[libcxxabi] Add macro for changing functions to support the relative vtables ABI
Under the relative vtables ABI, __dynamic_cast will not work since it assumes
the vtable pointer is 2 ptrdiff_ts away from the start of the vtable (8-byte
offset to top + 8-byte pointer to typeinfo) when it is actually 8 bytes away
(4-byte offset to top + 4-byte offset to typeinfo). This adjusts the logic under
__dynamic_cast and other areas vtable calculations are done to support this ABI
when it's used.
Differential Revision: https://reviews.llvm.org/D77606
GitOrigin-RevId: 61aec69a65dec949f3d2556c4d0efaa87869e1ee
diff --git a/src/private_typeinfo.cpp b/src/private_typeinfo.cpp
index 3e8bdae..c77ad66 100644
--- a/src/private_typeinfo.cpp
+++ b/src/private_typeinfo.cpp
@@ -61,6 +61,16 @@
return x == y || strcmp(x->name(), y->name()) == 0;
}
+static inline ptrdiff_t update_offset_to_base(const char* vtable,
+ ptrdiff_t offset_to_base) {
+#if __has_feature(cxx_abi_relative_vtable)
+ // VTable components are 32 bits in the relative vtables ABI.
+ return *reinterpret_cast<const int32_t*>(vtable + offset_to_base);
+#else
+ return *reinterpret_cast<const ptrdiff_t*>(vtable + offset_to_base);
+#endif
+}
+
namespace __cxxabiv1
{
@@ -297,7 +307,7 @@
if (__offset_flags & __virtual_mask)
{
const char* vtable = *static_cast<const char*const*>(adjustedPtr);
- offset_to_base = *reinterpret_cast<const ptrdiff_t*>(vtable + offset_to_base);
+ offset_to_base = update_offset_to_base(vtable, offset_to_base);
}
}
__base_type->has_unambiguous_public_base(
@@ -615,10 +625,26 @@
// Possible future optimization: Take advantage of src2dst_offset
// Get (dynamic_ptr, dynamic_type) from static_ptr
+#if __has_feature(cxx_abi_relative_vtable)
+ // The vtable address will point to the first virtual function, which is 8
+ // bytes after the start of the vtable (4 for the offset from top + 4 for the typeinfo component).
+ const int32_t* vtable =
+ *reinterpret_cast<const int32_t* const*>(static_ptr);
+ int32_t offset_to_derived = vtable[-2];
+ const void* dynamic_ptr = static_cast<const char*>(static_ptr) + offset_to_derived;
+
+ // The typeinfo component is now a relative offset to a proxy.
+ int32_t offset_to_ti_proxy = vtable[-1];
+ const uint8_t* ptr_to_ti_proxy =
+ reinterpret_cast<const uint8_t*>(vtable) + offset_to_ti_proxy;
+ const __class_type_info* dynamic_type =
+ *(reinterpret_cast<const __class_type_info* const*>(ptr_to_ti_proxy));
+#else
void **vtable = *static_cast<void ** const *>(static_ptr);
ptrdiff_t offset_to_derived = reinterpret_cast<ptrdiff_t>(vtable[-2]);
const void* dynamic_ptr = static_cast<const char*>(static_ptr) + offset_to_derived;
const __class_type_info* dynamic_type = static_cast<const __class_type_info*>(vtable[-1]);
+#endif
// Initialize answer to nullptr. This will be changed from the search
// results if a non-null answer is found. Regardless, this is what will
@@ -1267,7 +1293,7 @@
if (__offset_flags & __virtual_mask)
{
const char* vtable = *static_cast<const char*const*>(current_ptr);
- offset_to_base = *reinterpret_cast<const ptrdiff_t*>(vtable + offset_to_base);
+ offset_to_base = update_offset_to_base(vtable, offset_to_base);
}
__base_type->search_above_dst(info, dst_ptr,
static_cast<const char*>(current_ptr) + offset_to_base,
@@ -1287,7 +1313,7 @@
if (__offset_flags & __virtual_mask)
{
const char* vtable = *static_cast<const char*const*>(current_ptr);
- offset_to_base = *reinterpret_cast<const ptrdiff_t*>(vtable + offset_to_base);
+ offset_to_base = update_offset_to_base(vtable, offset_to_base);
}
__base_type->search_below_dst(info,
static_cast<const char*>(current_ptr) + offset_to_base,
