[lldb] Consider OP_addrx in DWARFExpression::Update_DW_OP_addr

[fdeazeve]

Summary

This uses rewrites DW_OP_addrx as a DW_OP_addr so that we can update addresses
that are originally located in the debug_addr section.

This is posted here for analysis in the context of https://discourse.llvm.org/t/dwarfexpression-and-dw-op-addrx/71627

[fdeazeve]

Now that we have agreement on the discourse thread, I'll close this, polish it, and post again with tests

lldb/source/Expression/DWARFExpression.cpp

if (op == DW_OP_addr) {
       // Replace the address in the new buffer
       if (encoder.PutAddress(offset, file_addr) == UINT32_MAX)
         return false;
 
       // All went well, so now we can reset the data using a shared pointer to
       // the heap data so "m_data" will now correctly manage the heap data.
       m_data.SetData(encoder.GetDataBuffer());
       return true;
-    } else {
-      const offset_t op_arg_size =
-          GetOpcodeDataSize(m_data, offset, op, dwarf_cu);
-      if (op_arg_size == LLDB_INVALID_OFFSET)
-        break;
-      offset += op_arg_size;
-    }
-  }
+    }
+    if (op == DW_OP_addrx) {
+      // Replace DW_OP_addrx with DW_OP_addr, since we can't modify the
+      // read-only debug_addr table.
+      llvm::ArrayRef data_before_op = m_data.GetData().take_front(offset - 1);
+
+      // Read out the old index to determine how many bytes it needs.
+      const lldb::offset_t old_offset = offset;
+      m_data.GetULEB128(&offset);
+      if (old_offset == offset)
+        return false;
+      llvm::ArrayRef data_after_op = m_data.GetData().drop_front(offset);
+
+      DataEncoder encoder(m_data.GetByteOrder(), m_data.GetAddressByteSize());
+      encoder.AppendData(data_before_op);
+      encoder.AppendU8(DW_OP_addr);
+      encoder.AppendAddress(file_addr);
+      encoder.AppendData(data_after_op);
+      m_data.SetData(encoder.GetDataBuffer());
+      return true;
+    }
   return false;
 }
 
 bool DWARFExpression::ContainsThreadLocalStorage(
     const DWARFUnit *dwarf_cu) const {
   lldb::offset_t offset = 0;
   while (m_data.ValidOffset(offset)) {
     const uint8_t op = m_data.GetU8(&offset);