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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Mar 04, 2025 at 02:40:31PM +0100, Philippe Mathieu-Daudé wrote:
> 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.

We don't have --enable-debug in Fedora/RHEL, and I doubt other distros
would enable it given it turns off all optimization as well as introducing
other static debug overheads.

With regards,
Daniel
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