[systemd-devel] [PATCH] build-sys: remove --gc-sections to fix debugging

[David Herrmann]

Hi

On Wed, Nov 26, 2014 at 6:40 PM, Gustavo Sverzut Barbieri
<gustavo.barbieri at intel.com> wrote:
> On Wed, Nov 26, 2014 at 06:33:31PM +0100, Lennart Poettering wrote:
>> On Wed, 26.11.14 15:16, Gustavo Sverzut Barbieri (gustavo.barbieri at intel.com) wrote:
>>
>> > I'm okay with the change to remove gc-sections.
>> >
>> > systemd is pretty good at not leaving crap in its code, then results
>> > are pretty small as we saw.
>> >
>> > My only comment is that we should also remove:
>> >         -ffunction-sections
>> >         -fdata-sections
>> > those are only useful to enable gc-sections.
>> >
>> > From time to time someone should run a build with
>> >         -ffunction-sections
>> >         -fdata-sections
>> >         -Wl,--gc-sections
>> >         -Wl,--print-gc-sections
>> > so we print out sections that are dangling and then we could remove
>> > them from source code if needed.
>>
>> This is not that simple. We have a large set of shared functions in
>> src/shared/*.c, and we link that into pretty much any binary we build,
>> even though each binary only needs a small subset of them. We rely on
>> the GC logic to deal with this and remove the functions that are
>> unused by the specific program. Howver, that is not an indication that
>> we can remove the function, that's simply an indication that that one
>> binary of the 90 or so we build doesn't need it. Other binaries might
>> need it still, but print-gc-sections won't tell you about that.
>
> For shared libraries it won't remove exported (visible) symbols. Only
> stuff (functions, variables) that have not reference are removed.
>
> Of course those local to a file (ie: static) are always eliminated,
> that's why we see minor benefit with gc-sections as the major bulk are
> marked as "static". As I said, if the functions are exported
> (visibility=default), then they are not GC, what leaves us to "extern"
> symbols, those that are visible across files but not exported in the elf.

We _really_ depend on --gc-sections. We link a lot of stuff from
src/shared/ statically into all binaries and expect the linker to drop
any unused symbols. Note that none of the symbols from src/shared/ are
exported, so the linker can drop them safely.

I don't get why we work around linker bugs in systemd? The bug was
fixed upstream, so make them backport it, if it's a critical bug. If
they refuse to do so, blacklist the given version. But I dislike
carrying fixes for other software in systemd-git.

Thanks
David