Define __GCC_CONSTRUCTIVE/DESTRUCTIVE_SIZE on macOS [GH #319]

bases/cmake-native/macos-arm/clang/lib/predefined_macros.txt

@@ -186,6 +186,8 @@ clang no  __GCC_ATOMIC_POINTER_LOCK_FREE 2
 clang no  __GCC_ATOMIC_SHORT_LOCK_FREE 2
 clang no  __GCC_ATOMIC_TEST_AND_SET_TRUEVAL 1
 clang no  __GCC_ATOMIC_WCHAR_T_LOCK_FREE 2
+clang no  __GCC_CONSTRUCTIVE_SIZE 64
+clang no  __GCC_DESTRUCTIVE_SIZE 256
 clang no  __GCC_HAVE_DWARF2_CFI_ASM 1
 clang no  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
 clang no  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1

bases/cmake-native/macos-arm/clang/lib_macos_arm64/predefined_macros.txt

@@ -186,6 +186,8 @@ clang no  __GCC_ATOMIC_POINTER_LOCK_FREE 2
 clang no  __GCC_ATOMIC_SHORT_LOCK_FREE 2
 clang no  __GCC_ATOMIC_TEST_AND_SET_TRUEVAL 1
 clang no  __GCC_ATOMIC_WCHAR_T_LOCK_FREE 2
+clang no  __GCC_CONSTRUCTIVE_SIZE 64
+clang no  __GCC_DESTRUCTIVE_SIZE 256
 clang no  __GCC_HAVE_DWARF2_CFI_ASM 1
 clang no  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
 clang no  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1

bases/cmake-native/macos-x86_64/clang/lib/predefined_macros.txt

@@ -137,6 +137,8 @@ clang no  __GCC_ATOMIC_POINTER_LOCK_FREE 2
 clang no  __GCC_ATOMIC_SHORT_LOCK_FREE 2
 clang no  __GCC_ATOMIC_TEST_AND_SET_TRUEVAL 1
 clang no  __GCC_ATOMIC_WCHAR_T_LOCK_FREE 2
+clang no  __GCC_CONSTRUCTIVE_SIZE 64
+clang no  __GCC_DESTRUCTIVE_SIZE 64
 clang no  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 1
 clang no  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
 clang no  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1

bases/cmake-native/macos-x86_64/clang/lib_macos_x86_64/predefined_macros.txt

@@ -137,6 +137,8 @@ clang no  __GCC_ATOMIC_POINTER_LOCK_FREE 2
 clang no  __GCC_ATOMIC_SHORT_LOCK_FREE 2
 clang no  __GCC_ATOMIC_TEST_AND_SET_TRUEVAL 1
 clang no  __GCC_ATOMIC_WCHAR_T_LOCK_FREE 2
+clang no  __GCC_CONSTRUCTIVE_SIZE 64
+clang no  __GCC_DESTRUCTIVE_SIZE 64
 clang no  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 1
 clang no  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
 clang no  __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1

tests/tests/changes/.GH319.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --clang --c++17 Test_name.c

tests/tests/changes/GH319.sft.cpp

@@ -0,0 +1,49 @@
+//type:fp
+//options_all:--clang --c++17
+//remark:[GH #319] The macOS clang bases define __GCC_CONSTRUCTIVE_SIZE and __GCC_DESTRUCTIVE_SIZE
+// 10/15/26  [GH #319]
+//
+// Clang 19 and later predefine __GCC_CONSTRUCTIVE_SIZE and
+// __GCC_DESTRUCTIVE_SIZE, and libc++ (<__new/interference_size.h>, reached by
+// <new>, <string>, <vector>, <algorithm>, ...) uses them unconditionally from
+// C++17 on to define std::hardware_constructive_interference_size and
+// std::hardware_destructive_interference_size.
+//
+// The values depend on the target (Apple clang 21 gives 64 and 256 for arm64,
+// 64 and 64 for x86_64), so the test checks that the macros are defined and
+// usable the way libc++ uses them, not their values.
+
+#if defined(__APPLE__) && defined(__clang__)
+
+#ifndef __GCC_DESTRUCTIVE_SIZE
+#error "__GCC_DESTRUCTIVE_SIZE is not defined"
+#endif
+#ifndef __GCC_CONSTRUCTIVE_SIZE
+#error "__GCC_CONSTRUCTIVE_SIZE is not defined"
+#endif
+
+typedef decltype(sizeof(0)) size_t;
+
+// As in libc++'s <__new/interference_size.h>.
+inline constexpr size_t hardware_destructive_interference_size =
+    __GCC_DESTRUCTIVE_SIZE;
+inline constexpr size_t hardware_constructive_interference_size =
+    __GCC_CONSTRUCTIVE_SIZE;
+
+// Both are positive powers of two, and the destructive size (the one that
+// avoids false sharing) is not smaller than the constructive one.
+static_assert(hardware_constructive_interference_size > 0 &&
+              (hardware_constructive_interference_size &
+               (hardware_constructive_interference_size - 1)) == 0,
+              "constructive size is a power of two");
+static_assert(hardware_destructive_interference_size > 0 &&
+              (hardware_destructive_interference_size &
+               (hardware_destructive_interference_size - 1)) == 0,
+              "destructive size is a power of two");
+static_assert(hardware_destructive_interference_size >=
+              hardware_constructive_interference_size,
+              "destructive size is not smaller than constructive size");
+
+#endif
+
+int main() { return 0; }