[llvm-branch-commits] [llvm] 25c1578 - Fix llvm::Optional build breaks in MSVC using std::is_trivially_copyable
Alexandre Ganea via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Sat Jan 16 06:42:17 PST 2021
Author: James Player
Date: 2021-01-16T09:37:04-05:00
New Revision: 25c1578a46ff93f920b7ad4e3057465902ced8f5
URL: https://github.com/llvm/llvm-project/commit/25c1578a46ff93f920b7ad4e3057465902ced8f5
DIFF: https://github.com/llvm/llvm-project/commit/25c1578a46ff93f920b7ad4e3057465902ced8f5.diff
LOG: Fix llvm::Optional build breaks in MSVC using std::is_trivially_copyable
Current code breaks this version of MSVC due to a mismatch between `std::is_trivially_copyable` and `llvm::is_trivially_copyable` for `std::pair` instantiations. Hence I was attempting to use `std::is_trivially_copyable` to set `llvm::is_trivially_copyable<T>::value`.
I spent some time root causing an `llvm::Optional` build error on MSVC 16.8.3 related to the change described above:
```
62>C:\src\ocg_llvm\llvm-project\llvm\include\llvm/ADT/BreadthFirstIterator.h(96,12): error C2280: 'llvm::Optional<std::pair<std::pair<unsigned int,llvm::Graph<4>::NodeSubset> *,llvm::Optional<llvm::Graph<4>::ChildIterator>>> &llvm::Optional<std::pair<std::pair<unsigned int,llvm::Graph<4>::NodeSubset> *,llvm::Optional<llvm::Graph<4>::ChildIterator>>>::operator =(const llvm::Optional<std::pair<std::pair<unsigned int,llvm::Graph<4>::NodeSubset> *,llvm::Optional<llvm::Graph<4>::ChildIterator>>> &)': attempting to reference a deleted function (compiling source file C:\src\ocg_llvm\llvm-project\llvm\unittests\ADT\BreadthFirstIteratorTest.cpp)
...
```
The "trivial" specialization of `optional_detail::OptionalStorage` assumes that the value type is trivially copy constructible and trivially copy assignable. The specialization is invoked based on a check of `is_trivially_copyable` alone, which does not imply both `is_trivially_copy_assignable` and `is_trivially_copy_constructible` are true.
[[ https://en.cppreference.com/w/cpp/named_req/TriviallyCopyable | According to the spec ]], a deleted assignment operator does not make `is_trivially_copyable` false. So I think all these properties need to be checked explicitly in order to specialize `OptionalStorage` to the "trivial" version:
```
/// Storage for any type.
template <typename T, bool = std::is_trivially_copy_constructible<T>::value
&& std::is_trivially_copy_assignable<T>::value>
class OptionalStorage {
```
Above fixed my build break in MSVC, but I think we need to explicitly check `is_trivially_copy_constructible` too since it might be possible the copy constructor is deleted. Also would be ideal to move over to `std::is_trivially_copyable` instead of the `llvm` namespace verson.
Reviewed By: dblaikie
Differential Revision: https://reviews.llvm.org/D93510
Added:
Modified:
llvm/include/llvm/ADT/Optional.h
llvm/unittests/ADT/OptionalTest.cpp
Removed:
################################################################################
diff --git a/llvm/include/llvm/ADT/Optional.h b/llvm/include/llvm/ADT/Optional.h
index daa9ee627fa9..a285c81d1be8 100644
--- a/llvm/include/llvm/ADT/Optional.h
+++ b/llvm/include/llvm/ADT/Optional.h
@@ -33,7 +33,30 @@ namespace optional_detail {
struct in_place_t {};
/// Storage for any type.
-template <typename T, bool = is_trivially_copyable<T>::value>
+//
+// The specialization condition intentionally uses
+// llvm::is_trivially_copy_constructible instead of
+// std::is_trivially_copy_constructible. GCC versions prior to 7.4 may
+// instantiate the copy constructor of `T` when
+// std::is_trivially_copy_constructible is instantiated. This causes
+// compilation to fail if we query the trivially copy constructible property of
+// a class which is not copy constructible.
+//
+// The current implementation of OptionalStorage insists that in order to use
+// the trivial specialization, the value_type must be trivially copy
+// constructible and trivially copy assignable due to =default implementations
+// of the copy/move constructor/assignment. It does not follow that this is
+// necessarily the case std::is_trivially_copyable is true (hence the expanded
+// specialization condition).
+//
+// The move constructible / assignable conditions emulate the remaining behavior
+// of std::is_trivially_copyable.
+template <typename T, bool = (llvm::is_trivially_copy_constructible<T>::value &&
+ std::is_trivially_copy_assignable<T>::value &&
+ (std::is_trivially_move_constructible<T>::value ||
+ !std::is_move_constructible<T>::value) &&
+ (std::is_trivially_move_assignable<T>::value ||
+ !std::is_move_assignable<T>::value))>
class OptionalStorage {
union {
char empty;
diff --git a/llvm/unittests/ADT/OptionalTest.cpp b/llvm/unittests/ADT/OptionalTest.cpp
index c7fa796a2d7f..d40e21255f80 100644
--- a/llvm/unittests/ADT/OptionalTest.cpp
+++ b/llvm/unittests/ADT/OptionalTest.cpp
@@ -8,6 +8,7 @@
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringMap.h"
#include "llvm/Support/raw_ostream.h"
#include "gtest/gtest-spi.h"
#include "gtest/gtest.h"
@@ -390,6 +391,143 @@ TEST(OptionalTest, ImmovableEmplace) {
EXPECT_EQ(0u, Immovable::Destructions);
}
+// Craft a class which is_trivially_copyable, but not
+// is_trivially_copy_constructible.
+struct NonTCopy {
+ NonTCopy() = default;
+
+ // Delete the volatile copy constructor to engage the "rule of 3" and delete
+ // any unspecified copy assignment or constructor.
+ NonTCopy(volatile NonTCopy const &) = delete;
+
+ // Leave the non-volatile default copy constructor unspecified (deleted by
+ // rule of 3)
+
+ // This template can serve as the copy constructor, but isn't chosen
+ // by =default in a class with a 'NonTCopy' member.
+ template <typename Self = NonTCopy>
+ NonTCopy(Self const &Other) : Val(Other.Val) {}
+
+ NonTCopy &operator=(NonTCopy const &) = default;
+
+ int Val{0};
+};
+
+#if defined(_MSC_VER) && _MSC_VER >= 1927 && !defined(__clang__)
+// Currently only true on recent MSVC releases.
+static_assert(std::is_trivially_copyable<NonTCopy>::value,
+ "Expect NonTCopy to be trivially copyable");
+
+static_assert(!std::is_trivially_copy_constructible<NonTCopy>::value,
+ "Expect NonTCopy not to be trivially copy constructible.");
+#endif // defined(_MSC_VER) && _MSC_VER >= 1927
+
+TEST(OptionalTest, DeletedCopyConstructor) {
+
+ // Expect compile to fail if 'trivial' version of
+ // optional_detail::OptionalStorage is chosen.
+ using NonTCopyOptT = Optional<NonTCopy>;
+ NonTCopyOptT NonTCopy1;
+
+ // Check that the Optional can be copy constructed.
+ NonTCopyOptT NonTCopy2{NonTCopy1};
+
+ // Check that the Optional can be copy assigned.
+ NonTCopy1 = NonTCopy2;
+}
+
+// Craft a class which is_trivially_copyable, but not
+// is_trivially_copy_assignable.
+class NonTAssign {
+public:
+ NonTAssign() = default;
+ NonTAssign(NonTAssign const &) = default;
+
+ // Delete the volatile copy assignment to engage the "rule of 3" and delete
+ // any unspecified copy assignment or constructor.
+ NonTAssign &operator=(volatile NonTAssign const &) = delete;
+
+ // Leave the non-volatile default copy assignment unspecified (deleted by rule
+ // of 3).
+
+ // This template can serve as the copy assignment, but isn't chosen
+ // by =default in a class with a 'NonTAssign' member.
+ template <typename Self = NonTAssign>
+ NonTAssign &operator=(Self const &Other) {
+ A = Other.A;
+ return *this;
+ }
+
+ int A{0};
+};
+
+#if defined(_MSC_VER) && _MSC_VER >= 1927 && !defined(__clang__)
+// Currently only true on recent MSVC releases.
+static_assert(std::is_trivially_copyable<NonTAssign>::value,
+ "Expect NonTAssign to be trivially copyable");
+
+static_assert(!std::is_trivially_copy_assignable<NonTAssign>::value,
+ "Expect NonTAssign not to be trivially assignable.");
+#endif // defined(_MSC_VER) && _MSC_VER >= 1927
+
+TEST(OptionalTest, DeletedCopyAssignment) {
+
+ // Expect compile to fail if 'trivial' version of
+ // optional_detail::OptionalStorage is chosen.
+ using NonTAssignOptT = Optional<NonTAssign>;
+ NonTAssignOptT NonTAssign1;
+
+ // Check that the Optional can be copy constructed.
+ NonTAssignOptT NonTAssign2{NonTAssign1};
+
+ // Check that the Optional can be copy assigned.
+ NonTAssign1 = NonTAssign2;
+}
+
+struct NoTMove {
+ NoTMove() = default;
+ NoTMove(NoTMove const &) = default;
+ NoTMove &operator=(NoTMove const &) = default;
+
+ // Delete move constructor / assignment. Compiler should fall-back to the
+ // trivial copy constructor / assignment in the trivial OptionalStorage
+ // specialization.
+ NoTMove(NoTMove &&) = delete;
+ NoTMove &operator=(NoTMove &&) = delete;
+
+ int Val{0};
+};
+
+TEST(OptionalTest, DeletedMoveConstructor) {
+ using NoTMoveOptT = Optional<NoTMove>;
+
+ NoTMoveOptT NonTMove1;
+ NoTMoveOptT NonTMove2{std::move(NonTMove1)};
+
+ NonTMove1 = std::move(NonTMove2);
+
+ static_assert(
+ std::is_trivially_copyable<NoTMoveOptT>::value,
+ "Expect Optional<NoTMove> to still use the trivial specialization "
+ "of OptionalStorage despite the deleted move constructor / assignment.");
+}
+
+class NoCopyStringMap {
+public:
+ NoCopyStringMap() = default;
+
+private:
+ llvm::StringMap<std::unique_ptr<int>> Map;
+};
+
+TEST(OptionalTest, DeletedCopyStringMap) {
+ // Old versions of gcc (7.3 and prior) instantiate the copy constructor when
+ // std::is_trivially_copyable is instantiated. This test will fail
+ // compilation if std::is_trivially_copyable is used in the OptionalStorage
+ // specialization condition by gcc <= 7.3.
+ Optional<NoCopyStringMap> TestInstantiation;
+}
+
#if LLVM_HAS_RVALUE_REFERENCE_THIS
TEST(OptionalTest, MoveGetValueOr) {
More information about the llvm-branch-commits
mailing list