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081211.1 Ron Brender Allow DW_AT_bit_size like DW_AT_byte_size Enhancement Accepted Ron Brender


Revised Dec 21, 2008.

PROBLEM
-------

In general, the size of an object or type is described as a given
number of bytes or a given number of bits. A byte size is preferred
when the size is a integral number of whole bytes. A bit size is
required when the size is not a integral number of bytes.

DWARF DIEs for types allow use of the DW_AT_byte_size attribute for
specifing a size in integral bytes. But not all also allow use of the
DW_AT_bit_size as an alternative size specification.

This proposal corrects this lack of parallel and consistent treatment.


PROPOSAL
--------

This proposal has several parts.

Part 1: In Figure 2, change the description of DW_AT_bit_size from

    "Base type bit size" & "Data member bit size"
to

    "Data type size in bits"

Also change the description of DW_AT_byte_size from

    "Data object and data type size"
to

    "Data type size in bytes"

Note: DW_AT_byte_size (and DW_AT_bit_size) occur only as attributes
of types, not objects.


Part 2: Add new Section 2.21 as follows:

|   "Section 2.21 Bit and Byte Sizes
|
|   "Many debugging information entities allow either a DW_AT_byte_size
|   attribute or a DW_AT_bit_size attribute, whose integer constant
|   value (see Section 2.19) speficies an amount of storage. The value
|   of the DW_AT_byte_size attribute is interpreted in bytes and the
|   value of the DW_AT_bit_size attribute is interpreted in bits."


Part 3: In Section 5.5 regarding array types, replace paragraph 5 on
page 76 which reads

    "The array type entry may have a DW_AT_byte_size attribute, whose
    value (see Section 2.19) is the total size in bytes of an instance
    of the array type."

with

    "The array type entry may have either a DW_AT_byte_size or a
    DW_AT_bit_size attribute (see Section 2.21), whose value is the
    amount of storage needed to hold an instance of the array type."


Part 4: In Section 5.6.1 regarding structure, union or class types,
replace paragraph 3 on page 77 which reads

    "If the size of an instance of the structure type, union type or
    class type can be determined statically, the entry has a
    DW_AT_byte_size attribute whose integer constant value is the
    number of bytes required to hold an instance of the structure,
    union, or class, including any padding bytes. Otherwise, the
    entry may have a DW_AT_byte_size attribute whose value (see
    Section 2.19) is the dynamic number of bytes required."

with

    "A structure type, union type or class type entry may have
    either a DW_AT_byte_size attribute or a DW_AT_bit_size attribute
    (see Section 2.21), whose value is the amount of storage needed
    to hold an instance of the structure, union or class type,
    including any padding."


Part 5: In Section 5.8 regarding enumeration types, replace the
second sentence of the first paragraph on page 87 which reads

    "The entry also has a DW_AT_byte_size attribute whose integer
    constant value is the number of bytes required to hold an
    instance of the enumeration.

with

    <new paragraph>
    "An enumeration type entry has either a DW_AT_byte_size
    attribute or a DW_AT_bit_size attribute (see Section 2.21), whose
    value is the amount of storage needed to hold an instance of the
    enumeration type."


Part 6: In Section 5.10 regarding string types, replace the third
and fourth paragraphs which read

    "The string type entry may have a DW_AT_string_length attribute
    whose value is a location description where the length of the
    string is stored in the program. The string type may also have a
    DW_AT_byte_size attribute, whose value (see Section 2.19) is the
    size in bytes of the data to be retrieved from the location
    referenced by the string length attribute. If no byte size
    attribute is present, the size of the data to be retrieved is the
    same as the size of an address on the target machine.

    "If no string length attribute is present, the string type entry
    may have a DW_AT_byte_size attribute, whose value (see Section 2.19)
    is the length in bytes of the string."

with

    "The string type entry may have a DW_AT_string_length attribute
    whose value is a location description where the length of the
    string is stored in the program. The string type may also have
    either a DW_AT_byte_size attribute or a DW_AT_bit_size attribute
    (see Section 2.21), whose value is the size of the data to be
    retrieved from the location referenced by the string length
    attribute. If no (byte or bit) size attribute is present, the
    size of the data to be retrieved is the same as the size of an
    address on the target machine.

    "If no string length attribute is present, the string type entry
    may have either a DW_AT_byte_size attribute or a DW_AT_bit_size
    attribute (see Section 2.21), whose value is the length of the
    string."


Part 7: In Section 5.11 regarding set types, replace the last paragraph
of the section which reads

    "If the amount of storage allocated to hold each element of an
    object of the given set type is different from the amount of
    storage that is normally allocated to hold an individual object
    of the indicated element type, then the set type has a
    DW_AT_byte_size attribute whose value (see Section 2.19) is the
    size in bytes of an instance of the set type."

with

    "If the amount of storage allocated to hold each element of an
    object of the given set type is different from the amount of
    storage that is normally allocated to hold an individual object
    of the indicated element type, then the set type has either a
    DW_AT_byte_size attribute or a DW_AT_bit_size attribute (see
    Section 2.21), whose value is the amount of storage needed to hold
    an instance of the set type."


Part 8: In section 5.12 regarding subrange types, replace the fourth paragraph (page 89) which reads

    "If the amount of storage allocated to hold each element of the
    given subrange type is different from the amount of storage that
    is normally allocated to hold an individual object of the
    indicated element type, then the subrange has a DW_AT_byte_size
    attribute, whose value (see Section 2.19) is the size in bytes
    of each element of the subrange type."

with

    "If the amount of storage allocated to hold each element of the
    given subrange type is different from the amount of storage that
    is normally allocated to hold an individual object of the
    indicated element type, then the subrange has either a
    DW_AT_byte attribute or a DW_AT_bit_size attribute (see Section
    2.21), whose value is the amount of storage needed to hold an
    instance of the subrange type."


Part 9: In Section 5.14 regarding file types, replace the last
paragraph which reads

    "The file type entry also has a DW_AT_byte_size attribute, whose
    value (see Section 2.19) is the size in bytes of an instance of
    this file type."

with

    "The file type entry also has either a DW_AT_byte_size attribute
    or a DW_AT_bit_size attribute (see Section 2.21), whose value
    is the amount of storage needed to hold an instance of
    the file type."


Part 10: In Appendix A, Figure 42, add the DW_AT_bit_size attribute
to the list of Applicable Attributes for each of the following:

    DW_TAG_array_type
    DW_TAG_class_type
    DW_TAG_enumeration_type
    DW_TAG_file_type
    DW_TAG_set_type
    DW_TAG_string_type
    DW_TAG_structure_type
    DW_TAG_subrange_type
    DW_TAG_union_type


DISCUSSION
----------

This proposal attempts to cover all uses of DW_AT_byte_size in the
DWARF document. Places where changes are not proposed are the
following:

  5.1   Base Types              Covered by separate proposal
  5.2   Unspecified Types       n/a
  5.3   Modified types          n/a
  5.4   Typedef                 n/a
  5.6.6 Data Members            Covered by separate proposal
  5.7   Condition               n/a
  5.9   Subroutine              n/a
  5.13  Pointer to member       n/a


Changes to Draft of Dec 11, 2008:

  . Add new Section 2.21 to factor the common business about byte
    size is bytes and bit size is bits into a single place.
  . Include string types and file types for completeness and
    consistency
  . Drop suggestion to change wording regarding padding in structures,
    unions and classes.

---
Accepted Jan. 6, 2009


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