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DWARF

InputOutputAlias
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Description​

The DWARF format parses DWARF debug symbols from an ELF file (executable, library, or object file). It is similar to dwarfdump, but much faster (hundreds of MB/s) and supporting SQL. It produces one row for each Debug Information Entry (DIE) in the .debug_info section and includes "null"-entries that the DWARF encoding uses to terminate lists of children in the tree.

Info

.debug_info consists of units, which correspond to compilation units:

  • Each unit is a tree of DIEs, with a compile_unit DIE as its root.
  • Each DIE has a tag and a list of attributes.
  • Each attribute has a name and a value (and also a form, which specifies how the value is encoded).

The DIEs represent things from the source code, and their tag tells you what kind of thing it is. For example, there are:

  • functions (tag = subprogram)
  • classes/structs/enums (class_type/structure_type/enumeration_type)
  • variables (variable)
  • function arguments (formal_parameter).

The tree structure mirrors the corresponding source code. For example, a class_type DIE can contain subprogram DIEs representing methods of the class.

The DWARF format outputs the following columns:

  • offset - position of the DIE in the .debug_info section
  • size - number of bytes in the encoded DIE (including attributes)
  • tag - type of the DIE; the conventional "DW_TAG_" prefix is omitted
  • unit_name - name of the compilation unit containing this DIE
  • unit_offset - position of the compilation unit containing this DIE in the .debug_info section
  • ancestor_tags - array of tags of the ancestors of the current DIE in the tree, in order from innermost to outermost
  • ancestor_offsets - offsets of ancestors, parallel to ancestor_tags
  • a few common attributes duplicated from the attributes array for convenience:
    • name
    • linkage_name - mangled fully qualified name; typically only functions have it (but not all functions)
    • decl_file - name of the source code file where this entity was declared
    • decl_line - line number in the source code where this entity was declared
  • parallel arrays describing attributes:
    • attr_name - name of the attribute; the conventional "DW_AT_" prefix is omitted
    • attr_form - how the attribute is encoded and interpreted; the conventional DW_FORM_ prefix is omitted
    • attr_int - integer value of the attribute; 0 if the attribute doesn't have a numeric value
    • attr_str - string value of the attribute; empty if the attribute doesn't have a string value

Example Usage​

The DWARF format can be used to find compilation units that have the most function definitions (including template instantiations and functions from included header files):

Query
SELECT
    unit_name,
    count() AS c
FROM file('programs/clickhouse', DWARF)
WHERE tag = 'subprogram' AND NOT has(attr_name, 'declaration')
GROUP BY unit_name
ORDER BY c DESC
LIMIT 3
Response
┌─unit_name──────────────────────────────────────────────────┬─────c─┐
│ ./src/Core/Settings.cpp                                    │ 28939 │
│ ./src/AggregateFunctions/AggregateFunctionSumMap.cpp       │ 23327 │
│ ./src/AggregateFunctions/AggregateFunctionUniqCombined.cpp │ 22649 │
└────────────────────────────────────────────────────────────┴───────┘

3 rows in set. Elapsed: 1.487 sec. Processed 139.76 million rows, 1.12 GB (93.97 million rows/s., 752.77 MB/s.)
Peak memory usage: 271.92 MiB.

Format Settings​