Issue 221114.1: DW_FORM_implicit_const and DW_FORM_indirect

Author:	David Anderson
Champion:	David Anderson
Date submitted:	2022-11-14
Date revised:	2023-07-08
Date closed:	2023-06-26
Type:	Error
Status:	Accepted with editorial changes
DWARF version:	6

DWARF5 Section 7.5.3, pg 207

BACKGROUND

DW_FORM_indirect and DW_FORM_implicit_const are not described correctly and this can lead to an abbreviation section being impossible to read. Because the standard says to put the implicit_const in .debug_abbrev but in a DW_FORM_indirect leading to .debug_abbrev DW_FORM_implicit_const there is nothing in .debug_abbrev that would allow the abbrev reader to understand .debug_abbrev when searching for a particular abbrev code.

A simple clarification/generalization resolves all the issues while keeping compatibility with DWARF5 wording.

From the index:

DW_FORM_implicit_const, pages 9,209,214,221
DW_FORM_indirect, pages 207,220,288.

the descriptions are in section 7.5.3 page 207:

(DWARF 5 page 207 starting at line 6:)
The attribute form DW_FORM_indirect is a special case. For attributes with this form, the attribute value itself in the .debug_info section begins with an unsigned LEB128 number that represents its form. This allows producers to choose forms for particular attributes dynamically, without having to add a new entry to the abbreviations table.

The attribute form DW_FORM_implicit_const is another special case. For attributes with this form, the attribute specification contains a third part, which is a signed LEB128 number. The value of this number is used as the value of the attribute, and no value is stored in the .debug_info section.

Add to the first paragraph:

A sequence of form DW_FORM_indirect in .debug_info is allowed.

Revise the second paragraph to read:

The attribute form DW_FORM_implicit_const is another special case. For attributes with this form, the attribute specification contains a third part which follows the DW_FORM_implicit_const unsigned LEB128 and is a signed LEB128 number. The value of this number is used as the value of the attribute.

Revised Proposal (2023-07-08)

DW5: page 207 lines 1-16

In DWARF6, replace those lines with proper latex saying:

Finally, the child encoding is followed by a series of attribute specifications. Each attribute specification consists of two parts (except for DW_FORM_implicit_const, DW_FORM_addrx_offset and DW_FORM_indirect, see below). The first part is an unsigned LEB128 number representing the attribute’s name. The second part is an unsigned LEB128 number representing the attribute’s form. The series of attribute specifications ends with an entry containing 0 for the name and 0 for the form.

For attributes with the form DW_FORM_implicit_const, in addition to the attribute name and form values, the attribute specification contains a third part, which is a signed LEB128 number. The value of this number is used as the value of the attribute.

For attributes with the form DW_FORM_addrx_offset, following the attribute name, the attribute specification contains two unsigned LEB128 numbers, each representing a form. The first form must be of class address and the second of class constant. Values using this form in the .debug_info section contain a value for the first form followed by a value for the second form. The total value of the DW_FORM_addrx_offset is then computed by adding those two values together (if the first value is an indirect address, that is resolved first before adding it to the second value).

For attributes with the form DW_FORM_indirect, the actual attribute form value itself is in the .debug_info section which begins with an unsigned LEB128 number that specifies the actual form, followed by the value according to that form. This allows producers to choose forms for particular attributes dynamically, without having to add a new entry to the abbreviations table.

If the actual attribute form is DW_FORM_implicit_const, the form is (still) followed by a signed LEB128 number.

If the actual attribute form is itself DW_FORM_indirect, the indirection repeats. There may be one or more occurrences of DW_FORM_indirect in sequence until a non-DW_FORM_indirect form is reached. The sequence of DW_FORM_indirect forms does not have any effect other than to use up space.

The abbreviations for a given compilation unit end with an entry consisting of a 0 byte for the abbreviation code.

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2022-11-21: Revised
2023-04-10: Revised
2023-06-26: Accepted with editorial changes
2023-07-08: Editorial changes finalized