[Dwarf-discuss] Proposal: Add Local and Indirect Strings to Name Index

[Cary Coutant]

Over on the generic-abi mailing list, Fang-Rui Song recently proposed a new
compressed relocation format for ELF, motivated in part by the number of
relocations for string in the .debug_names tables. Regardless of the
resolution of that proposal, some simple changes to the DWARF spec could
help eliminate a substantial number of those relocations. In fact, parts of
the split DWARF series of proposals in DWARF 5 were all about minimizing
the quantity of relocations (e.g., DW_FORM_addrx and DW_FORM_strx).

This proposal adds an indirect string form to the name table (for use when
a non-split DWARF compilation is already using .debug_str_offsets), and a
local string pool in the name table (for use in a split-DWARF compilation).

-cary
 Add Local and Indirect Strings to Name IndexBackground

The name table in the .debug_names section currently references strings via
offsets to the .debug_str section. Each string offset, therefore, requires
a relocation unless some under-the-table agreement is made between the
compiler and linker. This can get expensive, as the number of names can be
quite large.

For non-split DWARF, the use of .debug_str makes sense, as many or most of
the names in the name index are also referenced from entries in the
.debug_info section. If the .debug_info section is using DW_FORM_strx and a
.debug_str_offsets section, however, it would also be beneficial for the
name index to use string references in the style ofDW_FORM_strx, which
would eliminate the need for any relocations in the name table.

For split DWARF, there is little overlap between the strings referenced by
the skeleton .debug_info section and those referenced by the
.debug_names section,
and there is no advantage gained by storing the strings in the separate
section.
Overview

This proposal adds two alternative string representations to the name
index: one in the style of DW_FORM_strx for non-split DWARF compilation
units that use .debug_str_offsets, and one with a local string table stored
directly in the .debug_namessection that requires no relocations.
Proposed Changes

In Section 6.1.1.2 Structure of the Name Index, change “eight individual
parts” to “nine individual parts,” and add the following to the enumerated
list of parts between items 6 and 7:


   1. An optional local string pool.

(Renumber the last two items.)

In Figure 6.1 Name Index Layout (part 1), under “Name Index”, add a box for
“Local String Pool” between “Name Table” and “Abbrev Table”. The box
expands to a box on the right labeled “Strings”.

Also in Figure 6.1 Name Index Layout (part 1), change “String Offsets” to
“String Pointers or Indexes” in the expansion of “Name Table”.

In Section 6.1.1.4.1 Section Header, replace field 3 (“padding”) with the
following:


   1.

   str_format (ubyte)

   An enumerated constant that specifies the representation of string
   references in the name index. The possible values are: DW_FORM_strp,
   DW_FORM_strp8, and DW_FORM_strx4 (see Section 7.5.5 Classes and Forms).
   2.

   padding (ubyte)

   Reserved to DWARF (must be zero).

Renumber fields 4-8, and add the following fields after “name_count”:


   1.

   local_str_pool_size (uword)

   Size of the local string pool. If this value is non-zero, string offsets
   (when str_format is DW_FORM_strp or DW_FORM_strp8) reference the local
   string pool. If this value is 0, string offsets reference the .debug_str
   section. If str_format is DW_FORM_strx4, this field should be 0.
   2.

   str_offsets (section offset)

   A 4-byte or 8-byte unsigned offset that points to the header of the
   compilation unit’s contribution to the .debug_str_offsets section.
   Indirect string references (when str_format is DW_FORM_strx4) are
   interpreted as zero-based indexes into the array of offsets following the
   header. If str_format is DW_FORM_strp or DW_FORM_strp8, this field
   should be 0.

In Section 6.1.1.4.6 Name Table, replace the first paragraph with the
following:

The name table immediately follows the hash lookup table. It consists of
two arrays: an array of string pointers or indexes, followed immediately by
an array of entry offsets. The items in the first array are determined by
the str_format field in the section header, and may be 4-byte or 8-byte
offsets into either the .debug_str section or the local string pool, or
4-byte indexes into the array of offsets in the .debug_str_offsets section.
The items in the second array are section offsets: 4-byte unsigned integers
for the DWARF-32 format or 8-byte unsigned integers for the DWARF-64
format. The entry offsets in the second array refer to index entries, and
are relative to the start of the entry pool area.

Following Section 6.1.1.4.6, add a new section:

Section 6.1.1.4.7 Local String Pool

The local string pool, if present, immediately follows the name table. It
consists of a series of null-terminated strings. Its size is given by
local_str_pool_size.

[non-normative] *For non-split DWARF compilation units, strings used by the
name table will have significant overlap with strings used by
the .debug_info section, and a local string pool is not advisable.
Relocations for the string references may be minimized by using the
indirect string forms in both .debug_info and .debug_names. For split DWARF
compilation units, there is likely little overlap, and relocations for
string references in the name table can be minimized by using the local
string pool.*

In Section 6.1.1.4.8 Abbreviations Table (was 6.1.1.4.7), change the first
sentence to:

The abbreviations table immediately follows the local string pool (or, if
the local string pool is absent, the name table).

In Appendix B, Figure B.1, add an arc from .debug_names to .debug_str and to
 .debug_str_offsets. (There should have already been an arc to .debug_str in
the DWARF 5 spec.)

In Figure B.2, add similar arcs.
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