Issue 141212.1: DW_AT_export_symbols

Michael Eager

Proposal 
========

Add DW_AT_export_symbols attribute to indicate that the symbols
defined within the current scope are to be exported into the enclosing
scope.

This proposal is an alternate to 141020.1.  

Background
==========

C++11 adds a new use for the keword "inline" as a modifier for
namespace, which exports the symbols defined within the namespace
into the enclosing scope.  From the proposed extension
Section 7.3.1:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf

  "Members of an inline namespace can be used in most respects as
  though they were members of the enclosing namespace."

This is similar to the use of anonymous structs in C/C++, where symbols 
within the interior struct are referenced as if they were defined 
in the enclosing structure.  

Example -- C++ inline namespace

namespace A {
  inline namespace B {   // (1) inline namespace
    int a;
  }
}

void foo (void)
{
  using A::B::a;
  a = 1;
}

void bar (void)
{
  using A::a;
  a = 2;
}

<1> DW_TAG_namespace
      DW_AT_name        : A        
  <2> DW_TAG_namespace
        DW_AT_name        : B        
        DW_AT_export_symbols     
    <3> DW_TAG_variable)
          DW_AT_name        : a        

In this example, 'a' may be referenced either as a member of 
the fully qualified namespace A::B, or as if it were defined
in the enclosing namespace, A.

Example -- C/C++ anonymous structs

struct foo {
  int a;
  struct {
    int b;
  };
} x;

void bar(void)
{
  struct foo t;
  t.a = 1;
  t.b = 2;
}

<1> DW_TAG_structure_type  
      DW_AT_name (foo)
  <2> DW_TAG_member
        DW_AT_name (a)
  <3> DW_TAG_structure_type
        DW_AT_export_symbols
    <4> DW_TAG_member
          DW_AT_name (b)

In this example, 'b' is referenced as if it were defined in the 
enclosing struct 'foo'. 

Note:  GCC currently generates the following DWARF, with the nested
anonymous struct described as being at top-level, rather than within
struct foo, and with an unnamed member within struct foo with the
type of the anonymous struct.

<1> DW_TAG_structure_type
  <2> DW_TAG_member
        DW_AT_name (b)
<3> DW_TAG_structure_type
      DW_AT_name (foo)
  <4> DW_TAG_member
        DW_AT_name (a)
  <5> DW_TAG_member
        DW_AT_type <1>   

Document changes
================

Add to Table 2.2 (Attribute names)

  DW_AT_export_symbols  Namespace Entries
            Structure, Union and Class Type Entries

Add to end of 3.2.2 (Namespace Entries)

  A namespace may have a DW_AT_export_symbols attribute which indicates
  that all member names defined within the namespace may be referenced 
  as if they were defined within the containing namespace. 
  *This may be used to describe inline namespaces in C++.*

Add to end of 5.7.1 (Structure, Union and Class Type Entries)

  A structure, union, or class type may have a DW_AT_export_symbols
  attribute which indicates that all member names defined within 
  the structure, union, or class may be referenced as if they were
  defined within the containing structure, union, or class. 
  *This may be used to describe anonymous structs, etc., in C or C++.*

Add to Table 7.5 (Attribute encodings)

  DW_AT_export_symbols  0x89  flag

Modify Table A.1
  DW_TAG_class_type     -- Add DW_AT_export_symbols
  DW_TAG_namespace      -- Add DW_AT_export_symbols
  DW_TAG_structure_type -- Add DW_AT_export_symbols
  DW_TAG_union_type     -- Add DW_AT_export_symbols

Author:	Michael Eager
Champion:	Michael Eager
Date submitted:	2014-12-12
Date revised:
Date closed:
Type:	Enhancement
Status:	Accepted
DWARF Version:	5