diff --git a/llvm/include/llvm/IR/InstrTypes.h b/llvm/include/llvm/IR/InstrTypes.h
--- a/llvm/include/llvm/IR/InstrTypes.h
+++ b/llvm/include/llvm/IR/InstrTypes.h
@@ -1768,6 +1768,9 @@
 
   /// Returns true if this function is guaranteed to return.
   bool willReturn() const { return hasFnAttr(Attribute::WillReturn); }
+  void setWillReturn() {
+    addAttribute(AttributeList::FunctionIndex, Attribute::WillReturn);
+  }
 
   void setOnlyReadsMemory() {
     addAttribute(AttributeList::FunctionIndex, Attribute::ReadOnly);
diff --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -1433,7 +1433,46 @@
   return Changed;
 }
 
-static bool functionWillReturn(const Function &F) {
+namespace {
+/// Helper to indicate whether all call sites in a function are willreturn, may
+/// return or that status is unknown because no analysis was done.
+enum class CallsiteStatusTy { AllWillReturn, MayNotReturn, Unknown };
+} // namespace
+
+// Add willreturn to callsites in F, if possible.
+static std::pair<bool, CallsiteStatus> callsitesWillReturn(Function &F) {
+  // Can only analyze functions with a definition.
+  if (F.isDeclaration())
+    return {false, CallsiteStatus::Unknown};
+
+  bool FnWillReturn = F.willReturn();
+  // Unless F must make progress or willreturn, we cannot add willreturn at
+  // callsites in the function.
+  if (!F.mustProgress() && !FnWillReturn)
+    return {false, CallsiteStatus::Unknown};
+
+  bool Changed = false;
+  CallsiteStatus AllCallsWillReturn = CallsiteStatus::AllWillReturn;
+  // Add willreturn to all callsites that only read memory or if the function
+  // itself is willreturn.
+  for (Instruction &I : instructions(F)) {
+    auto *CB = dyn_cast<CallBase>(&I);
+    if (!CB)
+      continue;
+
+    if (FnWillReturn || CB->onlyReadsMemory()) {
+      if (CB->willReturn())
+        continue;
+      CB->setWillReturn();
+      Changed = true;
+    } else {
+      AllCallsWillReturn = CallsiteStatus::MayNotReturn;
+    }
+  }
+  return {Changed, AllCallsWillReturn};
+}
+
+static bool functionWillReturn(const Function &F, CallsiteStatus CS) {
   // Must-progress function without side-effects must return.
   if (F.mustProgress() && F.onlyReadsMemory())
     return true;
@@ -1451,10 +1490,12 @@
 
   // If there are no loops, then the function is willreturn if all calls in
   // it are willreturn.
-  return all_of(instructions(F), [](const Instruction &I) {
-    const auto *CB = dyn_cast<CallBase>(&I);
-    return !CB || CB->hasFnAttr(Attribute::WillReturn);
-  });
+  if (CS == CallsiteStatus::Unknown)
+    return all_of(instructions(F), [](const Instruction &I) {
+      const auto *CB = dyn_cast<CallBase>(&I);
+      return !CB || CB->willReturn();
+    });
+  return CS == CallsiteStatus::AllWillReturn;
 }
 
 // Set the willreturn function attribute if possible.
@@ -1462,9 +1503,15 @@
   bool Changed = false;
 
   for (Function *F : SCCNodes) {
-    if (!F || F->willReturn() || !functionWillReturn(*F))
+    if (!F)
       continue;
+    bool Modified;
+    CallsiteStatus CSStatus;
+    std::tie(Modified, CSStatus) = callsitesWillReturn(*F);
+    Changed |= Modified;
 
+    if (F->willReturn() || !functionWillReturn(*F, CSStatus))
+      continue;
     F->setWillReturn();
     NumWillReturn++;
     Changed = true;
diff --git a/llvm/test/Transforms/FunctionAttrs/willreturn-callsites.ll b/llvm/test/Transforms/FunctionAttrs/willreturn-callsites.ll
--- a/llvm/test/Transforms/FunctionAttrs/willreturn-callsites.ll
+++ b/llvm/test/Transforms/FunctionAttrs/willreturn-callsites.ll
@@ -8,8 +8,8 @@
 define void @test_fn_mustprogress(i32* %ptr) mustprogress {
 ; CHECK: Function Attrs: mustprogress
 ; CHECK-LABEL: @test_fn_mustprogress(
-; CHECK-NOT:     call void @decl_readonly() #
-; CHECK-NOT:     call void @decl_readnone() #
+; CHECK-NEXT:    call void @decl_readonly() [[WILLRETURN:#.*]]
+; CHECK-NEXT:    call void @decl_readnone() [[WILLRETURN]]
 ; CHECK-NOT:     call void @decl_unknown() #
 ; CHECK-NOT:     call void @decl_argmemonly(i32* [[PTR:%.*]]) #
 ; CHECK:         ret void
@@ -24,11 +24,11 @@
 define void @test_fn_willreturn(i32* %ptr) willreturn {
 ; CHECK: Function Attrs: willreturn
 ; CHECK-LABEL: @test_fn_willreturn(
-; CHECK-NOT:     call void @decl_readonly() #
-; CHECK-NOT :    call void @decl_readnone() #
-; CHECK-NOT:     call void @decl_unknown() #
-; CHECK-NOT:     call void @decl_argmemonly(i32* [[PTR:%.*]]) #
-; CHECK:         ret void
+; CHECK-NEXT:    call void @decl_readonly() [[WILLRETURN]]
+; CHECK-NEXT:    call void @decl_readnone() [[WILLRETURN]]
+; CHECK-NEXT:    call void @decl_unknown() [[WILLRETURN]]
+; CHECK-NEXT:    call void @decl_argmemonly(i32* [[PTR:%.*]]) [[WILLRETURN]]
+; CHECK-NEXT:    ret void
 ;
   call void @decl_readonly()
   call void @decl_readnone()
@@ -40,9 +40,9 @@
 define void @test_fn_mustprogress_readonly_calls(i32* %ptr) mustprogress {
 ; CHECK: Function Attrs: readonly willreturn mustprogress
 ; CHECK-LABEL: @test_fn_mustprogress_readonly_calls(
-; CHECK-NOT:     call void @decl_readonly() #
-; CHECK-NOT:     call void @decl_readnone() #
-; CHECK:         ret void
+; CHECK-NEXT:    call void @decl_readonly() [[WILLRETURN]]
+; CHECK-NEXT:    call void @decl_readnone() [[WILLRETURN]]
+; CHECK-NEXT:    ret void
 ;
   call void @decl_readonly()
   call void @decl_readnone()
@@ -50,11 +50,11 @@
 }
 
 define void @test_fn_mustprogress_readonly_calls_but_stores(i32* %ptr) mustprogress {
-; CHECK: Function Attrs: nofree mustprogress
+; CHECK: Function Attrs: nofree willreturn mustprogress
 ; CHECK-LABEL: @test_fn_mustprogress_readonly_calls_but_stores(
-; CHECK-NOT:     call void @decl_readonly() #
-; CHECK-NOT:     call void @decl_readnone() #
-; CHECK:         store i32 0, i32* [[PTR:%.*]], align 4
+; CHECK-NEXT:    call void @decl_readonly() [[WILLRETURN]]
+; CHECK-NEXT:    call void @decl_readnone() [[WILLRETURN]]
+; CHECK-NEXT:    store i32 0, i32* [[PTR:%.*]], align 4
 ; CHECK-NEXT:    ret void
 ;
   call void @decl_readonly()
@@ -62,3 +62,5 @@
   store i32 0, i32* %ptr
   ret void
 }
+
+; CHECK: attributes [[WILLRETURN]] = { willreturn }