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[libc++] P2747R2: constexpr placement new (library part) (#105768)
This patch implements https://wg21.link/P2747R2. The library changes affect direct `operator new` and `operator new[]` calls even when the core language changes are absent. The changes are not available for MS ABI because the `operator new` and `operator new[]` are from VCRuntime's `<vcruntime_new.h>`. A feature request was submitted for that [1]. As a drive-by change, the patch reformatted the whole `new.pass.cpp` and `new_array.pass.cpp` tests. Closes #105427 [1]: https://developercommunity.visualstudio.com/t/constexpr-for-placement-operator-newope/10730304.
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‎libcxx/docs/FeatureTestMacroTable.rst‎

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@@ -406,7 +406,7 @@ Status
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---------------------------------------------------------- -----------------
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``__cpp_lib_bitset`` ``202306L``
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---------------------------------------------------------- -----------------
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``__cpp_lib_constexpr_new`` *unimplemented*
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``__cpp_lib_constexpr_new`` ``202406L``
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---------------------------------------------------------- -----------------
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``__cpp_lib_constrained_equality`` *unimplemented*
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---------------------------------------------------------- -----------------

‎libcxx/docs/ReleaseNotes/20.rst‎

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@@ -38,6 +38,7 @@ What's New in Libc++ 20.0.0?
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Implemented Papers
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------------------
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- P2747R2: ``constexpr`` placement new (`Github <https://github.com/llvm/llvm-project/issues/105427>`__)
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- P2609R3: Relaxing Ranges Just A Smidge (`Github <https://github.com/llvm/llvm-project/issues/105253>`__)
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- P2985R0: A type trait for detecting virtual base classes (`Github <https://github.com/llvm/llvm-project/issues/105432>`__)
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‎libcxx/docs/Status/Cxx2cPapers.csv‎

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@@ -63,7 +63,7 @@
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"`P2642R6 <https://wg21.link/P2642R6>`__","Padded ``mdspan`` layouts","2024-03 (Tokyo)","","",""
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"`P3029R1 <https://wg21.link/P3029R1>`__","Better ``mdspan``'s CTAD","2024-03 (Tokyo)","|Complete|","19.0",""
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"","","","","",""
66-
"`P2747R2 <https://wg21.link/P2747R2>`__","``constexpr`` placement new","2024-06 (St. Louis)","","",""
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"`P2747R2 <https://wg21.link/P2747R2>`__","``constexpr`` placement new","2024-06 (St. Louis)","|Complete|","20.0",""
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"`P2997R1 <https://wg21.link/P2997R1>`__","Removing the common reference requirement from the indirectly invocable concepts","2024-06 (St. Louis)","|Complete| [#note-P2997R1]_","19.0",""
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"`P2389R2 <https://wg21.link/P2389R2>`__","``dextents`` Index Type Parameter","2024-06 (St. Louis)","|Complete|","19.0",""
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"`P3168R2 <https://wg21.link/P3168R2>`__","Give ``std::optional`` Range Support","2024-06 (St. Louis)","","","|ranges|"

‎libcxx/include/__config‎

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@@ -788,6 +788,12 @@ typedef __char32_t char32_t;
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# define _LIBCPP_CONSTEXPR_SINCE_CXX23
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# endif
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791+
# if _LIBCPP_STD_VER >= 26
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# define _LIBCPP_CONSTEXPR_SINCE_CXX26 constexpr
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# else
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# define _LIBCPP_CONSTEXPR_SINCE_CXX26
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# endif
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# ifndef _LIBCPP_WEAK
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# define _LIBCPP_WEAK __attribute__((__weak__))
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# endif

‎libcxx/include/new‎

Lines changed: 10 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -79,8 +79,8 @@ void operator delete[](void* ptr, const std::nothrow_t&) noexcept; // repla
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void operator delete[](void* ptr, std::align_val_t alignment,
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const std::nothrow_t&) noexcept; // replaceable, C++17
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82-
void* operator new (std::size_t size, void* ptr) noexcept; // nodiscard in C++20
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void* operator new[](std::size_t size, void* ptr) noexcept; // nodiscard in C++20
82+
void* operator new (std::size_t size, void* ptr) noexcept; // nodiscard in C++20, constexpr since C++26
83+
void* operator new[](std::size_t size, void* ptr) noexcept; // nodiscard in C++20, constexpr since C++26
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void operator delete (void* ptr, void*) noexcept;
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void operator delete[](void* ptr, void*) noexcept;
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@@ -242,8 +242,14 @@ _LIBCPP_OVERRIDABLE_FUNC_VIS void operator delete[](void* __p, std::size_t __sz,
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# endif
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# endif
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245-
_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI void* operator new(std::size_t, void* __p) _NOEXCEPT { return __p; }
246-
_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI void* operator new[](std::size_t, void* __p) _NOEXCEPT { return __p; }
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_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void*
246+
operator new(std::size_t, void* __p) _NOEXCEPT {
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return __p;
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}
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_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void*
250+
operator new[](std::size_t, void* __p) _NOEXCEPT {
251+
return __p;
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}
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inline _LIBCPP_HIDE_FROM_ABI void operator delete(void*, void*) _NOEXCEPT {}
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inline _LIBCPP_HIDE_FROM_ABI void operator delete[](void*, void*) _NOEXCEPT {}
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‎libcxx/include/version‎

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@@ -513,7 +513,9 @@ __cpp_lib_void_t 201411L <type_traits>
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# undef __cpp_lib_bind_front
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# define __cpp_lib_bind_front 202306L
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# define __cpp_lib_bitset 202306L
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// # define __cpp_lib_constexpr_new 202406L
516+
# if !defined(_LIBCPP_ABI_VCRUNTIME)
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# define __cpp_lib_constexpr_new 202406L
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# endif
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// # define __cpp_lib_constrained_equality 202403L
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// # define __cpp_lib_copyable_function 202306L
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// # define __cpp_lib_debugging 202311L

‎libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.placement/new.pass.cpp‎

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@@ -15,19 +15,30 @@
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int A_constructed = 0;
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struct A
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{
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A() {++A_constructed;}
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~A() {--A_constructed;}
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struct A {
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A() { ++A_constructed; }
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~A() { --A_constructed; }
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};
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24-
int main(int, char**)
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{
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char buf[sizeof(A)];
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TEST_CONSTEXPR_OPERATOR_NEW void test_direct_call() {
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assert(::operator new(sizeof(int), &A_constructed) == &A_constructed);
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28-
A* ap = new(buf) A;
29-
assert((char*)ap == buf);
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assert(A_constructed == 1);
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char ch = '*';
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assert(::operator new(1, &ch) == &ch);
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assert(ch == '*');
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}
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#ifdef __cpp_lib_constexpr_new
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static_assert((test_direct_call(), true));
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#endif
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int main(int, char**) {
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char buf[sizeof(A)];
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38+
A* ap = new (buf) A;
39+
assert((char*)ap == buf);
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assert(A_constructed == 1);
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test_direct_call();
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return 0;
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}

‎libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.placement/new_array.pass.cpp‎

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@@ -15,23 +15,34 @@
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int A_constructed = 0;
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struct A
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{
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A() {++A_constructed;}
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~A() {--A_constructed;}
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struct A {
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A() { ++A_constructed; }
20+
~A() { --A_constructed; }
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};
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24-
int main(int, char**)
25-
{
26-
const std::size_t Size = 3;
27-
// placement new might require additional space.
28-
const std::size_t ExtraSize = 64;
29-
char buf[Size*sizeof(A) + ExtraSize];
23+
TEST_CONSTEXPR_OPERATOR_NEW void test_direct_call() {
24+
assert(::operator new[](sizeof(int), &A_constructed) == &A_constructed);
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31-
A* ap = new(buf) A[Size];
32-
assert((char*)ap >= buf);
33-
assert((char*)ap < (buf + ExtraSize));
34-
assert(A_constructed == Size);
26+
char ch = '*';
27+
assert(::operator new[](1, &ch) == &ch);
28+
assert(ch == '*');
29+
}
30+
31+
#ifdef __cpp_lib_constexpr_new
32+
static_assert((test_direct_call(), true));
33+
#endif
34+
35+
int main(int, char**) {
36+
const std::size_t Size = 3;
37+
// placement new might require additional space.
38+
const std::size_t ExtraSize = 64;
39+
char buf[Size * sizeof(A) + ExtraSize];
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41+
A* ap = new (buf) A[Size];
42+
assert((char*)ap >= buf);
43+
assert((char*)ap < (buf + ExtraSize));
44+
assert(A_constructed == Size);
3545

46+
test_direct_call();
3647
return 0;
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}

‎libcxx/test/std/language.support/support.limits/support.limits.general/new.version.compile.pass.cpp‎

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#elif TEST_STD_VER > 23
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174-
# if !defined(_LIBCPP_VERSION)
174+
# if !defined(_LIBCPP_ABI_VCRUNTIME)
175175
# ifndef __cpp_lib_constexpr_new
176176
# error "__cpp_lib_constexpr_new should be defined in c++26"
177177
# endif
178178
# if __cpp_lib_constexpr_new != 202406L
179179
# error "__cpp_lib_constexpr_new should have the value 202406L in c++26"
180180
# endif
181-
# else // _LIBCPP_VERSION
181+
# else
182182
# ifdef __cpp_lib_constexpr_new
183-
# error "__cpp_lib_constexpr_new should not be defined because it is unimplemented in libc++!"
183+
# error "__cpp_lib_constexpr_new should not be defined when the requirement '!defined(_LIBCPP_ABI_VCRUNTIME)' is not met!"
184184
# endif
185185
# endif
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‎libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp‎

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@@ -6555,16 +6555,16 @@
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# error "__cpp_lib_constexpr_memory should have the value 202202L in c++26"
65566556
# endif
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6558-
# if !defined(_LIBCPP_VERSION)
6558+
# if !defined(_LIBCPP_ABI_VCRUNTIME)
65596559
# ifndef __cpp_lib_constexpr_new
65606560
# error "__cpp_lib_constexpr_new should be defined in c++26"
65616561
# endif
65626562
# if __cpp_lib_constexpr_new != 202406L
65636563
# error "__cpp_lib_constexpr_new should have the value 202406L in c++26"
65646564
# endif
6565-
# else // _LIBCPP_VERSION
6565+
# else
65666566
# ifdef __cpp_lib_constexpr_new
6567-
# error "__cpp_lib_constexpr_new should not be defined because it is unimplemented in libc++!"
6567+
# error "__cpp_lib_constexpr_new should not be defined when the requirement '!defined(_LIBCPP_ABI_VCRUNTIME)' is not met!"
65686568
# endif
65696569
# endif
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