P1007R3 assume_aligned() (#1306)

Co-authored-by: Stephan T. Lavavej <[email protected]>
Co-authored-by: Casey Carter <[email protected]>

Author: 3 people

stl/inc/memory

@@ -3687,15 +3687,17 @@ inline void* align(size_t _Bound, size_t _Size, void*& _Ptr, size_t& _Space) noe
     return _Ptr;
 }
 
-#if _HAS_CXX20 && 0
+#if _HAS_CXX20
 template <size_t _Nx, class _Ty>
-_NODISCARD constexpr _Ty* assume_aligned(_Ty* _Ptr) noexcept /* strengthened */ {
-    // this enforces the requirement that _Nx be a power of two
-    __builtin_assume_aligned(_Ptr, _Nx);
-
-    return _Ptr;
+_NODISCARD constexpr _Ty* assume_aligned(_Ty* const _Ptr) noexcept /* strengthened */ {
+    if (_STD is_constant_evaluated()) {
+        return _Ptr;
+    } else {
+        // this enforces the requirement that _Nx be a power of two
+        return static_cast<_Ty*>(__builtin_assume_aligned(_Ptr, _Nx));
+    }
 }
-#endif // _HAS_CXX20 && 0
+#endif // _HAS_CXX20
 
 // SPIN LOCKS
 _EXTERN_C

stl/inc/yvals_core.h

@@ -177,6 +177,7 @@
 // P0966R1 string::reserve() Should Not Shrink
 // P1001R2 execution::unseq
 // P1006R1 constexpr For pointer_traits<T*>::pointer_to()
+// P1007R3 assume_aligned()
 // P1023R0 constexpr For std::array Comparisons
 // P1024R3 Enhancing span Usability
 // P1032R1 Miscellaneous constexpr
@@ -1150,6 +1151,7 @@
 #define __cpp_lib_atomic_value_initialization 201911L
 
 #if _HAS_CXX20
+#define __cpp_lib_assume_aligned                201811L
 #define __cpp_lib_atomic_flag_test              201907L
 #define __cpp_lib_atomic_float                  201711L
 #define __cpp_lib_atomic_lock_free_type_aliases 201907L

tests/std/test.lst

@@ -348,6 +348,7 @@ tests\P0898R3_identity
 tests\P0912R5_coroutine
 tests\P0919R3_heterogeneous_unordered_lookup
 tests\P0966R1_string_reserve_should_not_shrink
+tests\P1007R3_assume_aligned
 tests\P1023R0_constexpr_for_array_comparisons
 tests\P1032R1_miscellaneous_constexpr
 tests\P1135R6_atomic_flag_test

tests/std/tests/P1007R3_assume_aligned/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_latest_matrix.lst

tests/std/tests/P1007R3_assume_aligned/test.cpp

@@ -0,0 +1,43 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <assert.h>
+#include <memory>
+#include <stddef.h>
+
+constexpr void can_change_value_via_assume_aligned(int* p) {
+    int* p1 = std::assume_aligned<256>(p);
+    p1[0]   = 10;
+    p1[1]   = -15;
+}
+
+constexpr int can_read_value_via_assume_aligned_and_sum_it(int const* p, size_t size) {
+    int const* p1 = std::assume_aligned<256>(p);
+    int result    = 0;
+    for (size_t i = 0; i < size; ++i) {
+        result += p1[i];
+    }
+    return result;
+}
+
+constexpr bool assume_aligned_can_change_value() {
+    alignas(256) int arr[2] = {0, 0};
+    can_change_value_via_assume_aligned(&arr[0]);
+    assert(arr[0] == 10);
+    assert(arr[1] == -15);
+    return true;
+}
+
+constexpr bool assume_aligned_can_read_value() {
+    alignas(256) constexpr int arr[3] = {10, 11, 9};
+    assert(can_read_value_via_assume_aligned_and_sum_it(arr, 3) == 30);
+    return true;
+}
+
+int main() {
+    assume_aligned_can_change_value();
+    static_assert(assume_aligned_can_change_value());
+
+    assume_aligned_can_read_value();
+    static_assert(assume_aligned_can_read_value());
+}

tests/std/tests/VSO_0157762_feature_test_macros/test.cpp

@@ -83,6 +83,20 @@ STATIC_ASSERT(__cpp_lib_array_constexpr == 201803L);
 STATIC_ASSERT(__cpp_lib_as_const == 201510L);
 #endif
 
+#if _HAS_CXX20
+#ifndef __cpp_lib_assume_aligned
+#error __cpp_lib_assume_aligned is not defined
+#elif __cpp_lib_assume_aligned != 201811L
+#error __cpp_lib_assume_aligned is not 201811L
+#else
+STATIC_ASSERT(__cpp_lib_assume_aligned == 201811L);
+#endif
+#else
+#ifdef __cpp_lib_assume_aligned
+#error __cpp_lib_assume_aligned is defined
+#endif
+#endif
+
 #if _HAS_CXX20
 #ifndef __cpp_lib_atomic_flag_test
 #error __cpp_lib_atomic_flag_test is not defined