Re-commit rL353927, patch included

Make llvm::Optional<T> trivially copyable when T is trivially copyable

This is an ever-recurring issue (see https://bugs.llvm.org/show_bug.cgi?id=39427 and https://bugs.llvm.org/show_bug.cgi?id=35978)
but I believe that thanks to https://reviews.llvm.org/D54472 we can now ship a decent implementation of this.

Basically the fact that llvm::is_trivially_copyable has a consistent behavior across compilers should prevent any ABI issue,
and using in-place new instead of memcpy should keep compiler bugs away.

This patch is slightly different from the original revision https://reviews.llvm.org/rL353927 but achieves the same goal. It just avoids
going through std::conditional which may the code more explicit.

llvm-svn: 353962

include/llvm/ADT/Optional.h

@@ -29,33 +29,61 @@ namespace llvm {
 class raw_ostream;
 
 namespace optional_detail {
-/// Storage for any type.
-template <typename T, bool = is_trivially_copyable<T>::value> struct OptionalStorage {
+template <typename T> struct OptionalStorageBase {
   AlignedCharArrayUnion<T> storage;
   bool hasVal = false;
 
+  OptionalStorageBase() = default;
+  OptionalStorageBase(const T &y) : hasVal(true) { new (storage.buffer) T(y); }
+  OptionalStorageBase(T &&y) : hasVal(true) {
+    new (storage.buffer) T(std::move(y));
+  }
+
+  T *getPointer() {
+    assert(hasVal);
+    return reinterpret_cast<T *>(storage.buffer);
+  }
+  const T *getPointer() const {
+    assert(hasVal);
+    return reinterpret_cast<const T *>(storage.buffer);
+  }
+  OptionalStorageBase &operator=(T &&y) {
+    hasVal = true;
+    new (this->storage.buffer) T(std::move(y));
+    return *this;
+  }
+  OptionalStorageBase &operator=(const T &y) {
+    hasVal = true;
+    new (this->storage.buffer) T(y);
+    return *this;
+  }
+  void reset() { this->hasVal = false; }
+};
+
+/// Storage for any type.
+template <typename T, bool = is_trivially_copyable<T>::value>
+struct OptionalStorage : OptionalStorageBase<T> {
   OptionalStorage() = default;
 
-  OptionalStorage(const T &y) : hasVal(true) { new (storage.buffer) T(y); }
-  OptionalStorage(const OptionalStorage &O) : hasVal(O.hasVal) {
-    if (hasVal)
-      new (storage.buffer) T(*O.getPointer());
+  OptionalStorage(const T &y) : OptionalStorageBase<T>(y) {}
+  OptionalStorage(const OptionalStorage &O) : OptionalStorageBase<T>() {
+    this->hasVal = O.hasVal;
+    if (this->hasVal)
+      new (this->storage.buffer) T(*O.getPointer());
   }
-  OptionalStorage(T &&y) : hasVal(true) {
-    new (storage.buffer) T(std::forward<T>(y));
-  }
-  OptionalStorage(OptionalStorage &&O) : hasVal(O.hasVal) {
+  OptionalStorage(T &&y) : OptionalStorageBase<T>(std::move(y)) {}
+  OptionalStorage(OptionalStorage &&O) : OptionalStorageBase<T>() {
+    this->hasVal = O.hasVal;
     if (O.hasVal) {
-      new (storage.buffer) T(std::move(*O.getPointer()));
+      new (this->storage.buffer) T(std::move(*O.getPointer()));
     }
   }
 
   OptionalStorage &operator=(T &&y) {
-    if (hasVal)
-      *getPointer() = std::move(y);
+    if (this->hasVal)
+      *this->getPointer() = std::move(y);
     else {
-      new (storage.buffer) T(std::move(y));
-      hasVal = true;
+      OptionalStorageBase<T>::operator=(std::move(y));
     }
     return *this;
   }
@@ -74,11 +102,10 @@ template <typename T, bool = is_trivially_copyable<T>::value> struct OptionalSto
   // requirements (notably: the existence of a default ctor) when implemented
   // in that way. Careful SFINAE to avoid such pitfalls would be required.
   OptionalStorage &operator=(const T &y) {
-    if (hasVal)
-      *getPointer() = y;
+    if (this->hasVal)
+      *this->getPointer() = y;
     else {
-      new (storage.buffer) T(y);
-      hasVal = true;
+      OptionalStorageBase<T>::operator=(y);
     }
     return *this;
   }
@@ -93,20 +120,30 @@ template <typename T, bool = is_trivially_copyable<T>::value> struct OptionalSto
   ~OptionalStorage() { reset(); }
 
   void reset() {
-    if (hasVal) {
-      (*getPointer()).~T();
-      hasVal = false;
+    if (this->hasVal) {
+      (*this->getPointer()).~T();
     }
+    OptionalStorageBase<T>::reset();
   }
+};
 
-  T *getPointer() {
-    assert(hasVal);
-    return reinterpret_cast<T *>(storage.buffer);
+template <typename T> struct OptionalStorage<T, true> : OptionalStorageBase<T> {
+  OptionalStorage() = default;
+  OptionalStorage(const T &y) : OptionalStorageBase<T>(y) {}
+  OptionalStorage(const OptionalStorage &O) = default;
+  OptionalStorage(T &&y) : OptionalStorageBase<T>(std::move(y)) {}
+  OptionalStorage(OptionalStorage &&O) = default;
+  OptionalStorage &operator=(T &&y) {
+    OptionalStorageBase<T>::operator=(std::move(y));
+    return *this;
   }
-  const T *getPointer() const {
-    assert(hasVal);
-    return reinterpret_cast<const T *>(storage.buffer);
+  OptionalStorage &operator=(OptionalStorage &&O) = default;
+  OptionalStorage &operator=(const T &y) {
+    OptionalStorageBase<T>::operator=(y);
+    return *this;
   }
+  OptionalStorage &operator=(const OptionalStorage &O) = default;
+  ~OptionalStorage() = default;
 };
 
 } // namespace optional_detail

unittests/ADT/OptionalTest.cpp

@@ -18,6 +18,12 @@ using namespace llvm;
 
 namespace {
 
+static_assert(llvm::is_trivially_copyable<Optional<int>>::value,
+              "trivially copyable");
+
+static_assert(llvm::is_trivially_copyable<Optional<std::array<int, 3>>>::value,
+              "trivially copyable");
+
 struct NonDefaultConstructible {
   static unsigned CopyConstructions;
   static unsigned Destructions;
@@ -45,6 +51,10 @@ unsigned NonDefaultConstructible::CopyConstructions = 0;
 unsigned NonDefaultConstructible::Destructions = 0;
 unsigned NonDefaultConstructible::CopyAssignments = 0;
 
+static_assert(
+    !llvm::is_trivially_copyable<Optional<NonDefaultConstructible>>::value,
+    "not trivially copyable");
+
 // Test fixture
 class OptionalTest : public testing::Test {
 };
@@ -203,6 +213,10 @@ struct MultiArgConstructor {
 };
 unsigned MultiArgConstructor::Destructions = 0;
 
+static_assert(
+    !llvm::is_trivially_copyable<Optional<MultiArgConstructor>>::value,
+    "not trivially copyable");
+
 TEST_F(OptionalTest, Emplace) {
   MultiArgConstructor::ResetCounts();
   Optional<MultiArgConstructor> A;
@@ -250,6 +264,9 @@ unsigned MoveOnly::MoveConstructions = 0;
 unsigned MoveOnly::Destructions = 0;
 unsigned MoveOnly::MoveAssignments = 0;
 
+static_assert(!llvm::is_trivially_copyable<Optional<MoveOnly>>::value,
+              "not trivially copyable");
+
 TEST_F(OptionalTest, MoveOnlyNull) {
   MoveOnly::ResetCounts();
   Optional<MoveOnly> O;
@@ -351,6 +368,9 @@ private:
 unsigned Immovable::Constructions = 0;
 unsigned Immovable::Destructions = 0;
 
+static_assert(!llvm::is_trivially_copyable<Optional<Immovable>>::value,
+              "not trivially copyable");
+
 TEST_F(OptionalTest, ImmovableEmplace) {
   Optional<Immovable> A;
   Immovable::ResetCounts();