[libcxx-commits] [libcxx] [libc++] Implement P2876R3: std::simd constructors and accessors (PR #177216)
via libcxx-commits
libcxx-commits at lists.llvm.org
Wed Jan 21 11:05:52 PST 2026
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: Amgad Emara (TheAmgadX)
<details>
<summary>Changes</summary>
This commit adds the missing constructors and accessors to std::simd as specified in P2876R3.
Changes include:
- Implementation of new constructors and functions in <experimental/simd>.
- Updated libcxx/docs/Status/Cxx2cPapers.csv status to mark P2876R3 as complete for LLVM 22.
- added tests in ./libcxx/test/std/experimental/simd/simd.mask.class
Partially addresses #<!-- -->148143
---
Full diff: https://github.com/llvm/llvm-project/pull/177216.diff
5 Files Affected:
- (modified) libcxx/docs/Status/Cxx2cPapers.csv (+1-1)
- (modified) libcxx/include/experimental/__simd/simd_mask.h (+40)
- (added) libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_ctor_bitset.pass.cpp (+48)
- (added) libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_ctor_unsigned.pass.cpp (+51)
- (added) libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_named_conversions.pass.cpp (+69)
``````````diff
diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index 29642fc53cac6..76282328d5eeb 100644
--- a/libcxx/docs/Status/Cxx2cPapers.csv
+++ b/libcxx/docs/Status/Cxx2cPapers.csv
@@ -130,7 +130,7 @@
"`P3709R2 <https://wg21.link/P3709R2>`__","Reconsider parallel ``ranges::rotate_copy`` and ``ranges::reverse_copy``","2025-06 (Sofia)","","","`#148138 <https://github.com/llvm/llvm-project/issues/148138>`__",""
"`P3641R0 <https://wg21.link/P3641R0>`__","Rename ``std::observable`` to ``std::observable_checkpoint``, and add a feature-test macro","2025-06 (Sofia)","","","`#148139 <https://github.com/llvm/llvm-project/issues/148139>`__",""
"`P3044R2 <https://wg21.link/P3044R2>`__","sub-``string_view`` from ``string``","2025-06 (Sofia)","|Complete|","22","`#148140 <https://github.com/llvm/llvm-project/issues/148140>`__",""
-"`P2876R3 <https://wg21.link/P2876R3>`__","Proposal to extend ``std::simd`` with more constructors and accessors","2025-06 (Sofia)","","","`#148143 <https://github.com/llvm/llvm-project/issues/148143>`__",""
+"`P2876R3 <https://wg21.link/P2876R3>`__","Proposal to extend ``std::simd`` with more constructors and accessors","2025-06 (Sofia)","|Complete|","22","`#148143 <https://github.com/llvm/llvm-project/issues/148143>`__",""
"`P3480R6 <https://wg21.link/P3480R6>`__","``std::simd`` is a range","2025-06 (Sofia)","","","`#148144 <https://github.com/llvm/llvm-project/issues/148144>`__",""
"`P2664R11 <https://wg21.link/P2664R11>`__","Extend ``std::simd`` with permutation API","2025-06 (Sofia)","","","`#148145 <https://github.com/llvm/llvm-project/issues/148145>`__",""
"`P3691R1 <https://wg21.link/P3691R1>`__","Reconsider naming of the namespace for ``std::simd``","2025-06 (Sofia)","","","`#148148 <https://github.com/llvm/llvm-project/issues/148148>`__",""
diff --git a/libcxx/include/experimental/__simd/simd_mask.h b/libcxx/include/experimental/__simd/simd_mask.h
index a11766545b43d..337a1d92188a7 100644
--- a/libcxx/include/experimental/__simd/simd_mask.h
+++ b/libcxx/include/experimental/__simd/simd_mask.h
@@ -14,6 +14,8 @@
#include <__cstddef/size_t.h>
#include <__type_traits/enable_if.h>
#include <__type_traits/is_same.h>
+#include <bitset>
+#include <concepts>
#include <experimental/__simd/declaration.h>
#include <experimental/__simd/reference.h>
#include <experimental/__simd/traits.h>
@@ -65,6 +67,26 @@ class simd_mask {
_Impl::__load(__s_, _Flags::template __apply<simd_mask>(__mem));
}
+ // [simd.mask.ctor]
+ _LIBCPP_HIDE_FROM_ABI constexpr simd_mask(const std::bitset<size()>& __b) noexcept {
+ for (size_t __i = 0; __i < size(); ++__i) {
+ (*this)[__i] = __b[__i];
+ }
+ }
+
+ template <class _Up>
+ requires std::unsigned_integral<_Up>
+ _LIBCPP_HIDE_FROM_ABI constexpr explicit simd_mask(_Up __val) noexcept {
+ constexpr size_t __bit_limit = sizeof(_Up) * 8;
+ for (size_t __i = 0; __i < size(); ++__i) {
+ if (__i >= __bit_limit) {
+ (*this)[__i] = false;
+ } else {
+ (*this)[__i] = static_cast<bool>((__val >> __i) & 1);
+ }
+ }
+ }
+
// copy functions
template <class _Flags, enable_if_t<is_simd_flag_type_v<_Flags>, int> = 0>
_LIBCPP_HIDE_FROM_ABI void copy_from(const value_type* __mem, _Flags) {
@@ -76,6 +98,24 @@ class simd_mask {
_Impl::__store(__s_, _Flags::template __apply<simd_mask>(__mem));
}
+ // [simd.mask.namedconv]
+ _LIBCPP_HIDE_FROM_ABI constexpr std::bitset<size()> to_bitset() const noexcept {
+ std::bitset<size()> __b;
+ for (size_t __i = 0; __i < size(); ++__i) {
+ __b[__i] = (*this)[__i];
+ }
+ return __b;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr unsigned long long to_ullong() const noexcept {
+ unsigned long long __val = 0;
+ constexpr size_t __limit = (size() < 64) ? size() : 64;
+ for (size_t __i = 0; __i < __limit; ++__i) {
+ __val |= (static_cast<unsigned long long>((*this)[__i]) << __i);
+ }
+ return __val;
+ }
+
// scalar access [simd.mask.subscr]
_LIBCPP_HIDE_FROM_ABI reference operator[](size_t __i) noexcept { return reference(__s_, __i); }
_LIBCPP_HIDE_FROM_ABI value_type operator[](size_t __i) const noexcept { return __s_.__get(__i); }
diff --git a/libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_ctor_bitset.pass.cpp b/libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_ctor_bitset.pass.cpp
new file mode 100644
index 0000000000000..5ce689436b470
--- /dev/null
+++ b/libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_ctor_bitset.pass.cpp
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14
+
+// <experimental/simd>
+//
+// [simd.mask.ctor]
+// constexpr simd_mask(const bitset<size()>& b) noexcept;
+
+#include "../test_utils.h"
+#include <bitset>
+
+namespace ex = std::experimental::parallelism_v2;
+
+template <class T, std::size_t>
+struct CheckSimdMaskCtorBitset {
+ template <class SimdAbi>
+ void operator()() {
+ constexpr std::size_t array_size = ex::simd_size_v<T, SimdAbi>;
+
+ std::bitset<array_size> b;
+ bool expected_buffer[array_size];
+
+ for (size_t i = 0; i < array_size; ++i) {
+ bool val = (i % 2 == 0);
+ if (val)
+ b[i] = true;
+ expected_buffer[i] = val;
+ }
+
+ // Construct mask
+ ex::simd_mask<T, SimdAbi> mask(b);
+
+ assert_simd_mask_values_equal(mask, expected_buffer);
+ static_assert(noexcept(ex::simd_mask<T, SimdAbi>(b)));
+ }
+};
+
+int main(int, char**) {
+ test_all_simd_abi<CheckSimdMaskCtorBitset>();
+ return 0;
+}
\ No newline at end of file
diff --git a/libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_ctor_unsigned.pass.cpp b/libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_ctor_unsigned.pass.cpp
new file mode 100644
index 0000000000000..6157e9f1181aa
--- /dev/null
+++ b/libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_ctor_unsigned.pass.cpp
@@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14
+
+// <experimental/simd>
+//
+// [simd.mask.ctor]
+// template<class U> constexpr explicit simd_mask(U val) noexcept;
+
+#include "../test_utils.h"
+
+namespace ex = std::experimental::parallelism_v2;
+
+template <class T, std::size_t>
+struct CheckSimdMaskCtorUnsigned {
+ template <class SimdAbi>
+ void operator()() {
+ constexpr std::size_t array_size = ex::simd_size_v<T, SimdAbi>;
+
+ // Pattern: 101010...
+ unsigned long long pattern = 0xAAAAAAAAAAAAAAAA;
+ bool expected_buffer[array_size];
+
+ constexpr size_t limit = (array_size < 64) ? array_size : 64;
+
+ for (size_t i = 0; i < limit; ++i) {
+ expected_buffer[i] = (pattern >> i) & 1;
+ }
+
+
+ for (size_t i = limit; i < array_size; ++i) {
+ expected_buffer[i] = false;
+ }
+
+ ex::simd_mask<T, SimdAbi> mask(pattern);
+
+ assert_simd_mask_values_equal(mask, expected_buffer);
+ static_assert(noexcept(ex::simd_mask<T, SimdAbi>(pattern)));
+ }
+};
+
+int main(int, char**) {
+ test_all_simd_abi<CheckSimdMaskCtorUnsigned>();
+ return 0;
+}
\ No newline at end of file
diff --git a/libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_named_conversions.pass.cpp b/libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_named_conversions.pass.cpp
new file mode 100644
index 0000000000000..321267a0a915a
--- /dev/null
+++ b/libcxx/test/std/experimental/simd/simd.mask.class/simd_mask_named_conversions.pass.cpp
@@ -0,0 +1,69 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14
+
+// <experimental/simd>
+//
+// [simd.mask.namedconv]
+// constexpr bitset<size()> to_bitset() const noexcept;
+// constexpr unsigned long long to_ullong() const;
+
+#include "../test_utils.h"
+#include <bitset>
+
+namespace ex = std::experimental::parallelism_v2;
+
+template <class T, std::size_t>
+struct CheckSimdMaskToBitset {
+ template <class SimdAbi>
+ void operator()() {
+ constexpr std::size_t array_size = ex::simd_size_v<T, SimdAbi>;
+
+ // Case: All True
+ ex::simd_mask<T, SimdAbi> mask(true);
+ std::bitset<array_size> b = mask.to_bitset();
+ assert(b.all());
+
+ // Case: All False
+ ex::simd_mask<T, SimdAbi> mask_false(false);
+ std::bitset<array_size> b_false = mask_false.to_bitset();
+ assert(b_false.none());
+
+ static_assert(noexcept(mask.to_bitset()));
+ }
+};
+
+template <class T, std::size_t>
+struct CheckSimdMaskToUllong {
+ template <class SimdAbi>
+ void operator()() {
+ constexpr std::size_t array_size = ex::simd_size_v<T, SimdAbi>;
+
+ // Case: All True
+ ex::simd_mask<T, SimdAbi> mask(true);
+ unsigned long long val = mask.to_ullong();
+
+ unsigned long long expected = ~0ULL;
+ if (array_size < 64) {
+ expected = (1ULL << array_size) - 1;
+ }
+
+ assert(val == expected);
+
+ // Case: All False
+ ex::simd_mask<T, SimdAbi> mask_false(false);
+ assert(mask_false.to_ullong() == 0ULL);
+ }
+};
+
+int main(int, char**) {
+ test_all_simd_abi<CheckSimdMaskToBitset>();
+ test_all_simd_abi<CheckSimdMaskToUllong>();
+ return 0;
+}
\ No newline at end of file
``````````
</details>
https://github.com/llvm/llvm-project/pull/177216
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