[lldb] Don't read firstSubclass and nextSiblingClass from class_rw_t

[JDevlieghere]

We're considering modifying the ObjC runtime's class_rw_t structure to remove the firstSubclass and nextSiblingClass fields in some cases. LLDB is currently reading those but not actually using them. Stop doing that to avoid issues if they are removed by the runtime.

rdar://166084122

[llvmbot]

@llvm/pr-subscribers-lldb

Author: Jonas Devlieghere (JDevlieghere)

Changes

We're considering modifying the ObjC runtime's class_rw_t structure to remove the firstSubclass and nextSiblingClass fields in some cases. LLDB is currently reading those but not actually using them. Stop doing that to avoid issues if they are removed by the runtime.

rdar://166084122

---

Full diff: https://github.com/llvm/llvm-project/pull/171213.diff

2 Files Affected:

• (modified) lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp (+1-4)
• (modified) lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.h (-3)

diff --git a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp
index 954f269f8860b..ebde8892d8f62 100644
--- a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp
+++ b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp
@@ -111,7 +111,6 @@ bool ClassDescriptorV2::class_rw_t::Read(Process *process, lldb::addr_t addr) {
                           process->GetAddressByteSize());
 
   lldb::offset_t cursor = 0;
-
   m_flags = extractor.GetU32_unchecked(&cursor);
   m_version = extractor.GetU32_unchecked(&cursor);
   m_ro_ptr = extractor.GetAddress_unchecked(&cursor);
@@ -119,18 +118,16 @@ bool ClassDescriptorV2::class_rw_t::Read(Process *process, lldb::addr_t addr) {
     m_ro_ptr = abi_sp->FixCodeAddress(m_ro_ptr);
   m_method_list_ptr = extractor.GetAddress_unchecked(&cursor);
   m_properties_ptr = extractor.GetAddress_unchecked(&cursor);
-  m_firstSubclass = extractor.GetAddress_unchecked(&cursor);
-  m_nextSiblingClass = extractor.GetAddress_unchecked(&cursor);
 
   if (m_ro_ptr & 1) {
     DataBufferHeap buffer(ptr_size, '\0');
     process->ReadMemory(m_ro_ptr ^ 1, buffer.GetBytes(), ptr_size, error);
     if (error.Fail())
       return false;
-    cursor = 0;
     DataExtractor extractor(buffer.GetBytes(), ptr_size,
                             process->GetByteOrder(),
                             process->GetAddressByteSize());
+    lldb::offset_t cursor = 0;
     m_ro_ptr = extractor.GetAddress_unchecked(&cursor);
     if (ABISP abi_sp = process->GetABI())
       m_ro_ptr = abi_sp->FixCodeAddress(m_ro_ptr);
diff --git a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.h b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.h
index 0fff9af438367..8d19b00f1551f 100644
--- a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.h
+++ b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.h
@@ -133,9 +133,6 @@ class ClassDescriptorV2 : public ObjCLanguageRuntime::ClassDescriptor {
     lldb::addr_t m_properties_ptr;
     lldb::addr_t m_protocols_ptr;
 
-    ObjCLanguageRuntime::ObjCISA m_firstSubclass;
-    ObjCLanguageRuntime::ObjCISA m_nextSiblingClass;
-
     bool Read(Process *process, lldb::addr_t addr);
   };