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DWARF5

Interpreting The Line Table File Numbers

Summary:

It is impossible to obey all the DWARF5 rules on file numbering in the line table at the same time. They contradict each other.

A specific example is Section 2.14 (page 50) and point 20 in the line table header (page 157).

One way to make consistency possible is to declare that things outside the file table use 1 origin numbering (DW_AT_decl_file, DW_AT_call_file,DW_MACRO_start_file) and to declare that all things *inside* the line table use zero-based file indexes. This seems messy and confusing: would a dumper like readelf or dwarfdump somehow print both meanings of a number?

A simpler way is to say that the file number value 0 meaning of \‘no file\’ is eliminated so that part of Section 2.14 page 50 is invalidated. So then we have a zero-based file array along with all references to it. And declare that the \‘file\’ line table register defaults to zero (DWARF5 Table 6.4 Page 153). Then the standard is self-consistent.

There was some discussion of this in the dwarf-discuss list in mid-February 2020 and a few people contributed their thoughts and observations.

Lines below are direct quotes from the standard except for [comment: wording]

DW_AT_decl_file

  Documented in Sec 2.14 Declaration Coordinates(page 50)\  

  The value of the DW_AT_decl_file attribute corresponds \   to a file number from the line number information table \   for the compilation unit containing the debugging information \   entry and represents the source file in which the\   declaration appeared (see Section 6.2 on page 148). \   The value 0 indicates that no source file has been specified.

DW_AT_call_file

  Documented in 3.3.8.2 Concrete Instances (page 83) as\   being just like DW_AT_decl_file.

  The call file, call line, and call column attributes are\   interpreted in the same way as the declaration file, \   declaration line, and declaration column attributes, respectively\   (see Section 2.14 on page 50).

DW_MACRO_start_file

  Sec 6.3.3.1 page 169\   The source file name index is the file number\   in the line number information table for the\   compilation unit.\   [comment: Nothing is said about 0 meaning no source file.]\   [comment: Likely intended to be like Section 2.14.]

DW_LNS_set_file

  Defined in sec 6.2.5.2 page 162.\   Stores the file value into the file register of\   the state machine.\   [comment: Nothing is said about 0 meaning no source file.]

line table header, pg 157. 20. file_names (sequence of file name entries)

  The first entry in the sequence is the primary source file\   whose file name exactly matches that given in the DW_AT_name\   attribute in the compilation unit debugging information\   entry.

  The line number program references file names in this\   sequence beginning with 0, and uses those numbers instead\   of file names in the line number program that follows.\   [comment: This contradicts DW_AT_decl_file\   and DW_AT_call_file file numbers.]

Table 6.3 state machine registers page 150. file

  An unsigned integer indicating the identity of\   the source file corresponding to a machine\   instruction. While not stated it is clear this\   refers to the line table header file table.\   [comment: Nothing is said about 0 meaning no source file.]

  The first entry in the sequence is the primary source\   file whose file name exactly matches that given in the\   DW_AT_name attribute in the compilation unit debugging\   information entry.

Table 6.4 initial conditions of state machine Page 153 file 1

  [comment: Nothing is said about 0 meaning no source file.]\   [comment: There is an assumption that the file\   table has at least one entry, and that first\   entry is numbered 1.]\   [ Comment: this is consistent with the above quotes\   about the state machine registers, but\   inconsistent with the standard as a whole..\   Change the default to zero in DWARF5]

Section 6.2.4 on file numbers in the header page 158

  The line number program references file names in this\   sequence beginning with 0, and uses those numbers instead\   of file names in the line number program that follows.

Analysis David Anderson

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