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List:       busybox
Subject:    Re: [PATCH] tar: Fix build error when CONFIG_UNCOMPRESS is not selected
From:       Michael Tokarev <mjt () tls ! msk ! ru>
Date:       2015-06-28 7:44:54
Message-ID: 558FA5F6.8000107 () msgid ! tls ! msk ! ru
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[Rehashing a thread from 3 years ago]

28.01.2013 11:48, Denys Vlasenko wrote:
> On Monday 28 January 2013 00:17, Abdoulaye Walsimou GAYE wrote:

> > > > > diff --git a/archival/libarchive/Kbuild.src \
> > > > > b/archival/libarchive/Kbuild.src index 58457fc..87e1ab9 100644
> > > > > --- a/archival/libarchive/Kbuild.src
> > > > > +++ b/archival/libarchive/Kbuild.src
> > > > > -lib-$(CONFIG_TAR)                       += get_header_tar.o
> > > > > +lib-$(CONFIG_TAR)                       += get_header_tar.o \
> > > > > decompress_uncompress.o
> > 
> > /home/walsimou/embtoolkit.git/build/packages_build-mipsel-linux-mips32/busybox-1.20.2/archival/tar.c:1065: \
> >                 undefined reference to `unpack_Z_stream'
> > /home/walsimou/embtoolkit.git/build/packages_build-mipsel-linux-mips32/busybox-1.20.2/archival/tar.c:1065: \
> >                 undefined reference to `unpack_Z_stream'
> > /home/walsimou/embtoolkit.git/tools-mipsel-linux-mips32/bin/mipsisa32el-unknown-linux-uclibc-ld: \
> >                 busybox_unstripped: hidden symbol `unpack_Z_stream' isn't defined
> > /home/walsimou/embtoolkit.git/tools-mipsel-linux-mips32/bin/mipsisa32el-unknown-linux-uclibc-ld: \
> >                 final link failed: Bad value
> > mipsisa32el-unknown-linux-uclibc-clang: error: linker command failed with exit \
> > code 1 (use -v to see invocation)
> 
> 
> I was able to build busybox-1.20.2 with this .config
> without errors.
> 
> Looks like your compiler did not optimize this out:
> 
> if (opt & OPT_COMPRESS)
> USE_FOR_MMU(xformer = unpack_Z_stream;)
> USE_FOR_NOMMU(xformer_prog = "uncompress";)
> 
> 
> even though OPT_COMPRESS is a constant zero.

The same prob exists with clang, apparently,  Here's a bugreport
filed against debian package of busybox: http://bugs.debian.org/789499 .
The proposed fix is to apply the above patch only if building
with clang :)

Basically, I'm not sure relying on dead code elimination like this is
a good idea.  I mean, the code is eliminated, there's no if() statement
in the generated code, so it is okay, but it looks like clang still
records symbols referenced in the eliminated code.  Sometimes it
is actually a good idea to keep ref symbols, eg when you build an
executable which can load modules, so that modules will use symbols
in that executable.

Thanks,

/mjt

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