[libcxx-commits] [libcxx] [libc++] Properly implement array cookies in the ARM ABI (PR #160182)

[Louis Dionne via libcxx-commits]

================
@@ -26,28 +27,103 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // Trait representing whether a type requires an array cookie at the start of its allocation when
 // allocated as `new T[n]` and deallocated as `delete[] array`.
 //
-// Under the Itanium C++ ABI [1], we know that an array cookie is available unless `T` is trivially
-// destructible and the call to `operator delete[]` is not a sized operator delete. Under ABIs other
-// than the Itanium ABI, we assume there are no array cookies.
+// Under the Itanium C++ ABI [1] and the ARM ABI which derives from it, we know that an array cookie is available
+// unless `T` is trivially destructible and the call to `operator delete[]` is not a sized operator delete. Under
+// other ABIs, we assume there are no array cookies.
 //
 // [1]: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#array-cookies
-#ifdef _LIBCPP_ABI_ITANIUM
+#if defined(_LIBCPP_ABI_ITANIUM) || defined(_LIBCPP_ABI_ITANIUM_WITH_ARM_DIFFERENCES)
 // TODO: Use a builtin instead
-// TODO: We should factor in the choice of the usual deallocation function in this determination.
+// TODO: We should factor in the choice of the usual deallocation function in this determination:
+//       a cookie may be available in more cases but we ignore those for now.
 template <class _Tp>
 struct __has_array_cookie : _Not<is_trivially_destructible<_Tp> > {};
 #else
 template <class _Tp>
 struct __has_array_cookie : false_type {};
 #endif
 
+template <class _Tp, bool _HasPadding = (_LIBCPP_PREFERRED_ALIGNOF(_Tp) > sizeof(size_t))>
+struct [[__gnu__::__aligned__(_LIBCPP_PREFERRED_ALIGNOF(_Tp))]] __itanium_array_cookie {
+  size_t __element_count;
+};
+
+template <class _Tp>
+struct [[__gnu__::__aligned__(_LIBCPP_PREFERRED_ALIGNOF(_Tp))]] __itanium_array_cookie<_Tp, /* _HasPadding */ true> {
+  char __padding[_LIBCPP_PREFERRED_ALIGNOF(_Tp) - sizeof(size_t)];
+  size_t __element_count;
+};
+
+template <class _Tp>
+struct [[__gnu__::__aligned__(_LIBCPP_ALIGNOF(_Tp))]] __arm_array_cookie {
+  size_t __element_size;
+  size_t __element_count;
+};
+
+// Return the element count in the array cookie located before the given pointer.
+//
+// In the Itanium ABI [1]
+// ----------------------
+// The element count is stored immediately before the first element of the array. If the preferred alignment
+// of array elements (which is different from the ABI alignment) is more than that of size_t, additional
+// padding bytes exist at the beginning of the array cookie. Assuming array elements of size and alignment 16
----------------
ldionne wrote:

```suggestion
// padding bytes exist before the array cookie. Assuming array elements of size and alignment 16
```

https://github.com/llvm/llvm-project/pull/160182