Issue 160714.1: Location List and Range List Sections Improvement/Enhancement
Section Many, pg Many
This a replacement for the proposal in Issue 150607.1. It is closely
related to and helps enable Proposal 160123.1 Revision 1.
This proposal results from review of proposal 150607.1 at the June
2016 meeting and subsequent discussion with Cary Coutant. However,
while it grows out of the same original concerns, it takes a different
approach and extends the treatment of location and range list sections
in new ways as suggested at that meeting. As a result it is being
submitted as a new proposal. The original proposal 150607.1 is withdrawn.
Note
The companion Proposal 160123.1 Revision 1 refers to Revision 1 of Issue
150607.1. THIS is that revision, although it now has a new Issue number.
Hopefully, this will not result in confusion.
This proposal replaces the .debug_loc and .debug_ranges sections
with new sections .debug_loclists and .debug_rnglists (and
.debug_loclists.dwo and .debug_rnglists.dwo). These sections
are designed to work with the new location list and range list
entry formats defined in 160123.1 Revision 1.
Proposal Sketch
1. Headers (of all four sections) consist of the
following fields
a) unit_length (initial_length)
b) version (uhalf) = 5
c) address_size (ubyte)
d) segment_selector_size (ubyte)
The above are the same as for .debug_loc/.debug_ranges in earlier
drafts for V5 (Note that .debug_loc/.debug_ranges did not have
headers in V4.) This proposal adds:
e) entry_offset_count (uword)
The number of offsets, one for each entry, that immediately
follow the header.
2. Immediately following the header are entry_offset_oount offsets.
Each offset is 4 bytes in size in the DWARF-32 format and 8 bytes
in the DWARF-64 format. The contents contained in the location of
the i-th offset is the offset from the base of the section (the
location of the first offset following the header) to the beginning
of the i-th entry.
3. Immediately following the entry offsets are the entries themselves.
4. Entries are referenced by their index using the (new) form
DW_FORM_secx whose value is a ULEB unsigned integer. Given the
operand value i, the location of the entry within the corresponding
section is determined as follows:
a) Let BASE be the location of the first offset following the
header. For a section in a split unit, this is the beginning
of the section plus the size of the header. For a section
in a non-split unit, this is the value given in the applicable
DW_AT_loclists_base or DW_AT_rnglists_base attribute.
b) Let SIZE be the size of an offset (4 in 32-bit and 8 in 64-bit
format). Note that these will (MUST) be the same in both the
referencing and referenced section.
c) Then the location of the i-th offset is given by BASE+i*SIZE.
The location of the i-th entry is given by BASE plus the contents
at that location, that is BASE + [BASE+i*SIZE].
Note that the entry_offset_count is not needed to interpret
form DW_FORM_secx. But it does provide a way for dumpers to determine
the boundary between the offsets and entries.
5. Form DW_FORM_secx is defined as the only member of the new loclistx
and rnglistx classes (instead of DW_FORM_sec_offset).
6. Rename class rangelistptr to rnglistsptr. Note that the only form
contained in class rnglistsptr remains DW_FORM_sec_offset.
Similarly, rename class loclistptr to loclistsptr. The only form
contained in class loclistsptr remains DW_FORM_sec_offset.
7. Rename DW_AT_ranges_base to DW_AT_rnglists_base for naming
consistency. The class remains rnglistsptr.
Note: DW_AT_ranges_base was new in V5 so there is no need to
preserve the code used for DW_AT_ranges_base to facilitate
backwards compatibility.
8. Add DW_AT_loclists_base which is analogous to DW_AT_rnglists_base
but for the .debug_loclists section. The value is of class
loclistsptr.
9. Change the class of the operand of all attributes that allow
loclistsptr except DW_AT_loclists_base to rnglistx.
10. Change the class of the operand of all attributes that allow
rnglistsptr except DW_AT_rnglists_base to rnglistx.
These changes (together with Proposal 160123.1 Revision 1), let us:
A. Extend the size benefits of the new location list representation
in split units to non-split units as well.
B. Extend the size benefits of location lists to range lists (in both
split and non-split contexts) in a manner consistent with location
lists.
C. Solve certain late optimization problems (both compilation and link-
time) related to referencing location and range list entries in a
simple consistent manner.
D. Get rid of the excessively complicated and contorted range
representation scheme used in V4 and before.
TEXTUAL CHANGES
---------------
Page numbers are from the May 20, 2016 draft.
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1. S7.28 & S7.29, P235 & P236 (parallel change in both):
Add bullet and new following text:
5. offet_entry_count (uword)
A 4-byte count of the number of offsets and entries
that follow the header.
Immediately following the header is an array of offsets.
This array is followed by a series of [range|location] list
entries.
There is one offset for each entry. The contents
of the location for the i-th offset is the offset from the
beginning of the offset array to the location of the i-th
list entry.
In the following paragraph (L1067 in S7.28, L1092 in S7.29): replace
"This header" with "The array of offsets".
Replace the last paragraph (L1072-1073 in 7.28, L1097-1098 in S7.29)
with
The [DW_AT_rnglists_base|DW_AT_loclists_base] attribute points
to the first offset following the header. The entries are referenced
by the index of the position of their corresponding offset in the
array of offsets, which indirectly specifies the offset to the
target entry.
--------
2. S7.5.4, P209, L756: Insert immediately following the description of
loclistsptr:
o loclistx
This is an index into the .debug_loclists
section. The unsigned ULEB operand identifies an offset location
relative to the base of that section (the location of the first offset
in the section, not the first byte of the section). The contents of
that location is then added to the base to determine the location of
the target list of entries.
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3. S7.5.4, P209, L768-770: Delete the sentence that reads "If a
DW_AT_ranges_base ... DW_AT_ranges_base.".
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4. S7.5.4, P210, L773: Insert imediately following the description of
rnglistsptr:
o rnglistx
This is an index into the .debug_rnglists
section. The unsigned ULEB operand identifies an offset location
relative to the base of that section (the location of the first offset
in the section, not the first byte of the section). The contents of
that location is then added to the base to determine the location of
the target list of entries.
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5. In Table 7.5, P203-207:
5a) Rename DW_AT_range_base to DW_AT_rnglists_base.
5b) Replace all occurences of rangelistptr except for DW_AT_rnglists_base
(namely for DW_AT_start_scope and DW_AT_ranges) with class rnglistx.
5c) Add (new) attribute DW_AT_loclists_base whose value is of class
loclistsptr.
5d) Replace all occurences of loclistptr except for DW_AT_loclists_base
(namely for DW_AT_location, DW_AT_string_length, DW_AT_return_addr,
DW_AT_data_member_location, DW_AT_frame_base, DW_AT_segment,
DW_AT_static_link, DW_AT_use_location, and DW_AT_vtable_elem_location)
with class loclistx.
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6. In Table 2.2, P19:
6A) Insert new row:
DW_AT_loclists_base | Base offset for location lists
6B) Change DW_AT_ranges_base to DW_AT_rnglists_base.
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7. In Table 2.3, P21-22: Add the following two new classes (in alphabetical
order):
loclistx Specifies a location list indirectly by use of an index.
rnglistx Specifies a range list indirectly by use of an index.
Also, rename rangelistptr to rnglistsptr.
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8. S3.1.1, P63, L168-172: Replace bullet 15 with the following:
15. A DW_AT_rnglists_base attribute, whose value is of class
rnglistsptr. This attribute points to the base of the compilation
unit’s contribution to the .debug_rnglists section. References
to range lists (using DW_FORM_secx) within the compilation
unit are interpreted relative to this base.
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9. S3.1.1, P63, L172: Insert the following new bullet 16:
16. A DW_AT_loclists_base attribute, whose value is of class
loclistsptr. This attribute points to the base of the compilation
unit’s contribution to the .debug_loclists section. References
to location lists (using DW_FORM_secx) within the compilation
unit are interpreted relative to this base.
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10. S3.1.2, P65, L214: Delete bullet 8.
Note that range lists can now be completedly contained in a split
unit which, like location lists, may reference addresses via the
DW_AT_addr_base attribute of the skeleton unit.
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11. S7.3.2.2, P184, L167-170: Delete--just like location lists, this
is no longer a special case.
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12. SF, Table F.1, P387:
12A) For DW_AT_ranges: Remove check for skeleton units and add for spilt
full unit.
12B) For DW_AT_rnglists_base (was DW_AT_ranges_base): Remove check for
skeleton unit.
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13. SF.2.1, P391, L136: Delete
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14. Sf.2.1, P391, Figure F.4: Delete DW_AT_ranges and DW_AT_ranges_base.
***********************************************************************
***
*** The above textual changes convey the heart of the proposal.
*** The following are remaining editorial cleanups.
***
15. S1.4, P8, L237: Insert new bullet:
o Add new location list and range list representations that
save space and processing time by eliminating most related
object file relocations.
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16. S1.4, P9, L273: After DW_FORM_ref_sup insert "DW_FORM_secx, ".
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17. S2.6, P35, L503: Change "loclistptr (which serves as an offset" to
"loclistx (which seres as an index".
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18. S3.1.2, P65, L219: Delete ", DW_AT_range_base,".
--------
19. S3.1.3, P65, L241: Replace ", DW_AT_addr_base and DW_AT_ranges_base"
with " and DW_AT_addr_base".
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20. S7.3.1, P182, L91-93: Delete discussion--no special case remains.
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21. S7.4, P193, L443: Replace ".debug_loc and .debug_ranges" with
".debug_loclists and .debug_rnglists".
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22. S7.3.1, P182, L102-104: Replace this bullet with:
o Entries in the .debug_addr and .debug_aranges sections may contain
references to locations within the virtual address space of the
program, and thus require relocation.
o Entries in the .debug_loclists and .debug_rnglists sections may
contain references to locations within the virtual address space of the
program depending on whether certain kinds of location or range
list entries are used, and thus require relocation.
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23. S7.3.2.1, P183, L136: Replace .debug_ranges with .debug_rnglists.
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24. S7.5.4, Table 7.5, P206: Add row immediately before DW_AT_lo_user:
DW_AT_rnglists_base<ddag> | loclistsptr
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25. S7.5.4, Table 7.6, P213:
25A) In the row for DW_FORM_sec_offset, change loclistptr to loclistsptr
and rangelistptr to rnglistsptr.
25B) Add new row at the end as in
DW_FORM_secx<ddag> | loclistx, rnglistx
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26. S7.28, P233, L1092: Change .debug_ranges to .debug_rnglists.
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27. Appendix A: Change all occurences of DW_AT_ranges_base to
DW_AT_rnglists_base except for DW_TAG_skeleton_unit from which it is
deleted.
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28. Appendix B: Rename...
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29: Appendix F: Rename...
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30. SF.1, P386, L95: Delete DW_AT_ranges_base.
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31. SF.1, P386, L111-113: Delete bullet 4.
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--
08/02/2016 - Accepted (requires 160123.1).
