[IR] Avoid UB in SymbolTableListTraits (llvm#139096)

This patch fixes the "dereferencing null" UB. Unfortunately, C++ doesn't
provide an inverse operation for `p->*pmf`.
See also llvm#130952.

Author: dtcxzyw

llvm/include/llvm/IR/BasicBlock.h

@@ -546,6 +546,10 @@ class BasicBlock final : public Value, // Basic blocks are data objects also
     return &BasicBlock::InstList;
   }
 
+  static size_t getSublistOffset(Instruction *) {
+    return offsetof(BasicBlock, InstList);
+  }
+
   /// Dedicated function for splicing debug-info: when we have an empty
   /// splice (i.e. zero instructions), the caller may still intend any
   /// debug-info in between the two "positions" to be spliced.

llvm/include/llvm/IR/Function.h

@@ -811,6 +811,10 @@ class LLVM_ABI Function : public GlobalObject, public ilist_node<Function> {
     return &Function::BasicBlocks;
   }
 
+  static size_t getSublistOffset(BasicBlock *) {
+    return offsetof(Function, BasicBlocks);
+  }
+
 public:
   const BasicBlock       &getEntryBlock() const   { return front(); }
         BasicBlock       &getEntryBlock()         { return front(); }

llvm/include/llvm/IR/Module.h

@@ -608,6 +608,9 @@ class LLVM_ABI Module {
   static GlobalListType Module::*getSublistAccess(GlobalVariable*) {
     return &Module::GlobalList;
   }
+  static size_t getSublistOffset(GlobalVariable *) {
+    return offsetof(Module, GlobalList);
+  }
   friend class llvm::SymbolTableListTraits<llvm::GlobalVariable>;
 
 public:
@@ -618,6 +621,9 @@ class LLVM_ABI Module {
   static FunctionListType Module::*getSublistAccess(Function*) {
     return &Module::FunctionList;
   }
+  static size_t getSublistOffset(Function *) {
+    return offsetof(Module, FunctionList);
+  }
 
   /// Detach \p Alias from the list but don't delete it.
   void removeAlias(GlobalAlias *Alias) { AliasList.remove(Alias); }
@@ -657,6 +663,9 @@ class LLVM_ABI Module {
   static AliasListType Module::*getSublistAccess(GlobalAlias*) {
     return &Module::AliasList;
   }
+  static size_t getSublistOffset(GlobalAlias *) {
+    return offsetof(Module, AliasList);
+  }
   friend class llvm::SymbolTableListTraits<llvm::GlobalAlias>;
 
   /// Get the Module's list of ifuncs (constant).
@@ -667,6 +676,9 @@ class LLVM_ABI Module {
   static IFuncListType Module::*getSublistAccess(GlobalIFunc*) {
     return &Module::IFuncList;
   }
+  static size_t getSublistOffset(GlobalIFunc *) {
+    return offsetof(Module, IFuncList);
+  }
   friend class llvm::SymbolTableListTraits<llvm::GlobalIFunc>;
 
   /// Get the Module's list of named metadata (constant).

llvm/include/llvm/IR/SymbolTableListTraits.h

@@ -77,10 +77,8 @@ class SymbolTableListTraits : public ilist_alloc_traits<ValueSubClass> {
   /// getListOwner - Return the object that owns this list.  If this is a list
   /// of instructions, it returns the BasicBlock that owns them.
   ItemParentClass *getListOwner() {
-    size_t Offset = reinterpret_cast<size_t>(
-        &((ItemParentClass *)nullptr->*ItemParentClass::getSublistAccess(
-                                           static_cast<ValueSubClass *>(
-                                               nullptr))));
+    size_t Offset = ItemParentClass::getSublistOffset(
+        static_cast<ValueSubClass *>(nullptr));
     ListTy *Anchor = static_cast<ListTy *>(this);
     return reinterpret_cast<ItemParentClass*>(reinterpret_cast<char*>(Anchor)-
                                               Offset);