[libcxx-commits] [libcxx] [libc++] Replace __is_trivially_relocatable by is_trivially_copyable (PR #124970)
Louis Dionne via libcxx-commits
libcxx-commits at lists.llvm.org
Tue Feb 4 11:45:19 PST 2025
https://github.com/ldionne updated https://github.com/llvm/llvm-project/pull/124970
>From 6da2fc4d827722f0adc5e445232d62fb191d3a61 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Wed, 29 Jan 2025 13:18:11 -0500
Subject: [PATCH 1/3] [libc++] Replace __is_trivially_relocatable by
is_trivially_copyable
The __is_trivially_relocatable builtin has semantics that do not
correspond to any current or future notion of trivial relocation.
Furthermore, it currently leads to incorrect optimizations for some
types on supported compilers:
- Clang on Windows where types with non-trivial destructors get
incorrectly optimized
- AppleClang where types with non-trivial move constructors get
incorrectly optimized
Until there is an agreed upon and bugfree implementation of what it
means to be trivially relocatable, it is safer to simply use trivially
copyable instead. This doesn't leave a lot of types behind and is
definitely correct.
---
.../__type_traits/is_trivially_relocatable.h | 7 --
.../is_trivially_relocatable.compile.pass.cpp | 10 +++
.../vector/trivial_relocation.pass.cpp | 67 +++++++++++++++++++
3 files changed, 77 insertions(+), 7 deletions(-)
create mode 100644 libcxx/test/std/containers/sequences/vector/trivial_relocation.pass.cpp
diff --git a/libcxx/include/__type_traits/is_trivially_relocatable.h b/libcxx/include/__type_traits/is_trivially_relocatable.h
index c0871731cc0016b..ecc97a41dfbdcbd 100644
--- a/libcxx/include/__type_traits/is_trivially_relocatable.h
+++ b/libcxx/include/__type_traits/is_trivially_relocatable.h
@@ -11,7 +11,6 @@
#include <__config>
#include <__type_traits/enable_if.h>
-#include <__type_traits/integral_constant.h>
#include <__type_traits/is_same.h>
#include <__type_traits/is_trivially_copyable.h>
@@ -23,14 +22,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
// A type is trivially relocatable if a move construct + destroy of the original object is equivalent to
// `memcpy(dst, src, sizeof(T))`.
-
-#if __has_builtin(__is_trivially_relocatable)
-template <class _Tp, class = void>
-struct __libcpp_is_trivially_relocatable : integral_constant<bool, __is_trivially_relocatable(_Tp)> {};
-#else
template <class _Tp, class = void>
struct __libcpp_is_trivially_relocatable : is_trivially_copyable<_Tp> {};
-#endif
template <class _Tp>
struct __libcpp_is_trivially_relocatable<_Tp,
diff --git a/libcxx/test/libcxx/type_traits/is_trivially_relocatable.compile.pass.cpp b/libcxx/test/libcxx/type_traits/is_trivially_relocatable.compile.pass.cpp
index 674df1d0219057d..e43f38aa26f7d0a 100644
--- a/libcxx/test/libcxx/type_traits/is_trivially_relocatable.compile.pass.cpp
+++ b/libcxx/test/libcxx/type_traits/is_trivially_relocatable.compile.pass.cpp
@@ -60,6 +60,16 @@ static_assert(std::__libcpp_is_trivially_relocatable<MoveOnlyTriviallyCopyable>:
static_assert(!std::__libcpp_is_trivially_relocatable<MoveOnlyTriviallyCopyable>::value, "");
#endif
+struct NonTrivialMoveConstructor {
+ NonTrivialMoveConstructor(NonTrivialMoveConstructor&&);
+};
+static_assert(!std::__libcpp_is_trivially_relocatable<NonTrivialMoveConstructor>::value, "");
+
+struct NonTrivialDestructor {
+ ~NonTrivialDestructor() {}
+};
+static_assert(!std::__libcpp_is_trivially_relocatable<NonTrivialDestructor>::value, "");
+
// library-internal types
// ----------------------
diff --git a/libcxx/test/std/containers/sequences/vector/trivial_relocation.pass.cpp b/libcxx/test/std/containers/sequences/vector/trivial_relocation.pass.cpp
new file mode 100644
index 000000000000000..e109a5034032ddb
--- /dev/null
+++ b/libcxx/test/std/containers/sequences/vector/trivial_relocation.pass.cpp
@@ -0,0 +1,67 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// Make sure we don't miscompile vector operations for types that shouldn't be considered
+// trivially relocatable.
+
+#include <vector>
+#include <cassert>
+#include <cstddef>
+
+#include "test_macros.h"
+
+struct Tracker {
+ std::size_t move_constructs = 0;
+};
+
+struct [[clang::trivial_abi]] Inner {
+ TEST_CONSTEXPR explicit Inner(Tracker* tracker) : tracker_(tracker) {}
+ TEST_CONSTEXPR Inner(const Inner& rhs) : tracker_(rhs.tracker_) { tracker_->move_constructs += 1; }
+ TEST_CONSTEXPR Inner(Inner&& rhs) : tracker_(rhs.tracker_) { tracker_->move_constructs += 1; }
+ Tracker* tracker_;
+};
+
+// Even though this type contains a trivial_abi type, it is not trivially move-constructible,
+// so we should not attempt to optimize its move construction + destroy using trivial relocation.
+struct NotTriviallyMovable {
+ TEST_CONSTEXPR explicit NotTriviallyMovable(Tracker* tracker) : inner_(tracker) {}
+ TEST_CONSTEXPR NotTriviallyMovable(NotTriviallyMovable&& other) : inner_(std::move(other.inner_)) {}
+ Inner inner_;
+};
+static_assert(!std::is_trivially_copyable<NotTriviallyMovable>::value, "");
+LIBCPP_STATIC_ASSERT(!std::__libcpp_is_trivially_relocatable<NotTriviallyMovable>::value, "");
+
+TEST_CONSTEXPR_CXX20 bool tests() {
+ Tracker track;
+ std::vector<NotTriviallyMovable> v;
+
+ // Fill the vector at its capacity, such that any subsequent push_back would require growing.
+ v.reserve(5);
+ for (std::size_t i = 0; i != 5; ++i) {
+ v.emplace_back(&track);
+ }
+ assert(track.move_constructs == 0);
+ assert(v.size() == 5);
+
+ // Force a reallocation of the buffer + relocalization of the elements.
+ // All the existing elements of the vector should be move-constructed to their new location.
+ v.emplace_back(&track);
+ assert(track.move_constructs == 5);
+
+ return true;
+}
+
+int main(int, char**) {
+ tests();
+#if TEST_STD_VER >= 20
+ static_assert(tests());
+#endif
+ return 0;
+}
>From 798bb8067126208fc665d23a0d1b46f5820c9e59 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Tue, 4 Feb 2025 14:41:28 -0500
Subject: [PATCH 2/3] Use && 0 and add a comment
---
libcxx/include/__type_traits/is_trivially_relocatable.h | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/libcxx/include/__type_traits/is_trivially_relocatable.h b/libcxx/include/__type_traits/is_trivially_relocatable.h
index ecc97a41dfbdcbd..9b0e240de55f4ec 100644
--- a/libcxx/include/__type_traits/is_trivially_relocatable.h
+++ b/libcxx/include/__type_traits/is_trivially_relocatable.h
@@ -22,8 +22,17 @@ _LIBCPP_BEGIN_NAMESPACE_STD
// A type is trivially relocatable if a move construct + destroy of the original object is equivalent to
// `memcpy(dst, src, sizeof(T))`.
+//
+// Note that we don't use the __is_trivially_relocatable Clang builtin right now because it does not
+// implement the semantics of any current or future trivial relocation proposal and it can lead to
+// incorrect optimizations on some platforms (Windows) and supported compilers (AppleClang).
+#if __has_builtin(__is_trivially_relocatable) && 0
+template <class _Tp, class = void>
+struct __libcpp_is_trivially_relocatable : integral_constant<bool, __is_trivially_relocatable(_Tp)> {};
+#else
template <class _Tp, class = void>
struct __libcpp_is_trivially_relocatable : is_trivially_copyable<_Tp> {};
+#endif
template <class _Tp>
struct __libcpp_is_trivially_relocatable<_Tp,
>From 55eaf470a98736e6ac2cfa7eb68d3a812bb0d233 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Tue, 4 Feb 2025 14:45:04 -0500
Subject: [PATCH 3/3] Make test portable
---
.../sequences/vector/trivial_relocation.pass.cpp | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/libcxx/test/std/containers/sequences/vector/trivial_relocation.pass.cpp b/libcxx/test/std/containers/sequences/vector/trivial_relocation.pass.cpp
index e109a5034032ddb..d093ae8b8771656 100644
--- a/libcxx/test/std/containers/sequences/vector/trivial_relocation.pass.cpp
+++ b/libcxx/test/std/containers/sequences/vector/trivial_relocation.pass.cpp
@@ -44,16 +44,18 @@ TEST_CONSTEXPR_CXX20 bool tests() {
// Fill the vector at its capacity, such that any subsequent push_back would require growing.
v.reserve(5);
- for (std::size_t i = 0; i != 5; ++i) {
+ std::size_t const capacity = v.capacity(); // could technically be more than 5
+ while (v.size() < v.capacity()) {
v.emplace_back(&track);
}
assert(track.move_constructs == 0);
- assert(v.size() == 5);
+ assert(v.capacity() == capacity);
+ assert(v.size() == capacity);
// Force a reallocation of the buffer + relocalization of the elements.
// All the existing elements of the vector should be move-constructed to their new location.
v.emplace_back(&track);
- assert(track.move_constructs == 5);
+ assert(track.move_constructs == capacity);
return true;
}
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