Re: [PATCH 1/5] kbuild: thin archives final link close --whole-archives option

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On Wed, Jun 21, 2017 at 11:16 AM, Nicholas Piggin <npiggin@xxxxxxxxx> wrote:
> On Wed, 21 Jun 2017 09:15:09 +0200
> Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
>> On Wed, Jun 21, 2017 at 6:04 AM, Nicholas Piggin <npiggin@xxxxxxxxx> wrote:
>> > On Wed, 21 Jun 2017 12:29:33 +0900
>> > Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> wrote:
>>
>> >> BTW, I saw abuse of lib.a in
>> >> https://patchwork.kernel.org/patch/9768439/
>> >>
>> >> I see it in linux-next.
>> >>
>> >> commit 06e226c7fb233f676b01b144d0b321ebe510fdcd
>> >> Author: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
>> >> Date:   Fri Jun 2 15:30:06 2017 -0700
>> >>
>> >>     clk: sunxi-ng: Move all clock types to a library
>> >>
>> >>
>> >>
>> >>
>> >> Now drivers/clk/sunxi-ng/lib.a
>> >> will go into thin archives.
>> >> The result might be different from what they expect...
>> >
>> > Yes I see. With thin archives, that is just going to cause the
>> > same behaviour as built-in.o (everything will be linked). So the
>> > build should not break, but they won't get savings.
>> >
>> > Does it even save space with incremental linking? If the lib.a gets
>> > linked into drivers/built-in.o, I wonder what happens then?
>>
>> Ah, too bad. I thought we had found a way to use a library correctly
>> here, but I just verified that indeed all the just gets linked into built-in.o
>>
>> I played around with it some more now, but without success: if I
>> build sunxi-ng as a loadable module (using a few modifications),
>> then the unneeded objects from lib.a are dropped as I had hoped,
>> but for built-in code we now always include everything.
>>
>> I suppose that we can ignore this once we get
>> LD_DEAD_CODE_DATA_ELIMINATION enabled on ARM, but
>> until then, we have a code size regression.
>
> I didn't follow the thread there, is it a regression caused by
> thin archives, or just by removing the Kconfig symbol from each
> file?

I thought it was the latter, but actually it only happens with thin
archives, so we are fine as long as we enable THIN_ARCHIVES
and LD_DEAD_CODE_DATA_ELIMINATION at the same time
on ARM.

        Arnd
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