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

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 4/3/25 13:11, Daniel P. Berrangé wrote:
> On Tue, Mar 04, 2025 at 11:33:44AM +0000, Alex Bennée wrote:
>> 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.
> 
> See this response from one of the upstream GCC maintainers pretty much
> recommending against (on by default) use of -gsplit-dwarf:
> 
>    https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/RXA55BTO62NWWHCTUFYGEVHJKZPL5EKE/

Jakub said:

  "it can be useful for fast modify/rebuild/test cycles during
   development, but is something that is really undesirable for
   the distro builds."

Do distro use --enable-debug by default? Otherwise it might be
useful to add it for developers.