Re: [PATCH 1/6] gdb/testsuite/dwarf: use single abbrev table in .dwo files

Andrew Burgess
2025-11-18T16:56:13Z

Simon Marchi <simon.marchi@efficios.com> writes:

> From: Simon Marchi <simon.marchi@polymtl.ca>
>
> When I wrote test gdb.dwarf2/fission-with-type-unit.exp, I did not use
> build_executable_and_dwo_files, because it wouldn't work to have
> multiple units in the .dwo file, each referring to their own abbrev
> table using labels. build_executable_and_dwo_files extracts the .dwo
> file content from the .o using objcopy (just like gcc does, I learned),
> meaning that the .dwo file never runs through a linker. Anything
> needing relocation (like labels pointing to abbrev tables) doesn't work.
>
> I instead opted to use gdb_compile_shlib to build the .dwo file on its
> own, so that those labels would get resolved. That causes problems now
> that I'm trying to write a test with multiple type units in a .dwo file,
> where each type unit should be in its own .debug_types section. Running
> the .dwo file through the linker causes all the .debug_types section to
> be collapsed into one. And generally, I think it was a bad idea to
> generate a .dwo file using the linker, since the idea behind .dwo files
> is that they do not need to be linked (therefore improving link
> times). We want to produce files as close to what an actual compiler
> would produce.
>
> This patch fixes this by doing what compilers do in the same situation:
> use a single abbrev table shared by all units in the .dwo file. This
> requires the following changes in lib/dwarf.exp:
>
> - Declare a new variable _dwo_abbrev_num, which holds the next
> abbrev number to use in the .dwo file's abbrev section
> (.debug_abbrev.dwo). Initialize this variable to 1.
> - When producing a CU or TU in a .dwo file, use 0 as the abbrev table
> offset.
> - When generating a DIE, return $_dwo_abbrev_num or $_abbrev_num,
> depending on whether the current CU is in a .dwo file.
> - After producing a CU or TU in a .dwo file, don't append the
> terminator byte.
> - After finishing producing the CUs and TUs, append the terminator byte
> in .debug_abbrev.dwo if we did output anything there.
>
> Update gdb.dwarf2/fission-with-type-units.exp to use
> build_executable_and_dwo_files, as it should.
>
> Change-Id: Iabbcf00db97faf2a4fa5fc71652ad273081189f9
> ---
> .../gdb.dwarf2/fission-with-type-unit.exp | 29 +++-----
> gdb/testsuite/lib/dwarf.exp | 69 +++++++++++++++----
> 2 files changed, 67 insertions(+), 31 deletions(-)
>
> diff --git a/gdb/testsuite/gdb.dwarf2/fission-with-type-unit.exp b/gdb/testsuite/gdb.dwarf2/fission-with-type-unit.exp
> index 58cda296c67b..5c73001ec66b 100644
> --- a/gdb/testsuite/gdb.dwarf2/fission-with-type-unit.exp
> +++ b/gdb/testsuite/gdb.dwarf2/fission-with-type-unit.exp
> @@ -21,25 +21,22 @@ load_lib dwarf.exp
> # This test can only be run on targets which support DWARF-2 and use gas.
> require dwarf2_support
>
> -standard_testfile .c -dw.S -dwo.S
> +standard_testfile .c -dw.S
>
> -set main_asm_file [standard_output_file $srcfile2]
> -set dwo_asm_file [standard_output_file $srcfile3]
> +set asm_file [standard_output_file $srcfile2]
>
> -# Debug info in the main file.
> -Dwarf::assemble $main_asm_file {
> +Dwarf::assemble $asm_file {
> + # In the main file.
> cu {
> version 5
> dwo_id 0xF00D
> } {
> compile_unit {
> - DW_AT_dwo_name ${::gdb_test_file_name}.dwo DW_FORM_strp
> + DW_AT_dwo_name ${::gdb_test_file_name}-dw.dwo DW_FORM_strp
> } {}
> }
> -}
>
> -# Debug info in the DWO file.
> -Dwarf::assemble $dwo_asm_file {
> + # In the .dwo file.
> tu {
> fission 1
> version 5
> @@ -79,15 +76,11 @@ Dwarf::assemble $dwo_asm_file {
> }
> }
>
> -# Build main file.
> -if { [build_executable "${testfile}.exp" $binfile \
> - [list ${srcfile} ${main_asm_file}] {nodebug}] } {
> - return
> -}
> -
> -# Build DWO file.
> -set dwo_file [standard_output_file ${testfile}.dwo]
> -if { [gdb_compile_shlib $dwo_asm_file $dwo_file nodebug] != "" } {
> +set obj [standard_output_file "${testfile}-dw.o"]
> +set dwo_file [standard_output_file "${testfile}.dwo"]

The name of the dwo file here is now wrong. The auto generated name
will be '${testfile}-dw.dwo', which is what you embed in the DWARF
above.

I was wondering why this wasn't causing problems for the later block in
this file (not touched in this patch):

if { [is_remote host] } {
gdb_remote_download host $dwo_file
}

But it turns out that ...

> +if {[build_executable_and_dwo_files "$testfile.exp" "${binfile}" {} \

... build_executable_and_dwo_files includes this check:

# Must be run on local host due to use of objcopy.
if {[is_remote host]} {
return -1
}

So this test never gets past this point for remote host boards.

I think you should:

1. Remove the 'set dwo_file ...' line.

2. Remove the 'gdb_remote_download host $dwo_file' line, and its
containing 'if' block.

3. Add a 'require {!is_remote host}' line at the start of the file.

Otherwise, this change looks good to me (along with the additional text
you posted in the follow up, which I also replied to).

Approved-By: Andrew Burgess <aburgess@redhat.com>

Thanks,
Andrew

> + [list $asm_file {nodebug split-dwo} $obj] \
> + [list $srcfile {nodebug}]]} {
> return
> }
>
> diff --git a/gdb/testsuite/lib/dwarf.exp b/gdb/testsuite/lib/dwarf.exp
> index ba8197d0bb8e..919e2e05687f 100644
> --- a/gdb/testsuite/lib/dwarf.exp
> +++ b/gdb/testsuite/lib/dwarf.exp
> @@ -563,6 +563,9 @@ namespace eval Dwarf {
> # table.
> variable _abbrev_num
>
> + # The next available abbrev number in the (single) DWO abbrev table.
> + variable _dwo_abbrev_num
> +
> # The string table for this assembly. The key is the string; the
> # value is the label for that string.
> variable _strings
> @@ -1008,8 +1011,17 @@ namespace eval Dwarf {
> # table.
> proc _get_abbrev_num {} {
> variable _abbrev_num
> - set res $_abbrev_num
> - incr _abbrev_num
> + variable _dwo_abbrev_num
> + variable _cu_is_fission
> +
> + if { $_cu_is_fission } {
> + set res $_dwo_abbrev_num
> + incr _dwo_abbrev_num
> + } else {
> + set res $_abbrev_num
> + incr _abbrev_num
> + }
> +
> return $res
> }
>
> @@ -1538,8 +1550,17 @@ namespace eval Dwarf {
> _section $section
>
> set cu_num [incr _cu_count]
> - set my_abbrevs [_compute_label "abbrev${cu_num}_begin"]
> - set _abbrev_num 1
> +
> + # Since .dwo files are not linked, we can't use a label to point to a
> + # specific place in the .debug_abbrev section. For .dwo files, we
> + # therefore use a single abbrev table (at offset 0) shared by all units
> + # in that file.
> + if { $_cu_is_fission } {
> + set my_abbrevs 0
> + } else {
> + set my_abbrevs [_compute_label "abbrev${cu_num}_begin"]
> + set _abbrev_num 1
> + }
>
> set _cu_label [_compute_label "cu${cu_num}_begin"]
> set start_label [_compute_label "cu${cu_num}_start"]
> @@ -1606,9 +1627,11 @@ namespace eval Dwarf {
>
> uplevel $_level $body
>
> - _defer_output $_abbrev_section {
> - # Emit the terminator.
> - _op .byte 0x0 "Abbrev end - Terminator"
> + if { !$_cu_is_fission } {
> + _defer_output $_abbrev_section {
> + # Emit the terminator.
> + _op .byte 0x0 "Abbrev end - Terminator"
> + }
> }
>
> define_label $end_label
> @@ -1681,8 +1704,17 @@ namespace eval Dwarf {
> _section $section
>
> set cu_num [incr _cu_count]
> - set my_abbrevs [_compute_label "abbrev${cu_num}_begin"]
> - set _abbrev_num 1
> +
> + # Since .dwo files are not linked, we can't use a label to point to a
> + # specific place in the .debug_abbrev section. For .dwo files, we
> + # therefore use a single abbrev table (at offset 0) shared by all units
> + # in that file.
> + if { $_cu_is_fission } {
> + set my_abbrevs 0
> + } else {
> + set my_abbrevs [_compute_label "abbrev${cu_num}_begin"]
> + set _abbrev_num 1
> + }
>
> set _cu_label [_compute_label "cu${cu_num}_begin"]
> set start_label [_compute_label "cu${cu_num}_start"]
> @@ -1736,9 +1768,11 @@ namespace eval Dwarf {
>
> uplevel $_level $body
>
> - _defer_output $_abbrev_section {
> - # Emit the terminator.
> - _op .byte 0x0 "Abbrev end - Terminator"
> + if { !$_cu_is_fission } {
> + _defer_output $_abbrev_section {
> + # Emit the terminator.
> + _op .byte 0x0 "Abbrev end - Terminator"
> + }
> }
>
> define_label $end_label
> @@ -3507,6 +3541,7 @@ namespace eval Dwarf {
> variable _debug_ranges_64_bit
> variable _debug_addr_index
> variable _level
> + variable _dwo_abbrev_num
>
> if { [llength $options] == 1 } {
> set options [list filename [lindex $options 0]]
> @@ -3539,8 +3574,8 @@ namespace eval Dwarf {
>
> set _line_count 0
> set _debug_ranges_64_bit [is_64_target]
> -
> set _debug_addr_index 0
> + set _dwo_abbrev_num 1
>
> # Dummy CU at the start to ensure that the first CU in $body is not
> # the first in .debug_info.
> @@ -3562,6 +3597,14 @@ namespace eval Dwarf {
> dummy_cu
> }
>
> + # If we wrote any abbrev in .debug_abbrev.dwo, write the terminator.
> + if { $_dwo_abbrev_num > 1 } {
> + _defer_output .debug_abbrev.dwo {
> + # Emit the terminator.
> + _op .byte 0x0 "Abbrev end - Terminator"
> + }
> + }
> +
> _write_deferred_output
>
> _section .note.GNU-stack "" progbits
> --
> 2.51.2