Issue 250311.1: Clarifying use of DW_AT_rnglists_base and other base attributes

Author:	Cary Coutant
Champion:
Date submitted:	2025-03-11
Date revised:
Date closed:	2025-03-17
Type:	Error
Status:	Accepted
DWARF version:	6

Submitted as a replacement for Issue 240618.2: Making sense of DW_AT_rnglists_base.

Background

The various _base attributes were added for two reasons. First, to collapse many relocations into one for non-split DWARF compilations. The CU could specify DW_AT_addr_base, DW_AT_loclists_base, and DW_AT_rnglists_base, and then all references to those tables could be relative to those base pointers and be free of any linker relocations. Second, to allow a DIE in a split DWARF (.dwo) to reference a table in the main program (.o, as linked into an exe). This second purpose is why DW_AT_addr_base needs to be in the skeleton CU and inherited by the split CU. But because location lists and range lists are both in the .dwo, those _base pointers do not need to be (and should not be) inherited by the split CU.

In 200709.1, we concluded that DW_AT_rnglists_base should be added to the list of attributes that a skeleton CU could have, based on the misunderstanding that the split CU should inherit it. But that was a relic from an early version of split DWARF where the range list table stayed in the .o and the references from the .dwo were to range lists in the .o. We changed that, and moved the range lists to the .dwo, and the split CU no longer needs to reference the executable and should not inherit that attribute.

It still makes sense for a skeleton CU to have a DW_AT_rnglists_base attribute, as the skeleton CU might still reference range lists in the .o/exe. In a split DWARF compilation, these would be just overall CU ranges to support lookup by address. Other ranges used by DIEs in the .dwo would reference ranges in the _debug_.rnglists.dwo section, and do not need a base address because the references do not need relocation. Note that it doesn't need to use a base attribute—it could just use DW_FORM_sec_offset to reference what is probably the one location list for the skeleton CU.

So the conclusion was:

Add "8. A DW_AT_rnglists_base attribute." to the list of attributes a skeleton unit may have.

This was OK, but somehow we ended up also adding "for range list entry references from the corresponding split full compilation unit," which was wrong. I suspect that extra phrase was copied from bullet 6, about DW_AT_str_offsets, which is also wrong.

The non-normative text below those bullets is also wrong:

The DW_AT_addr_base, DW_AT_str_offsets, and DW_AT_rnglists_base attributes provide context that may be necessary to interpret the contents of the corresponding split DWARF object file.

This is true only for the DW_AT_addr_base attribute.

In 210310.1, we (correctly) removed DW_AT_rnglists_base from the list of inherited attributes in 3.1.3 Split Full Compilation Unit Entries. This issue (incorrectly) left DW_AT_str_offsets_base in that list, but that was fixed in 221203.1.

I think it's therefore incorrect to have a DW_AT_rnglists_base or DW_AT_loclists_base attribute in a .dwo. If present, and they match sizeof(section header), it's harmless, but I don't think we should even suggest that a producer should emit them.

Proposal

In 3.1.2 Skeleton Compilation Unit Entries, change bullet items 6, 7, and 8 to:

6. A DW_AT_str_offsets attribute, for indirect string references from this skeleton compilation unit to a (non-split) .debug_str_offsets section.

7. A DW_AT_addr_base attribute, for indirect references from this skeleton compilation unit and from the corresponding split full compilation unit (See Section 3.1.3) to the compilation unit's contribution to the .debug_addr section. The DW_AT_addr_base attribute provides context that may be necessary to interpret the contents of the corresponding split DWARF object file.

8. A DW_AT_rnglists_base attribute, for range list entry references from this skeleton compilation unit to a (non-split) .debug_rnglists section.

And remove the non-normative paragraph:

The DW_AT_addr_base, DW_AT_str_offsets, and DW_AT_rnglists_base attributes provides context that may be necessary to interpret the contents of the corresponding split DWARF object file.

In 7.5.5 Classes and Forms, the loclist bullet should mention .debug_loclists.dwo in the first item:

• An index into the .debug_loclists or .debug_loclists.dwo section (DW_FORM_loclistx)....

Likewise, the rnglist bullet:

• An index into the .debug_rnglists or .debug_rnglists.dwo section (DW_FORM_rnglistx)....

In Appendix F, Section F.1 Overview, remove the checkmarks from the Split Full column of Table F.1 for DW_AT_loclists_base, DW_AT_rnglists_base, and DW_AT_str_offsets.

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2025-03-17: Accepted.