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[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
This commit is contained in:
Leonard Chan 2020-04-06 16:08:58 -07:00
parent e425d0b92a
commit 61aec69a65
1 changed files with 29 additions and 3 deletions
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32
libcxxabi/src/private_typeinfo.cpp
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@ -61,6 +61,16 @@ is_equal(const std::type_info* x, const std::type_info* y, bool use_strcmp)
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 @@ __base_class_type_info::has_unambiguous_public_base(__dynamic_cast_info* info,
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 @@ __dynamic_cast(const void *static_ptr, const __class_type_info *static_type,
// 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 @@ __base_class_type_info::search_above_dst(__dynamic_cast_info* info,
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 @@ __base_class_type_info::search_below_dst(__dynamic_cast_info* info,
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,