_control_scope : Error<
+  "%0 attribute can only appear inside an if, while, for, switch or catch">;
+def err_multiple_likelihood : Error<
+  "there can only be one %0 attribue in any attribute list">;
+def err_mutuably_exclusive_likelihood : Error<
+  "%0 and %1 are mutually exclusive">;
+def note_previous_likelihood : Note<
+  "previously used %0 attribue">;
+
 def warn_unreachable_default : Warning<
   "default label in switch which covers all enumeration values">,
   InGroup<CoveredSwitchDefault>, DefaultIgnore;
Index: clang/lib/Sema/SemaStmtAttr.cpp
===================================================================
--- clang/lib/Sema/SemaStmtAttr.cpp
+++ clang/lib/Sema/SemaStmtAttr.cpp
@@ -51,6 +51,31 @@
   return ::new (S.Context) auto(Attr);
 }
 
+static Attr *handleLikelihoodAttr(Sema &S, Stmt *St, const ParsedAttr &A,
+                                   SourceRange Range) {
+  LikelihoodAttr Attr(A.getRange(), S.Context,
+                       A.getAttributeSpellingListIndex());
+
+  Scope* scope = S.getCurScope();
+  Scope* previousScope = nullptr;
+
+  if (scope)
+    previousScope = scope->getParent();
+
+  //check that ths attribute is used in an if, while, for, switch or catch
+  if (!previousScope || 
+      !(previousScope->getFlags() & Scope::ControlScope) ||
+       previousScope->getFlags() & Scope::SEHExceptScope ||
+       previousScope->getFlags() & Scope::SEHTryScope ||
+       previousScope->getFlags() & Scope::FnTryCatchScope)
+         S.Diag(A.getLoc(), diag::err_likelihood_outside_control_scope) << A.getName();
+
+  if (!S.getLangOpts().CPlusPlus2a)
+    S.Diag(A.getLoc(), diag::ext_cxx2a_attr) << A.getName();
+
+  return ::new (S.Context) auto(Attr);
+}
+
 static Attr *handleSuppressAttr(Sema &S, Stmt *St, const ParsedAttr &A,
                                 SourceRange Range) {
   if (A.getNumArgs() < 1) {
@@ -201,7 +226,21 @@
   } HintAttrs[] = {{nullptr, nullptr}, {nullptr, nullptr}, {nullptr, nullptr},
                    {nullptr, nullptr}, {nullptr, nullptr}, {nullptr, nullptr}};
 
+  //there can be only one likelyhood attribute
+  const Attr* likelihoodAttr = nullptr;
+
   for (const auto *I : Attrs) {
+    if (llvm::isa<LikelihoodAttr>(I)) {
+      if (likelihoodAttr) {
+        if (std::strcmp(I->getSpelling(), likelihoodAttr->getSpelling()))
+          S.Diag(I->getLocation(), diag::err_mutuably_exclusive_likelihood) << I->getSpelling() << likelihoodAttr->getSpelling();
+        else
+          S.Diag(I->getLocation(), diag::err_multiple_likelihood) << I->getSpelling();
+        S.Diag(likelihoodAttr->getLocation(), diag::note_previous_likelihood) << likelihoodAttr->getSpelling();
+      }
+      likelihoodAttr = I;
+    }
+
     const LoopHintAttr *LH = dyn_cast<LoopHintAttr>(I);
 
     // Skip non loop hint attributes
@@ -336,6 +375,8 @@
     return nullptr;
   case ParsedAttr::AT_FallThrough:
     return handleFallThroughAttr(S, St, A, Range);
+  case ParsedAttr::AT_Likelihood:
+    return handleLikelihoodAttr(S, St, A, Range);
   case ParsedAttr::AT_LoopHint:
     return handleLoopHintAttr(S, St, A, Range);
   case ParsedAttr::AT_OpenCLUnrollHint:
Index: clang/test/SemaCXX/cxx2a-likelihood-attr.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaCXX/cxx2a-likelihood-attr.cpp
@@ -0,0 +1,51 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++2a
+
+int f(int i) {
+  if (i == 1) [[unlikely]]
+    {
+      return 0;
+    }
+  else if (i == 2) [[likely]]
+    return 1;
+  return 3;
+}
+
+[[likely]] typedef int n1; // expected-error {{'likely' attribute cannot be applied to a declaration}}
+typedef int [[likely]] n2; // expected-error {{'likely' attribute cannot be applied to types}}
+typedef int n3 [[likely]]; // expected-error {{'likely' attribute cannot be applied to a declaration}}
+
+enum [[likely]] E { // expected-error {{'likely' attribute cannot be applied to a declaration}}
+  One
+};
+
+[[likely]] // expected-error {{'likely' attribute cannot be applied to a declaration}}
+
+void test(int i) {
+  [[likely]] // expected-error {{'likely' attribute can only appear in if, while, switch and for}}
+    if (1) [[likely, likely]] {
+  // expected-error@-1 {{there can only be one likely attribue in any attribute list}}
+  // expected-note@-2 {{previously used likely attribue}}
+      [[unlikely]] return ; // expected-error {{'unlikely' attribute can only appear in if, while, switch and for}}
+    }
+  else [[unlikely]] if (1) {
+      while (1) [[likely]] {
+          switch (i) {
+            [[likely]] case 1:
+          default: [[likely]]
+            return ;
+          }
+        }
+      for (;;) [[likely, unlikely]]
+  // expected-error@-1 {{unlikely and likely are mutually exclusive}}
+  // expected-note@-2 {{previously used likely attribue}}
+        [[likely]] return ;
+  // expected-error@-1 {{likely and unlikely are mutually exclusive}}
+  // expected-note@-5 {{previously used unlikely attribue}}
+    }
+  try { // expected-error {{cannot use 'try' with exceptions disabled}}
+    [[likely]]; // expected-error {{'likely' attribute can only appear in if, while, switch and for}}
+  } catch (int) {
+      [[likely]] test: // expected-error {{'likely' attribute cannot be applied to a declaration}}
+      [[unlikely]] return ;
+  }
+}
\ No newline at end of file