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[libc++] constexpr is_sufficiently_aligned #146787

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[libc++] constexpr is_sufficiently_aligned #146787

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f6e9679
[libc++] constexpr is_sufficiently_aligned
dalg24 Jul 2, 2025
35ccbe6
Use __builtin_is_aligned with Clang and fix GCC implemtation
dalg24 Jul 3, 2025
bfaccf2
Prefer if consteval
dalg24 Jul 3, 2025
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12 changes: 10 additions & 2 deletions libcxx/include/__memory/is_sufficiently_aligned.h
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Original file line number Diff line number Diff line change
Expand Up @@ -23,8 +23,16 @@ _LIBCPP_BEGIN_NAMESPACE_STD
#if _LIBCPP_STD_VER >= 26

template <size_t _Alignment, class _Tp>
_LIBCPP_HIDE_FROM_ABI bool is_sufficiently_aligned(_Tp* __ptr) {
return reinterpret_cast<uintptr_t>(__ptr) % _Alignment == 0;
_LIBCPP_HIDE_FROM_ABI constexpr bool is_sufficiently_aligned(_Tp* __ptr) {
# ifdef _LIBCPP_COMPILER_CLANG_BASED
return __builtin_is_aligned(__ptr, _Alignment);
# else
if consteval {
return __builtin_constant_p(__builtin_assume_aligned(__ptr, _Alignment) != nullptr);
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I would prefer simply not supporting constexpr with GCC here.
It seems better to fail loudly than to give the wrong result.

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The goal was to determine whether or not it is implementable, and potentially opening a LWG issue before C++26 is shipped.

} else {
return reinterpret_cast<uintptr_t>(__ptr) % _Alignment == 0;
}
# endif
}

#endif // _LIBCPP_STD_VER >= 26
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9 changes: 5 additions & 4 deletions libcxx/test/std/utilities/memory/ptr.align/is_sufficiently_aligned.pass.cpp
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Expand Up @@ -21,7 +21,7 @@
#include "test_macros.h"

template <typename T>
void test_is_sufficiently_aligned() {
constexpr void test_is_sufficiently_aligned() {
constexpr std::size_t N = alignof(T);

alignas(8 * N) std::remove_cv_t<T> buf[5];
Expand Down Expand Up @@ -54,7 +54,7 @@ void test_is_sufficiently_aligned() {
}

template <typename T>
void check(T* p) {
constexpr void check(T* p) {
ASSERT_SAME_TYPE(bool, decltype(std::is_sufficiently_aligned<alignof(T)>(p)));
test_is_sufficiently_aligned<T>();
test_is_sufficiently_aligned<const T>();
Expand All @@ -73,7 +73,7 @@ struct alignas(1) X {
};
static_assert(sizeof(X) == 2 * alignof(X));

bool tests() {
constexpr bool test() {
char c;
int i;
long l;
Expand Down Expand Up @@ -107,7 +107,8 @@ bool tests() {
}

int main(int, char**) {
tests();
test();
static_assert(test());

return 0;
}
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