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

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On Wed, 21 Jun 2017 12:21:16 +0200
Arnd Bergmann <arnd@xxxxxxxx> wrote:

> 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, 

Is this including these changes now in the kbuild tree?

I can take a look at ARM and try to get it at least to parity with
incremental link. Any particular config options required?

> so we are fine as long as we enable THIN_ARCHIVES
> and LD_DEAD_CODE_DATA_ELIMINATION at the same time
> on ARM.

Well the current proposal is to unconditionally enable it for all archs
for 4.13. After that I'll submit patches to x86 and powerpc arch
maintainers to allow LD_DEAD_CODE_DATA_ELIMINATION as an option. I guess
you will do ARM and there have been MIPS guys looking at it too.

That leaves a window of one release. ARM could unselect thin archives if
necessary but I think it would be much better to enable it and flush out
any toolchain and build issues before doing the LD_DCDE option. Disabling
should be a last resort if we can't fix something in time for release.

Thanks,
Nick
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