[libcxx-commits] [PATCH] D77606: [libcxxabi] Add macro for changing functions to support the relative vtables ABI

[Leonard Chan via Phabricator via libcxx-commits]

leonardchan updated this revision to Diff 307467.
leonardchan marked an inline comment as done.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D77606/new/

https://reviews.llvm.org/D77606

Files:
  libcxxabi/src/private_typeinfo.cpp


Index: libcxxabi/src/private_typeinfo.cpp
===================================================================
--- libcxxabi/src/private_typeinfo.cpp
+++ libcxxabi/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 +625,28 @@
     // Possible future optimization:  Take advantage of src2dst_offset
 
     // Get (dynamic_ptr, dynamic_type) from static_ptr
-    void **vtable = *static_cast<void ** const *>(static_ptr);
-    ptrdiff_t offset_to_derived = reinterpret_cast<ptrdiff_t>(vtable[-2]);
+#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;
-    const __class_type_info* dynamic_type = static_cast<const __class_type_info*>(vtable[-1]);
+
+    // 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 +1295,7 @@
     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 +1315,7 @@
     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,


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