On 2/22/23 09:28, Florian Weimer via Gcc wrote:
> * Richard Biener:
> 
>> On Wed, Feb 22, 2023 at 9:19 AM Florian Weimer via Gcc <gcc@gcc.gnu.org> wrote:
>>>
>>> Can we use the COMMON symbol __gnu_lto_slim to detect
>>> -fno-fat-lto-objects on contemporary GNU/Linux (with the LTO linker
>>> plugin)?
>>
>> Yes.
> 
> Great, thanks.
> 
>>> We currently build the distribution with -ffat-lto-objects, and I want
>>> to switch away from that.  Packages will need to opt in to
>>> -ffat-lto-objects if static objects they build escape the buildroot.

Hello.

We use the same approach where only a selected packages enable FAT lto objects.
You can take a look at our META bug where such packages are linked (in most cases):
https://bugzilla.opensuse.org/show_bug.cgi?id=1133084

Or one can easily grep it from here:
https://github.com/bmwiedemann/openSUSE

$ git grep 'fat-lto-objects'
(see attachment)

>>> And to make sure that this opt-in happens, I want to fail the build if
>>> there would be any -fno-fat-lto-objects objects leaking.
>>
>> For SUSE we're checking that no LTO bytecode leaks instead, thus we check
>> for __gnu_lto_v? (I think).  The reason is that even for static libraries
>> we do not want to ship LTO bytecode.
> 
> We build with -ffat-lto-objects, and this means we can create perfectly
> fine object files by stripping the LTO data:
> 
> <https://src.fedoraproject.org/rpms/redhat-rpm-config/blob/rawhide/f/brp-strip-lto>

We strip this way:
https://github.com/openSUSE/brp-check-suse/blob/2f7add6f0b7f5c2e9698d180a4761a10fb808482/brp-15-strip-debug#L38

> 
> This means that so far, we only had to fix LTO compilation problems in
> the packages, but not teach individual packages about LTO and non-LTO
> object files.  Of course it's wasteful because few packages actually
> install the object files (without a final link into a program or shared
> object), and that's what I want to fix.

And we integrated a rpmlint check that checks we don't ship a LTO bytecode:
https://github.com/rpm-software-management/rpmlint/blob/aa1f710c1b3ff409e42068c2b067ab2eee200b1d/rpmlint/checks/BinariesCheck.py#L277-L283

and there's one related check where we look if a static library contains something:
https://github.com/rpm-software-management/rpmlint/blob/aa1f710c1b3ff409e42068c2b067ab2eee200b1d/rpmlint/checks/BinariesCheck.py#L230-L255

it can happen that it only contained bytecode that was eventually striped.

Cheers,
Martin

> 
> Florian
>