[llvm-dev] [llvm] r322838 - [ADT] Split optional to only include copy mechanics and dtor for non-trivial types.
Sam McCall via llvm-dev
llvm-dev at lists.llvm.org
Wed Jan 24 12:20:22 PST 2018
Hey Ben,
This change broke some clangd code (no failing test, mea culpa), because it
changed the move semantics.
Previously: a moved-from llvm::Optional was None (for all types).
Now: a moved-from llvm::Optional is None (for non-trivial types), and has
the old value (for trivial types).
FWIW, a moved-from std::optional is *not* nullopt, and contains the
moved-from value.
This seems sad to me, and kills a nice use of optional (this one
<https://reviews.llvm.org/source/clang-tools-extra/browse/clang-tools-extra/trunk/clangd/Function.h;323350$162>),
but there's some value in consistency with std.
Either way, I wanted to bring this up because
- I wasn't sure it was explicitly considered, and should probably be
documented
- I think we should have either the old llvm behavior or the std behavior,
the new hybrid is a gotcha I think.
WDYT?
On Thu, Jan 18, 2018 at 12:26 PM, Benjamin Kramer via llvm-commits <
llvm-commits at lists.llvm.org> wrote:
> Author: d0k
> Date: Thu Jan 18 03:26:24 2018
> New Revision: 322838
>
> URL: http://llvm.org/viewvc/llvm-project?rev=322838&view=rev
> Log:
> [ADT] Split optional to only include copy mechanics and dtor for
> non-trivial types.
>
> This makes uses of Optional more transparent to the compiler (and
> clang-tidy) and generates slightly smaller code.
>
> This is a re-land of r317019, which had issues with GCC 4.8 back then.
> Those issues don't reproduce anymore, but I'll watch the buildbots
> closely in case anything goes wrong.
>
> Modified:
> llvm/trunk/include/llvm/ADT/Optional.h
> llvm/trunk/unittests/ADT/OptionalTest.cpp
>
> Modified: llvm/trunk/include/llvm/ADT/Optional.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/
> llvm/ADT/Optional.h?rev=322838&r1=322837&r2=322838&view=diff
> ============================================================
> ==================
> --- llvm/trunk/include/llvm/ADT/Optional.h (original)
> +++ llvm/trunk/include/llvm/ADT/Optional.h Thu Jan 18 03:26:24 2018
> @@ -27,139 +27,173 @@
>
> namespace llvm {
>
> -template <typename T> class Optional {
> +namespace optional_detail {
> +/// Storage for any type.
> +template <typename T, bool IsPodLike> struct OptionalStorage {
> AlignedCharArrayUnion<T> storage;
> bool hasVal = false;
>
> -public:
> - using value_type = T;
> -
> - Optional(NoneType) {}
> - explicit Optional() {}
> -
> - Optional(const T &y) : hasVal(true) { new (storage.buffer) T(y); }
> + OptionalStorage() = default;
>
> - Optional(const Optional &O) : hasVal(O.hasVal) {
> + 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);
> + new (storage.buffer) T(*O.getPointer());
> }
> -
> - Optional(T &&y) : hasVal(true) { new (storage.buffer)
> T(std::forward<T>(y)); }
> -
> - Optional(Optional<T> &&O) : hasVal(O) {
> - if (O) {
> - new (storage.buffer) T(std::move(*O));
> + OptionalStorage(T &&y) : hasVal(true) {
> + new (storage.buffer) T(std::forward<T>(y));
> + }
> + OptionalStorage(OptionalStorage &&O) : hasVal(O.hasVal) {
> + if (O.hasVal) {
> + new (storage.buffer) T(std::move(*O.getPointer()));
> O.reset();
> }
> }
>
> - ~Optional() { reset(); }
> -
> - Optional &operator=(T &&y) {
> + OptionalStorage &operator=(T &&y) {
> if (hasVal)
> - **this = std::move(y);
> + *getPointer() = std::move(y);
> else {
> new (storage.buffer) T(std::move(y));
> hasVal = true;
> }
> return *this;
> }
> -
> - Optional &operator=(Optional &&O) {
> - if (!O)
> + OptionalStorage &operator=(OptionalStorage &&O) {
> + if (!O.hasVal)
> reset();
> else {
> - *this = std::move(*O);
> + *this = std::move(*O.getPointer());
> O.reset();
> }
> return *this;
> }
>
> - /// Create a new object by constructing it in place with the given
> arguments.
> - template <typename... ArgTypes> void emplace(ArgTypes &&... Args) {
> - reset();
> - hasVal = true;
> - new (storage.buffer) T(std::forward<ArgTypes>(Args)...);
> - }
> -
> - static inline Optional create(const T *y) {
> - return y ? Optional(*y) : Optional();
> - }
> -
> // 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.
> - Optional &operator=(const T &y) {
> + OptionalStorage &operator=(const T &y) {
> if (hasVal)
> - **this = y;
> + *getPointer() = y;
> else {
> new (storage.buffer) T(y);
> hasVal = true;
> }
> return *this;
> }
> -
> - Optional &operator=(const Optional &O) {
> - if (!O)
> + OptionalStorage &operator=(const OptionalStorage &O) {
> + if (!O.hasVal)
> reset();
> else
> - *this = *O;
> + *this = *O.getPointer();
> return *this;
> }
>
> + ~OptionalStorage() { reset(); }
> +
> void reset() {
> if (hasVal) {
> - (**this).~T();
> + (*getPointer()).~T();
> hasVal = false;
> }
> }
>
> - const T *getPointer() const {
> - assert(hasVal);
> - return reinterpret_cast<const T *>(storage.buffer);
> - }
> T *getPointer() {
> assert(hasVal);
> return reinterpret_cast<T *>(storage.buffer);
> }
> - const T &getValue() const LLVM_LVALUE_FUNCTION {
> + const T *getPointer() const {
> assert(hasVal);
> - return *getPointer();
> + return reinterpret_cast<const T *>(storage.buffer);
> }
> - T &getValue() LLVM_LVALUE_FUNCTION {
> - assert(hasVal);
> - return *getPointer();
> +};
> +
> +/// Storage for trivially copyable types only.
> +template <typename T> struct OptionalStorage<T, true> {
> + AlignedCharArrayUnion<T> storage;
> + bool hasVal = false;
> +
> + OptionalStorage() = default;
> +
> + OptionalStorage(const T &y) : hasVal(true) { new (storage.buffer) T(y);
> }
> + OptionalStorage &operator=(const T &y) {
> + new (storage.buffer) T(y);
> + hasVal = true;
> + return *this;
> }
>
> - explicit operator bool() const { return hasVal; }
> - bool hasValue() const { return hasVal; }
> - const T *operator->() const { return getPointer(); }
> - T *operator->() { return getPointer(); }
> - const T &operator*() const LLVM_LVALUE_FUNCTION {
> - assert(hasVal);
> - return *getPointer();
> + void reset() { hasVal = false; }
> +};
> +} // namespace optional_detail
> +
> +template <typename T> class Optional {
> + optional_detail::OptionalStorage<T, isPodLike<T>::value> Storage;
> +
> +public:
> + using value_type = T;
> +
> + constexpr Optional() {}
> + constexpr Optional(NoneType) {}
> +
> + Optional(const T &y) : Storage(y) {}
> + Optional(const Optional &O) = default;
> +
> + Optional(T &&y) : Storage(std::forward<T>(y)) {}
> + Optional(Optional &&O) = default;
> +
> + Optional &operator=(T &&y) {
> + Storage = std::move(y);
> + return *this;
> }
> - T &operator*() LLVM_LVALUE_FUNCTION {
> - assert(hasVal);
> - return *getPointer();
> + Optional &operator=(Optional &&O) = default;
> +
> + /// Create a new object by constructing it in place with the given
> arguments.
> + template <typename... ArgTypes> void emplace(ArgTypes &&... Args) {
> + reset();
> + Storage.hasVal = true;
> + new (getPointer()) T(std::forward<ArgTypes>(Args)...);
> }
>
> + static inline Optional create(const T *y) {
> + return y ? Optional(*y) : Optional();
> + }
> +
> + Optional &operator=(const T &y) {
> + Storage = y;
> + return *this;
> + }
> + Optional &operator=(const Optional &O) = default;
> +
> + void reset() { Storage.reset(); }
> +
> + const T *getPointer() const {
> + assert(Storage.hasVal);
> + return reinterpret_cast<const T *>(Storage.storage.buffer);
> + }
> + T *getPointer() {
> + assert(Storage.hasVal);
> + return reinterpret_cast<T *>(Storage.storage.buffer);
> + }
> + const T &getValue() const LLVM_LVALUE_FUNCTION { return *getPointer(); }
> + T &getValue() LLVM_LVALUE_FUNCTION { return *getPointer(); }
> +
> + explicit operator bool() const { return Storage.hasVal; }
> + bool hasValue() const { return Storage.hasVal; }
> + const T *operator->() const { return getPointer(); }
> + T *operator->() { return getPointer(); }
> + const T &operator*() const LLVM_LVALUE_FUNCTION { return *getPointer();
> }
> + T &operator*() LLVM_LVALUE_FUNCTION { return *getPointer(); }
> +
> template <typename U>
> constexpr T getValueOr(U &&value) const LLVM_LVALUE_FUNCTION {
> return hasValue() ? getValue() : std::forward<U>(value);
> }
>
> #if LLVM_HAS_RVALUE_REFERENCE_THIS
> - T &&getValue() && {
> - assert(hasVal);
> - return std::move(*getPointer());
> - }
> - T &&operator*() && {
> - assert(hasVal);
> - return std::move(*getPointer());
> - }
> + T &&getValue() && { return std::move(*getPointer()); }
> + T &&operator*() && { return std::move(*getPointer()); }
>
> template <typename U>
> T getValueOr(U &&value) && {
>
> Modified: llvm/trunk/unittests/ADT/OptionalTest.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/
> ADT/OptionalTest.cpp?rev=322838&r1=322837&r2=322838&view=diff
> ============================================================
> ==================
> --- llvm/trunk/unittests/ADT/OptionalTest.cpp (original)
> +++ llvm/trunk/unittests/ADT/OptionalTest.cpp Thu Jan 18 03:26:24 2018
> @@ -518,5 +518,14 @@ TEST_F(OptionalTest, OperatorGreaterEqua
> CheckRelation<GreaterEqual>(InequalityLhs, InequalityRhs, !IsLess);
> }
>
> +#if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION))
> || \
> + (defined(__GNUC__) && __GNUC__ >= 5)
> +static_assert(std::is_trivially_copyable<Optional<int>>::value,
> + "Should be trivially copyable");
> +static_assert(
> + !std::is_trivially_copyable<Optional<NonDefaultConstructible>>::
> value,
> + "Shouldn't be trivially copyable");
> +#endif
> +
> } // end anonymous namespace
>
>
>
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> llvm-commits at lists.llvm.org
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