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220304.1 Tony Tye Allow MD5 hash to be optional in the line table Enhancement Open


Section 6.2.4.1, pg 159
Allow MD5 hash to be optional in the line table
===============================================

PROBLEM DESCRIPTION
-------------------

The producer of different translation units may generate different properties
for the files of a compilation unit in the line table. For example, the presence
of the file size, time stamp, and MD5 hash in the file entries of the line table
may be different in different translation units.

Link time optimizations (LTO) can result in code from multiple translation units
being mixed into a single compilation unit. This can result in different files
in a single compilation unit line table having different file properties
present.

The file size and timestamp are defined to have a distinguished value to
indicate they are not present. However, there is no value for the MD5 hash that
can be used to indicate it is not present. This forces a producer to drop the
MD5 hash of all files of a compilation unit if it is not available for all
files. To avoid this it is proposed to add a way to indicate if the MD5 file
property is present.

PROPOSED RESOLUTION 1
---------------------

The first proposed solution defines a new file property to explicitly indicate
that the DW_LNCT_MD5 file property should be ignored. This allows the
DW_LNCT_MD5 file property to express the full range of hash values when present.

This augments DWARF Version 5 section 6.2.4.1.

1.  DW_LNCT_is_MD5

    DW_LNCT_is_MD5 indicates if the DW_LNCT_MD5 content kind, if
    present, is valid: when 0 it is not valid and when 1 it is valid. If
    DW_LNCT_is_MD5 content kind is not present, and DW_LNCT_MD5
    content kind is present, then the MD5 checksum is valid.

    DW_LNCT_is_MD5 is always paired with the DW_FORM_udata form.

    This allows a compilation unit to have a mixture of files with and without
    MD5 checksums. This can happen when link time optimization (LTO) generates code
    for a translation unit that includes contributions from other translation units
    that have different information about the source files.

This augments DWARF Version 5 section 7.22 and Table 7.27.

The following table gives the encoding of the additional line number header
entry formats.

  Table 7.27: Line number header entry format encodings

  ==================================== ====================
  Line number header entry format name Value
  ==================================== ====================
  DW_LNCT_is_MD5                       0x7
  ==================================== ====================

PROPOSED RESOLUTION 2
---------------------

The second proposed solution defines a distinguished hash value to indicate the
absence of the property. This is the same approach taken for the
DW_LNCT_timestamp and DW_LNCT_size file properties. Using a hash of 0 bytes is
the same approach taken by the Mercurial source control system (see
https://www.mercurial-scm.org/wiki/Nodeid).

This modifies DWARF Version 5 section 6.2.4.1.

Change:

5.  DW_LNCT_MD5

    DW_LNCT_MD5 indicates that the value is a 16-byte MD5 digest of the file
    contents. It is paired with form DW_FORM_data16.

To:

5.  DW_LNCT_MD5

    DW_LNCT_MD5 indicates that the value is a 16-byte MD5 digest of the file
    contents, or 16 bytes of 0 if not available. It is paired with form
    DW_FORM_data16.




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