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

Simon Marchi
2025-11-18T16:51:22Z

On 11/18/25 8:32 AM, Andrew Burgess wrote:
> Simon Marchi <simon.marchi@polymtl.ca> writes:
>
>> On 11/7/25 4:10 PM, Simon Marchi wrote:
>>> 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 $_abbrev_num,
>>> depending on whether the current CU is in a .dwo file.
>>> - After producing a CU or TU in a .dwo file, don't append the
>>> terminator byte.
>>> - After finishing producing the CUs and TUs, append the terminator byte
>>> in .debug_abbrev.dwo if we did output anything there.
>>>
>>> Update gdb.dwarf2/fission-with-type-units.exp to use
>>> build_executable_and_dwo_files, as it should.
>>
>> I spotted that running this test with the cc-with-gdb-index board now
>> shows a new internal error. I just wanted to note that this is not a
>> regression, it's actually the test going further and catching an actual
>> bug in GDB. Please consider that the following text is appended to the
>> commit message.
>>
>> ---
>> With this change, running with the cc-with-gdb-index board, I see:
>>
>> (gdb) maint expand-symtabs
>> /home/smarchi/src/binutils-gdb/gdb/dwarf2/read.c:3056: internal-error: cutu_reader: Assertion `sig_type->signature == cu->header.signature' failed.
>> A problem internal to GDB has been detected,
>> further debugging may prove unreliable.
>> ----- Backtrace -----
>> FAIL: gdb.dwarf2/fission-with-type-unit.exp: maint expand-symtabs (GDB internal error)
>>
>> This is actually an improvement, as the test case didn't run properly
>> before. The compilation failed with:
>>
>> gdb compile failed, During symbol reading: Could not find DWO CU fission-with-type-unit.dwo(0xf00d) referenced by CU at offset 0xc [in module /home/smarchi/build/binutils-gdb/gdb/testsuite/outputs/gdb.dwarf2/fission-with-type-unit/.tmp/fission-with-type-unit]
>>
>> The reason was that the old code would try to generate the GDB index
>> during this step:
>>
>> # Build main file.
>> if { [build_executable "${testfile}.exp" $binfile \
>> [list ${srcfile} ${main_asm_file}] {nodebug}] } {
>> return
>> }
>>
>> ... which is even the DWO file is even generated. With this patch
>> things are done in the correct order:
>
> I cannot parse the "which is even the DWO file is even generated" part
> here. Can you reword this?

Oops. I meant: ... which is before the DWO file is even generated.

If the main file exists but the DWO file doesn't exist yet, and you try
to generate an .gdb_index from that, it won't go very well, hopefully
that's clear.

>
>>
>> - The -dw.S file is generated
>> - The -dw.o file is compiled from the -dw.S
>> - The .dwo sections are extracted to the .dwo file, and stripped from
>> the -dw.o file
>> - The executable is linked from the .o and -dw.o
>> - gdb-add-index is ran on the executable
>
> Not critical, but I think s/ran/run/ would be better.

Noted, will fix.

Simon