[Lldb-commits] [PATCH] D134849: [LLDB][NativePDB] Fix struct layout when it has anonymous unions.

[Pavel Labath via Phabricator via lldb-commits]

labath added a comment.

The fact that MSVC does not store all of the anonymous union data is unfortunate, though not entirely surprising, given that the goal of debug info never was to offer an exact reconstruction of the source AST.

That, I'm not sure if checking for matching initial offsets is sufficient to create a semblance of a consistent structure definition. What will this code do with structures like this:

  struct S3 {
    char x[3];
  };
  struct A {
    union {
      struct {
        char c1;
        S3 s1;
      };
      struct {
        S3 s2;
        char c2;
      };
    };
  } a;

In this case, `a.s1` and `a.c2` overlap, but they don't share the same starting offset. If the compiler does not provide data for anonymous structs as well, then I think this algorithm will need to be more complicated. I'm not even sure they can be reconstructed correctly, as the anonymous structs and unions can nest indefinitely. Maybe it would be sufficient to represent this as a "union of structs", where each struct gets (greedily) packed with as many members as it can hold, and we create as many structs as we need (we can replace the struct with a simple member if it would hold just one member)?



================
Comment at: lldb/source/Plugins/SymbolFile/NativePDB/UdtRecordCompleter.cpp:321
+  // the artificial anonymous union.
+  lldb::user_id_t au_id = LLDB_INVALID_UID - toOpaqueUid(m_id);
+  uint64_t au_field_bitoffset = 0;
----------------
How are these opaque ids computed? Will they produce unique IDs if you have two structs like this close together? Might it be better to just make a global (local to a symbol file or something) counter and always increment/decrement that when creating another type ?


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  https://reviews.llvm.org/D134849/new/

https://reviews.llvm.org/D134849