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

[David Herrmann]

Hi

On Thu, Nov 27, 2014 at 4:42 PM, Gustavo Sverzut Barbieri
<gustavo.barbieri at intel.com> wrote:
> On Thu, Nov 27, 2014 at 03:42:37PM +0100, David Herrmann wrote:
>> 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.
>
> Did you confirm what you said or is it a supposition?
>
> I'm asking because from Peter Wu's email we're 26384 - 26380 = 4Kb
> heavier in the whole installed set of binaries. If this is correct I
> doubt the gc-sections is making any difference in the linkage of
> static libraries (src/shared). In this case my *assumption* (as I have
> no deep knowledge about the linker) is that it's selectively picking
> the symbols it uses and leaving the other stuff out of it.

-ffunction-section -fdata-section make the binaries slightly bigger,
but with --gc-sections I get a difference of >1MB for each of the
shared library objects (libudev.so.1.6.1, libsystemd.so.0.5.0, ...).

Thanks
David