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

[serge-sans-paille]

Summary

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.

[serge-sans-paille]

This validates with gcc 8.2. Given the history of llvm::Optional, I'm going to try to build with other compilers too.

[serge-sans-paille]

Also validates ok with clang 6.0.1 and gcc 4.8.5

[serge-sans-paille]

up!

[chandlerc]

I think you may be able to do this in a slightly nicer way.

If you move the definitions of the relevant special member functions to be out-of-line, I think you can remove the class-level specialization entirely, and simply explicitly specialize (out-of-line) the definitions of the relevant special member functions, with the trivial case specializing them to = default definitions, and the non-trivial case specializing to their current definitions. Make sense?

My only concern is that I think there were some dark corner cases around defined-as-defaulted but not declared-as-defaulted. I don't know if they impact triviality in ways that cause problems.

[serge-sans-paille]

@chandlerc there's indeed some redundancy that can be removed unfortunately, as you feared, defaulting seems incompatible with per-member specialization, or at least it hits my c++ knowledge limit.

Here is my attempt: https://godbolt.org/z/9_Lykd
I have an alternate version using inheritance under testing, I'll upload the patch during the day.

[serge-sans-paille]

@chandlerc patch updated, using inheritance to avoid too much duplication.

[chandlerc]

LGTM

Thanks for seeing through this .... very non-trivial effort! =D Also, as I'm sure you know by now, will need to watch the bots carefully. Sadly, some may fall over.

[serge-sans-paille]

Some follow-up on this review: I had to revert the associated commit because it was breaking on buildbot, on test case I failed to reproduce locally.

My educated guess on that subject is that original llvm::Optional implementation had some issues, namely:

• improper encapsulation (internal buffer exposed to llvm::Optional)
• UB as far as I understand this post https://stackoverflow.com/questions/27003727/does-this-really-break-strict-aliasing-rules
• some typo (std::forward used where std::move should be used)

This made implementing the property « optional<T> is trivially copyable if T is trivially copyable a very difficult task.
I experimented a bit and ended up commiting https://reviews.llvm.org/rL354246 that uses union as storage and provides cleaner encapsulation between
Optional and OptionalStorage, followed by https://reviews.llvm.org/rL354246 that implements the same idea as current review.

I'm pretty sure there's room for improvements, at least I hope we're on firmer ground now.

include/llvm/ADT/Optional.h

 #include <new>
 #include <utility>
 
 namespace llvm {
 
 class raw_ostream;
 
 namespace optional_detail {
+
 /// Storage for any type.
-template <typename T, bool = is_trivially_copyable<T>::value> struct OptionalStorage {
+//
+template <class T> struct OptionalTrivialStorage {
   AlignedCharArrayUnion<T> storage;
   bool hasVal = false;
+  OptionalTrivialStorage() = default;
+  OptionalTrivialStorage(OptionalTrivialStorage const &) = default;
+  OptionalTrivialStorage(OptionalTrivialStorage &&) = default;
+  OptionalTrivialStorage &operator=(OptionalTrivialStorage const &) = default;
+  OptionalTrivialStorage &operator=(OptionalTrivialStorage &&) = default;
+  ~OptionalTrivialStorage() = default;
 
-  OptionalStorage() = default;
+  OptionalTrivialStorage(const T &y) : hasVal(true) {
+    new (storage.buffer) T(y);
+  }
+  OptionalTrivialStorage(T &&y) : hasVal(true) {
+    new (storage.buffer) T(std::move(y));
+  }
+
+  OptionalTrivialStorage &operator=(const T &y) {
+    new (storage.buffer) T(y);
+    hasVal = true;
+    return *this;
+  }
+  OptionalTrivialStorage &operator=(T &&y) {
+    new (storage.buffer) T(std::move(y));
+    hasVal = true;
+    return *this;
+  }
 
-  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());
+  T *getPointer() {
+    assert(hasVal);
+    return reinterpret_cast<T *>(storage.buffer);
+  }
+  const T *getPointer() const {
+    assert(hasVal);
+    return reinterpret_cast<const T *>(storage.buffer);
   }
-  OptionalStorage(T &&y) : hasVal(true) {
-    new (storage.buffer) T(std::forward<T>(y));
+  void reset() { hasVal = false; }
+};
+
+template <typename T> struct OptionalStorage : OptionalTrivialStorage<T> {
+
+  OptionalStorage() = default;
+
+  OptionalStorage(const T &y) : OptionalTrivialStorage<T>(y) {}
+  OptionalStorage(T &&y) : OptionalTrivialStorage<T>(std::move(y)) {}
+
+  OptionalStorage(const OptionalStorage &O) : OptionalTrivialStorage<T>() {
+    this->hasVal = O.hasVal;
+    if (this->hasVal)
+      new (this->storage.buffer) T(*O.getPointer());
   }
-  OptionalStorage(OptionalStorage &&O) : hasVal(O.hasVal) {
+
+  OptionalStorage(OptionalStorage &&O) : OptionalTrivialStorage<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;
+      OptionalTrivialStorage<T>::operator=(std::move(y));
     }
     return *this;
   }
+
   OptionalStorage &operator=(OptionalStorage &&O) {
     if (!O.hasVal)
       reset();
     else {
       *this = std::move(*O.getPointer());
     }
     return *this;
   }
 
   // FIXME: these assignments (& the equivalent const T&/const Optional& ctors)
   // could be made more efficient by passing by value, possibly unifying them
   // with the rvalue versions above - but this could place a different set of
   // 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;
+      OptionalTrivialStorage<T>::operator=(y);
     }
     return *this;
   }
   OptionalStorage &operator=(const OptionalStorage &O) {
     if (!O.hasVal)
       reset();
     else
       *this = *O.getPointer();
     return *this;
   }
 
   ~OptionalStorage() { reset(); }
 
   void reset() {
-    if (hasVal) {
-      (*getPointer()).~T();
-      hasVal = false;
-    }
+    if (this->hasVal) {
+      (*this->getPointer()).~T();
+      OptionalTrivialStorage<T>::reset();
     }
-
-  T *getPointer() {
-    assert(hasVal);
-    return reinterpret_cast<T *>(storage.buffer);
-  }
-  const T *getPointer() const {
-    assert(hasVal);
-    return reinterpret_cast<const T *>(storage.buffer);
   }
 };
 
 } // namespace optional_detail
 
 template <typename T> class Optional {
-  optional_detail::OptionalStorage<T> Storage;
+  typename std::conditional<is_trivially_copyable<T>::value,
+                            optional_detail::OptionalTrivialStorage<T>,
+                            optional_detail::OptionalStorage<T>>::type Storage;
 
 public:
   using value_type = T;
 
   constexpr Optional() {}
   constexpr Optional(NoneType) {}
 
   Optional(const T &y) : Storage(y) {}

unittests/ADT/OptionalTest.cpp

 #include "llvm/Support/raw_ostream.h"
 #include "gtest/gtest-spi.h"
 #include "gtest/gtest.h"
 
 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;
   static unsigned CopyAssignments;
   explicit NonDefaultConstructible(int) {
   }
   NonDefaultConstructible(const NonDefaultConstructible&) {
     ++CopyConstructions;
static void ResetCounts() {
     CopyAssignments = 0;
   }
 };
 
 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 {
 };
 
 TEST_F(OptionalTest, NonDefaultConstructibleTest) {
   Optional<NonDefaultConstructible> O;
   EXPECT_FALSE(O);
 }
~MultiArgConstructor() {
     ++Destructions;
   }
   static void ResetCounts() {
     Destructions = 0;
   }
 };
 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;
   
   A.emplace(1, 2);
   EXPECT_TRUE(A.hasValue());
   EXPECT_EQ(1, A->x);
   EXPECT_EQ(2, A->y);
static void ResetCounts() {
     MoveAssignments = 0;
   }
 };
 
 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;
   EXPECT_EQ(0u, MoveOnly::MoveConstructions);
   EXPECT_EQ(0u, MoveOnly::MoveAssignments);
   EXPECT_EQ(0u, MoveOnly::Destructions);
 }
 

 private:
   // This should disable all move/copy operations.
   Immovable(Immovable&& other) = delete;
 };
 
 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();
   A.emplace(4);
   EXPECT_TRUE((bool)A);
   EXPECT_EQ(4, A->val);
   EXPECT_EQ(1u, Immovable::Constructions);
   EXPECT_EQ(0u, Immovable::Destructions);