Index: llvm/include/llvm/Transforms/IPO/Attributor.h
===================================================================
--- llvm/include/llvm/Transforms/IPO/Attributor.h
+++ llvm/include/llvm/Transforms/IPO/Attributor.h
@@ -582,6 +582,38 @@
   static constexpr Attribute::AttrKind ID = Attribute::Returned;
 };
 
+
+/// An abstract attribute for willreturn.
+struct AAWillReturn : public AbstractAttribute {
+
+  /// See AbstractAttribute::AbstractAttribute(...).
+  AAWillReturn(Function &F, InformationCache &InfoCache)
+      : AbstractAttribute(F, InfoCache) {}
+
+  /// Return the deduced attributes in \p Attrs.
+  virtual void
+  getDeducedAttributes(SmallVectorImpl<Attribute> &Attrs) const override {
+    LLVMContext &Ctx = AnchoredVal.getContext();
+    Attrs.emplace_back(Attribute::get(Ctx, "willreturn"));
+  }
+
+  /// See AbstractAttribute::getAttrKind()
+  virtual Attribute::AttrKind getAttrKind() const override {
+    return Attribute::None;
+  }
+
+  /// Return true if "willreturn" is known.
+  bool isKnownWillReturn() const;
+
+  /// Return true if "willreturn" is assumed.
+  bool isAssumedWillReturn() const;
+
+  /// The identifier used by the Attributor for this class of attributes.
+  /// FIXME: I use Attribute::None + 2 for now.
+  static constexpr Attribute::AttrKind ID =
+      Attribute::AttrKind(Attribute::None + 2);
+};
+
 } // end namespace llvm
 
 #endif // LLVM_TRANSFORMS_IPO_FUNCTIONATTRS_H
Index: llvm/lib/Transforms/IPO/Attributor.cpp
===================================================================
--- llvm/lib/Transforms/IPO/Attributor.cpp
+++ llvm/lib/Transforms/IPO/Attributor.cpp
@@ -14,6 +14,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/IPO/Attributor.h"
+#include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
@@ -1031,6 +1032,116 @@
   return ChangeStatus::CHANGED;
 }
 
+/// ------------------------ Will-Return Attributes ----------------------------
+
+struct AAWillReturnImpl : public AAWillReturn, BooleanState {
+
+  /// See AbstractAttribute::AbstractAttribute(...).
+  AAWillReturnImpl(Function &F, InformationCache &InfoCache)
+      : AAWillReturn(F, InfoCache) {}
+
+  /// See AAWillReturn::isKnownWillReturn().
+  bool isKnownWillReturn() const { return getKnown(); }
+
+  /// See AAWillReturn::isAssumedWillReturn().
+  bool isAssumedWillReturn() const { return getAssumed(); }
+
+  /// See AbstractAttribute::getState(...).
+  virtual AbstractState &getState() override { return *this; }
+
+  /// See AbstractAttribute::getState(...).
+  virtual const AbstractState &getState() const override { return *this; }
+
+  /// See AbstractAttribute::getAsStr()
+  virtual const std::string getAsStr() const override {
+    return getAssumed() ? "willreturn" : "may-noreturn";
+  }
+};
+
+struct AAWillReturnFunction final : AAWillReturnImpl {
+
+  /// See AbstractAttribute::AbstractAttribute(...).
+  AAWillReturnFunction(Function &F, InformationCache &InfoCache)
+      : AAWillReturnImpl(F, InfoCache) {}
+
+  /// See AbstractAttribute::getManifestPosition().
+  virtual ManifestPosition getManifestPosition() const override {
+    return MP_FUNCTION;
+  }
+
+  /// See AbstractAttribute::initialize(...).
+  virtual void initialize(Attributor &A) override;
+
+  /// See AbstractAttribute::updateImpl(...).
+  virtual ChangeStatus updateImpl(Attributor &A) override;
+};
+
+// Helper function that checks whether a function has any cycle.
+bool containsCycle(Function &F) {
+  SmallPtrSet<BasicBlock *, 32> Visited;
+
+  // Traverse BB by dfs and check whether successor is already visited.
+  for (BasicBlock *BB : depth_first(&F)) {
+    Visited.insert(BB);
+    for (auto *SuccBB : successors(BB)) {
+      if (Visited.count(SuccBB))
+        return true;
+    }
+  }
+  return false;
+}
+
+// Helper function that checks the function have a loop which might become an
+// endless loop
+// FIXME: Any cycle is regarded as endless loop for now.
+//        We have to allow some patterns.
+bool containsPossiblyEndlessLoop(Function &F) { return containsCycle(F); }
+
+void AAWillReturnFunction::initialize(Attributor &A) {
+  Function &F = getAnchorScope();
+
+  if (containsPossiblyEndlessLoop(F))
+    indicatePessimisticFixpoint();
+}
+
+ChangeStatus AAWillReturnFunction::updateImpl(Attributor &A) {
+  Function &F = getAnchorScope();
+
+  // The map from instruction opcodes to those instructions in the function.
+  auto &OpcodeInstMap = InfoCache.getOpcodeInstMapForFunction(F);
+
+  for (unsigned Opcode :
+       {(unsigned)Instruction::Invoke, (unsigned)Instruction::CallBr,
+        (unsigned)Instruction::Call}) {
+    for (Instruction *I : OpcodeInstMap[Opcode]) {
+      auto ICS = ImmutableCallSite(I);
+
+      // Assume that all intrinsic without noreturn are willreturn
+      if (isa<IntrinsicInst>(I) && !ICS.hasFnAttr(Attribute::NoReturn)) {
+        continue;
+      }
+
+      auto *WillReturnAA = A.getAAFor<AAWillReturnFunction>(*this, *I);
+      if ((!WillReturnAA || !WillReturnAA->isValidState() ||
+           !WillReturnAA->isAssumedWillReturn()) &&
+          !ICS.hasFnAttr("willreturn")) {
+        indicatePessimisticFixpoint();
+        return ChangeStatus::CHANGED;
+      }
+
+      // FIXME: Prohibit any recursion for now
+      auto *NoRecurseAA = A.getAAFor<AANoRecurseFunction>(*this, *I);
+      if ((!NoRecurseAA || !NoRecurseAA->isValidState() ||
+           !NoRecurseAA->isAssumedNoRecurse()) &&
+          !ICS.hasFnAttr(Attribute::NoRecurse)) {
+        indicatePessimisticFixpoint();
+        return ChangeStatus::CHANGED;
+      }
+    }
+  }
+
+  return ChangeStatus::UNCHANGED;
+}
 
 /// ----------------------------------------------------------------------------
 ///                               Attributor
@@ -1190,6 +1301,9 @@
   // Every function might be "no-return".
   registerAA(*new AANoReturnFunction(F, InfoCache));
 
+  // Every function might be "will-return".
+  registerAA(*new AAWillReturnFunction(F, InfoCache));
+
   // Walk all instructions to find more attribute opportunities and also
   // interesting instructions that might be queried by abstract attributes
   // during their initialization or update.
Index: llvm/test/Transforms/FunctionAttrs/willreturn.ll
===================================================================
--- llvm/test/Transforms/FunctionAttrs/willreturn.ll
+++ llvm/test/Transforms/FunctionAttrs/willreturn.ll
@@ -1,4 +1,6 @@
 ; RUN: opt -functionattrs -S < %s | FileCheck %s --check-prefix=FNATTR
+; RUN: opt -attributor -S < %s | FileCheck %s --check-prefix=ATTRIBUTOR
+
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 
@@ -7,9 +9,10 @@
 
 
 ; TEST 1 (positive case)
-; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: noinline norecurse nounwind readnone uwtable
 ; FNATTR-NEXT: define void @only_return()
+; ATTRIBUTOR: Function Attrs: noinline norecurse nounwind uwtable "nofree" "willreturn"
+; ATTRIBUTOR-NEXT: define void @only_return()
 define void @only_return() #0 {
     ret void
 }
@@ -25,6 +28,8 @@
 ; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: noinline nounwind readnone uwtable
 ; FNATTR-NEXT: define i32 @fib(i32)
+; ATTRIBUTOR: Function Attrs: noinline nounwind uwtable "nofree"
+; ATTRIBUTOR-NEXT: define i32 @fib(i32) local_unnamed_addr
 define i32 @fib(i32) local_unnamed_addr #0 {
   %2 = icmp slt i32 %0, 2
   br i1 %2, label %9, label %3
@@ -54,6 +59,9 @@
 ; FNATTR: Function Attrs: noinline norecurse nounwind readnone uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: define i32 @fact_maybe_not_halt(i32) local_unnamed_addr
+; ATTRIBUTOR: Function Attrs: noinline norecurse nounwind uwtable "nofree"
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define i32 @fact_maybe_not_halt(i32) local_unnamed_addr
 define i32 @fact_maybe_not_halt(i32) local_unnamed_addr #0 {
   %2 = icmp eq i32 %0, 0
   br i1 %2, label %11, label %3
@@ -87,6 +95,8 @@
 ; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: noinline norecurse nounwind readnone uwtable
 ; FNATTR-NEXT: define i32 @fact_loop(i32)
+; ATTRIBUTOR: Function Attrs: noinline norecurse nounwind uwtable "nofree"
+; ATTRIBUTOR-NEXT: define i32 @fact_loop(i32) local_unnamed_addr
 define i32 @fact_loop(i32) local_unnamed_addr #0 {
   %2 = icmp slt i32 %0, 1
   br i1 %2, label %3, label %5
@@ -116,6 +126,9 @@
 ; FNATTR: Function Attrs: noinline nounwind readnone uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: define void @mutual_recursion1()
+; ATTRIBUTOR: Function Attrs: noinline noreturn nounwind uwtable "nofree"
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define void @mutual_recursion1()
 define void @mutual_recursion1() #0 {
   call void @mutual_recursion2()
   ret void
@@ -125,6 +138,9 @@
 ; FNATTR: Function Attrs: noinline nounwind readnone uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: define void @mutual_recursion2()
+; ATTRIBUTOR: Function Attrs: noinline noreturn nounwind uwtable "nofree"
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define void @mutual_recursion2()
 define void @mutual_recursion2() #0 {
   call void @mutual_recursion1()
   ret void
@@ -135,11 +151,16 @@
 ; call exit/abort (has noreturn attribute)
 ; FNATTR: Function Attrs: noreturn
 ; FNATTR-NEXT: declare void @exit(i32) local_unnamed_addr
+; ATTRIBUTOR: Function Attrs: noreturn
+; ATTRIBUTOR-NEXT: declare void @exit(i32) local_unnamed_add
 declare void @exit(i32) local_unnamed_addr noreturn
 
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: define void @only_exit()
+; ATTRIBUTOR: Function Attrs: noinline noreturn nounwind uwtable
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define void @only_exit() local_unnamed_addr
 define void @only_exit() local_unnamed_addr #0 {
   tail call void @exit(i32 0)
   unreachable
@@ -158,6 +179,9 @@
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: define void @conditional_exit(i32, i32* nocapture readonly) local_unnamed_addr
+; ATTRIBUTOR: Function Attrs: noinline nounwind uwtable
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define void @conditional_exit(i32, i32* nocapture readonly) local_unnamed_addr
 define void @conditional_exit(i32, i32* nocapture readonly) local_unnamed_addr #0 {
   %3 = icmp eq i32 %0, 0
   br i1 %3, label %5, label %4
@@ -183,11 +207,14 @@
 ; Call intrinsic function
 ; FNATTRS: Function Attrs: noinline readnone speculatable
 ; FNATTRS-NEXT: declare float @llvm.floor.f32(float)
+; ATTRIBUTOR: Function Attrs: nounwind readnone speculatable
+; ATTRIBUTOR-NEXT: declare float @llvm.floor.f32(float
 declare float @llvm.floor.f32(float)
 
-; FIXME: missing "willreturn"
 ; FNATTRS: Function Attrs: noinline nounwind uwtable
 ; FNATTRS-NEXT: define void @call_floor(float %a)
+; ATTRIBUTOR: Function Attrs: noinline norecurse nounwind uwtable "willreturn"
+; ATTRIBUTOR-NEXT: define void @call_floor(float %a)
 define void @call_floor(float %a) #0 {
     tail call float @llvm.floor.f32(float %a)
     ret void
@@ -200,11 +227,17 @@
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: declare void @maybe_noreturn()
+; ATTRIBUTOR: Function Attrs: noinline nounwind uwtable
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: declare void @maybe_noreturn()
 declare void @maybe_noreturn() #0
 
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: define void @call_maybe_noreturn()
+; ATTRIBUTOR: Function Attrs: noinline nounwind uwtable
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define void @call_maybe_noreturn()
 define void @call_maybe_noreturn() #0 {
     tail call void @maybe_noreturn()
     ret void
@@ -216,11 +249,15 @@
 
 ; FNATTR: Function Attrs: "willreturn"
 ; FNATTR-NEXT: declare void @will_return()
+; ATTRIBUTOR: Function Attrs: "willreturn"
+; ATTRIBUTOR-NEXT: declare void @will_return()
 declare void @will_return() "willreturn"
 
 ; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NEXT: define void @f1()
+; ATTRIBUTOR: Function Attrs: noinline nounwind uwtable
+; ATTRIBUTOR-NEXT: define void @f1()
 define void @f1() #0 {
     tail call void @will_return()
     ret void
@@ -229,6 +266,8 @@
 ; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NEXT: define void @f2()
+; ATTRIBUTOR: Function Attrs: noinline nounwind uwtable
+; ATTRIBUTOR-NEXT: define void @f2()
 define void @f2() #0 {
     tail call void @f1()
     ret void
@@ -241,6 +280,9 @@
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: define void @call_will_return_but_has_loop()
+; ATTRIBUTOR: Function Attrs: noinline noreturn nounwind uwtable
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define void @call_will_return_but_has_loop()
 define void @call_will_return_but_has_loop() #0 {
   br label %label1
 label1:
@@ -256,11 +298,14 @@
 
 ; FNATTR: Function Attrs: noinline uwtable "willreturn"
 ; FNATTR-NEXT: declare i1 @maybe_raise_exception()
+; ATTRIBUTOR: Function Attrs: noinline uwtable "willreturn"
+; ATTRIBUTOR-NEXT: declare i1 @maybe_raise_exception()
 declare i1 @maybe_raise_exception() #1 "willreturn"
 
 ; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: nounwind
 ; FNATTR-NEXT: define void @invoke_test()
+; ATTRIBUTOR: define void @invoke_test()
 define void @invoke_test() personality i32 (...)* @__gxx_personality_v0 {
   invoke i1 @maybe_raise_exception()
 			to label %N unwind label %F
@@ -288,6 +333,8 @@
 ; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: noinline norecurse nounwind readonly uwtable
 ; FNATTR-NEXT: define i32 @loop_constant_trip_count(i32* nocapture readonly)
+; ATTRIBUTOR: Function Attrs: noinline norecurse nounwind uwtable "nofree"
+; ATTRIBUTOR-NEXT: define i32 @loop_constant_trip_count(i32* nocapture readonly)
 define i32 @loop_constant_trip_count(i32* nocapture readonly) #0 {
   br label %3
 
@@ -319,6 +366,9 @@
 ; FNATTR: Function Attrs: noinline norecurse nounwind readonly uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: define i32 @loop_trip_count_unbound(i32, i32, i32* nocapture readonly, i32) local_unnamed_addr
+; ATTRIBUTOR: Function Attrs: noinline norecurse nounwind uwtable "nofree"
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define i32 @loop_trip_count_unbound(i32, i32, i32* nocapture readonly, i32) local_unnamed_addr
 define i32 @loop_trip_count_unbound(i32, i32, i32* nocapture readonly, i32) local_unnamed_addr #0 {
   %5 = icmp eq i32 %0, %1
   br i1 %5, label %6, label %8
@@ -354,6 +404,8 @@
 ; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: noinline norecurse nounwind readonly uwtable
 ; FNATTR-NEXT: define i32 @loop_trip_dec(i32, i32* nocapture readonly)
+; ATTRIBUTOR: Function Attrs: noinline norecurse nounwind uwtable "nofree"
+; ATTRIBUTOR-NEXT: define i32 @loop_trip_dec(i32, i32* nocapture readonly) local_unnamed_addr
 
 define i32 @loop_trip_dec(i32, i32* nocapture readonly) local_unnamed_addr #0 {
   %3 = icmp sgt i32 %0, -1
@@ -384,6 +436,8 @@
 ; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: noinline norecurse nounwind readnone uwtable
 ; FNATTR-NEXT: define i32 @multiple_return(i32 %a)
+; ATTRIBUTOR: Function Attrs: noinline norecurse nounwind uwtable "nofree" "willreturn"
+; ATTRIBUTOR-NEXT: define i32 @multiple_return(i32 %a)
 define i32 @multiple_return(i32 %a) #0 {
   %b =  icmp eq i32 %a, 0
   br i1 %b, label %t, label %f
@@ -401,6 +455,8 @@
 ; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NEXT: define void @unreachable_exit_positive1()
+; ATTRIBUTOR: Function Attrs: noinline nounwind uwtable
+; ATTRIBUTOR-NEXT: define void @unreachable_exit_positive1()
 define void @unreachable_exit_positive1() #0 {
   tail call void @will_return()
   ret void
@@ -413,6 +469,8 @@
 ; FIXME: missing "willreturn"
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NEXT: define i32 @unreachable_exit_positive2(i32)
+; ATTRIBUTOR: Function Attrs: noinline nounwind uwtable
+; ATTRIBUTOR-NEXT: define i32 @unreachable_exit_positive2(i32)
 define i32 @unreachable_exit_positive2(i32) local_unnamed_addr #0 {
   %2 = icmp slt i32 %0, 1
   br i1 %2, label %3, label %5
@@ -439,6 +497,9 @@
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: define void @unreachable_exit_negative1()
+; ATTRIBUTOR: Function Attrs: noinline nounwind uwtable
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define void @unreachable_exit_negative1()
 define void @unreachable_exit_negative1() #0 {
   tail call void @exit(i32 0)
   ret void
@@ -451,6 +512,9 @@
 ; FNATTR: Function Attrs: noinline nounwind uwtable
 ; FNATTR-NOT: "willreturn"
 ; FNATTR-NEXT: define void @unreachable_exit_negative2()
+; ATTRIBUTOR: Function Attrs: noinline noreturn nounwind uwtable
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define void @unreachable_exit_negative2()
 define void @unreachable_exit_negative2() #0 {
 
   br label %L1
@@ -464,6 +528,23 @@
   unreachable
 }
 
+; FNATTR: Function Attrs: noreturn nounwind
+; FNATTR-NEXT: declare void @llvm.eh.sjlj.longjmp(i8*)
+; ATTRIBUTOR: Function Attrs: noreturn nounwind
+; ATTRIBUTOR-NEXT: declare void @llvm.eh.sjlj.longjmp(i8*)
+declare void @llvm.eh.sjlj.longjmp(i8*)
+
+; FNATTR: Function Attrs: noinline nounwind uwtable
+; FNATTR-NOT: "willreturn"
+; FNATTR-NEXT: define void @call_longjmp(i8* nocapture readnone) local_unnamed_addr #3 {
+; ATTRIBUTOR: Function Attrs: noinline nounwind uwtable
+; ATTRIBUTOR-NOT: "willreturn"
+; ATTRIBUTOR-NEXT: define void @call_longjmp(i8* nocapture readnone) local_unnamed_addr
+define void @call_longjmp(i8* nocapture readnone) local_unnamed_addr #0 {
+  tail call void @llvm.eh.sjlj.longjmp(i8* %0)
+  ret void
+}
+
 
 attributes #0 = { nounwind uwtable noinline }
 attributes #1 = { uwtable noinline }