Issue 210713.1: Fix "file 0"

Author: Paul Robinson
Champion: Paul Robinson
Date submitted: 2021-07-13
Date revised:
Date closed: 2022-07-25
Type: Improvement
Status: Accepted
DWARF Version: 6
Section 2.14, 6.2.2, 6.2.4, pg various

In DWARF v5, we changed the line-table header so that the directory and
file tables started with index 0 (not 1) and specifically required that
directory 0 match the CU's DW_AT_comp_dir, and file 0 describe the main
source file, matching the CU's DW_AT_name.  Thus, file 0 necessarily 
points to directory 0, and the combination matches DW_AT_comp_dir + 

However, we missed making two related changes.  First, the initial state
for the "file" register of the line-table state machine remained 1, even
though it would make more sense for it to be 0; second, the description
of DW_AT_decl_file said that 0 meant "no source file."  (Note that prior
DWARF versions didn't specify any particular file number for the main
source file, and presumably we picked 1 because what else would you do.)

While it is technically possible to conform to both the line table's
requirement that file 0 mean the primary source file, and DW_AT_decl_file
not using file 0 (by duplicating an entry for the primary source file),
this is wasteful and inconsistent.

Also, non-normative text introduced in DWARF v5 suggested that file 0
had always meant the primary source file; that's actually not true.

This proposal fixes all the above problems.


Section 2.14 Declaration Coordinates

- p.50 lines 20-21, delete the sentence starting "The value 0..."

Section 6.2.2 State Machine Registers

- p.151, in the description of the "file" register, add:

    Files are numbered beginning at 0.

Table 6.4 Line number program initial state

- change the initial value for "file" from 1 to 0

Section 6.2.4 The Line Number Program Header

- p.158, lines 4-8, edit the non-normative paragraph starting "Prior to
  DWARF Version 5, ..." so that it reads as follows.

*Prior to DWARF Version 5, the current compilation file name did not have
a specific entry in the file_names field.  Starting in DWARF Version 5,
the current compilation file name has index 0.*
2022-07-25:  Accepted.