[PATCH] D96035: [dsymutil][DWARFlinker] implement separate multi-thread processing for compile units.

[Alexey Lapshin via Phabricator via llvm-commits]

avl added a comment.

> ! In D96035#3232592 <https://reviews.llvm.org/D96035#3232592>, @dblaikie wrote:
>
> This doesn't address the case where struct `A` is a local type (in an anonymous namespace). Moving it out of the translation/compile unit it's defined it will break the representation (because the name is only valid within that compile unit - so you might end up pulling two types with the same name into the type table unit, making it hard for a DWARF consumer to do correct name lookup/know which version of the type the user is talking about in a given context).

This patch put such types into the type table separately. It creates two different namespaces with different DW_AT_decl_file attribute:

  DW_TAG_compile_unit
    DW_AT_name "__type_table"
    
    DW_TAG_namespace
      DW_AT_decl_file "compile_unit1.cpp"
      
      DW_TAG_class_type
        DW_AT_name "B"
      NULL
    NULL
  
    DW_TAG_namespace
      DW_AT_decl_file "compile_unit2.cpp"
      
      DW_TAG_class_type
        DW_AT_name "B"
      NULL
    NULL
  
  NULL



> What we could do is a third option - not duplicating the whole definition of A, but only part of the definition.
>
> Take a look at the DWARF Clang produces for something like this, for instance:
>
>   struct t1 {
>     virtual void f1(); // home this type into another translation unit
>     template<typename T>
>     void f1() { }
>   };
>   namespace {
>   struct t2 { };
>   }
>   int main() {
>     t1().f1<t2>();
>   }
>
> The DWARF for this will include a declaration of `t1` (so, some duplication - but not all the members, etc) with a declaration of the `f1<t2>` member in that `t1` declaration (& then an out of line definition that refers to that declaration) (in this case we could potentially also include a `DW_AT_specification` in the declaration that points to the definition in the type table, if that helps consumers significantly - so they don't have to do name lookup to figure out that they're the same thing).

yes. I assumed the similar approach here - https://reviews.llvm.org/D96035#3222090 (part about types splitting).

If type splitting were supported then this approach allows leaving anonymous types inside the compilation unit at the cost of little duplication and without unnecessary inter-CU references. It also allows decreasing the overall size of "type table". The current patch does not support type splitting. I suggest implementing it as an additional improvement.

Types splitting is necessary because originally generated "declaration of t1 (so, some duplication - but not all the members, etc) with a declaration of the f1<t2> member in that t1 declaration" might contain other members. So we need to move these other members into the type table and leave "declaration of the f1<t2> member" in place. Otherwise, (because other members will stay not deduplicated) the size of resulting DWARF would be bigger.

> Speaking of all that - how are member function definitions working in this current proposal? Do they use sec_offsets to refer to the declaration in the type table, or do they already do something like ^ with a type declaration in the same unit to house the member function declaration?

All member function definitions are merged into the type which is in type table. Later they are referenced(using DW_FORM_ref_addr) from compile units:

  DW_TAG_compile_unit
    DW_AT_name "__type_table"
  
    DW_TAG_structure_type
      DW_AT_name "t1"
        
      DW_TAG_subprogram
        DW_AT_name "t1"
      NULL
  
      DW_TAG_subprogram
        DW_AT_name "f1"
        
        DW_TAG_template_type_parameter
          DW_AT_type "__type_table::main.cpp::t2"  // I used names instead offsets, in fact this is DW_FORM_ref_addr referencing into "__type_table"
      NULL
    NULL
    
    DW_TAG_namespace
      DW_AT_decl_file "main.cpp"
      
      DW_TAG_struct_type
        DW_AT_name "t2"
      NULL
    NULL
  
  NULL
  
  DW_TAG_compile_unit
    DW_AT_name "main.cpp"
    
    DW_TAG_subprogram
      DW_AT_name "main"
    NULL
  
    DW_TAG_subprogram
      DW_AT_name "t1"
      DW_AT_specification "__type_table::t1::t1" // I used names instead offsets, in fact this is DW_FORM_ref_addr referencing into "__type_table"
    NULL
  
    DW_TAG_subprogram
      DW_AT_name "f1"
      DW_AT_specification "__type_table::t1::f1" // I used names instead offsets, in fact this is DW_FORM_ref_addr referencing into "__type_table"
    NULL
  
  NULL


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