I am in the process if writing a tool that can add .debug_names to a file that
contains DWARF but doesn’t have an accelerator table that is adequate for
debuggers to use. I was trying to see some example implementations of the
.debug_names section to see exactly what I should emit from this tool.
I have a question on the following section from the DWARF 5 specification:
6.1.1.4.7 Abbreviations Table. It contains "Table 6.1: Index attribute
encodings” which shows the available attributes for the abbreviations that can
be used for each index entry:
DW_IDX_compile_unit. // Index of CU
DW_IDX_type_unit // Index of TU
DW_IDX_die_offset // Offset of DIE within CU or TU
DW_IDX_parent // Index of name table entry for parent
DW_IDX_type_hash // Hash of type declaration
My main question is what are the best practices for how producers should emit
an entry for a specific DIE in this lookup table. One option is to specify both
a CU index and a relative DIE offset:
DW_IDX_compile_unit + DW_FORM_dataX form
DW_IDX_die_offset + DW_FORM_ref4
“DW_FORM_ref4” is known to be a CU relative offset.
Or we can simply emit a single attribute for the die_offset using an absolute
DIE offset reference?
DW_IDX_die_offset + DW_FORM_ref_addr
There isn’t much written up in the DWARF spec about these attributes except
that it seems to imply that DW_IDX_die_offset must also have a CU or TU since
the comment says "Offset of DIE within CU or TU”
I was looking at the output of dsymutil, a smart DWARF linker maintained by
Apple, and it can emit .debug_names with an option, but it emits the following
abbreviation, as seen by using “llvm-dwarfdump --debug-names”:
Abbreviation 0x16 {
Tag: DW_TAG_typedef
DW_IDX_die_offset: DW_FORM_ref4
}
And the in the Entry for each type, dsymutil only emits a DIE offset as a
DW_FORM_ref4, which seems to imply it is an absolute offset:
Bucket 0 [
Name 1 {
Hash: 0x4D51C8E0
String: 0x000010c4 "pthread_t"
Entry @ 0x5a7 {
Abbrev: 0x16
Tag: DW_TAG_typedef
DW_IDX_die_offset: 0x00002162
}
}
Name 2 {
Hash: 0x8FEC1B20
String: 0x00000323 "long int"
Entry @ 0x5ad {
Abbrev: 0x24
Tag: DW_TAG_base_type
DW_IDX_die_offset: 0x00000920
}
}
]
Should dsymutil be emitting both a CU index and a CU relative offset, or should
it just switch over to using a DW_FORM_ref_addr as the encoding for the DIE?
Do any current compilers support emitting .debug_names when “-gdwarf-5” is
produced?
Any clarification or pointers to other producers would be appreciated!
Greg Clayton
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