Re: [RFC PATCH] meson.build: add -gsplit-dwarf to default cflags

["Alex Bennée" <alex.bennee@linaro.org>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Mon, Mar 03, 2025 at 10:17:03PM +0000, Alex Bennée wrote:
>> This option is supported by both gcc (since 4.7) and clang (since
>> 7.0). Not only does this make the linkers job easier by reducing the
>> amount of ELF it needs to parse it also reduces the total build size
>> quite considerably. In my case a default build went from 5.8G to 3.9G.
>
> I've not come across this option before, but the docs say
>
>   ‘-gsplit-dwarf’
>      If DWARF debugging information is enabled, separate as much
>      debugging information as possible into a separate output file with
>      the extension ‘.dwo’.  This option allows the build system to avoid
>      linking files with debug information.  To be useful, this option
>      requires a debugger capable of reading ‘.dwo’ files.
>
> In Fedora and RHEL we build QEMU will full debug enabled, and then a feature
> of the distro RPM build config will post-process all ELF files to extract
> the debug info into files that we store under /usr/lib/debug. eg for
> /usr/bin/qemu-system-x86_64, we get a separate
> /usr/lib/debug/bin/qemu-system-x86_64-9.1.3-1.fc41.x86_64.debug, and
> tools like GDB, etc know to look for these separate files.

A modern gdb can certainly handle fetching the debug out of the .dwo
files when debugging.

>
> When I test our RPM build process with this -gsplit-dwarf flag added,
> we fail to find any debug symbols in the QEMU binaries and thus the
> whole RPM package build aborts.

Do you know if you build the kernel the same way? That has had the
option to do split dwarf files for awhile.

The tools dwp and dwz are used to collect the split files and generate
the final (optionally compressed) dwarf symbol file.

>
> IOW, if we add this to QEMU, my best guess currently is that we're
> going to have to revert it downstream.
>
>> 
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> 
>> ---
>> RFC:
>>   - currently and RFC because it should also apply to test binaries
>> ---
>>  meson.build | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/meson.build b/meson.build
>> index 4588bfd864..a936d1b422 100644
>> --- a/meson.build
>> +++ b/meson.build
>> @@ -363,7 +363,7 @@ endforeach
>>  
>>  qemu_common_flags = [
>>    '-D_GNU_SOURCE', '-D_FILE_OFFSET_BITS=64', '-D_LARGEFILE_SOURCE',
>> -  '-fno-strict-aliasing', '-fno-common', '-fwrapv' ]
>> +  '-fno-strict-aliasing', '-fno-common', '-fwrapv', '-gsplit-dwarf' ]
>>  qemu_cflags = []
>>  qemu_ldflags = []
>>  
>> -- 
>> 2.39.5
>> 
>
> With regards,
> Daniel

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro