RFR: 8234525: enable link-time section-gc for linux s390x to remove unused code

[Erik Joelsson]

Hello Matthias,

This looks like an interesting change. If you want to enable this for 
s390x, I'm ok with it. If it works out well, perhaps we can find a way 
to expand this to other architectures.

Do you really want to set --print-gc-sections by default? I would assume 
that makes the build quite chatty.

/Erik

On 2019-11-21 00:54, Baesken, Matthias wrote:
> Hello,
>
> gcc and ld can be instructed to work together to "garbage collect" unused input sections.
> This feature eliminates unused code from native libraries. As a prerequisite to take full advantage of the feature,
> the source files need to be compiled with "-ffunction-sections -fdata-sections".
>
> Details on what happens can be found in the ld documentation: https://linux.die.net/man/1/ld .
> See the description of --gc-sections and --print-gc-sections therein.
> For more detailed insights there is a talk available from ELC2010: https://elinux.org/images/2/2d/ELC2010-gc-sections_Denys_Vlasenko.pdf
>
> My change enables the unused code elimination on linux s390x .
> (on the other Linux platforms, there are still issues to be solved with the serviceability agent, but we do not have the serviceability agent  on linux s390x).
>
> The change has 2 benefits :
> - native libs with unused code get smaller (some get alot smaller)
> some example lib sizes  from  linuxs390x (product build) :
>                      default settings  / link-time gc-sections
> libmlib_image.so        556K                 536K
> libjavajpeg.so              300K                 292K
> libsplashscreen.so      412K                 268K
> libfontmanager.so       1.4M                 864K
> libjvm.so                        19M                  17M
>
> - the flag --print-gc-sections  outputs the removed sections when calling the linker;
> this helps a lot to find coding "waiting for" cross-platform removal.
>
>
> Here is an example output of --print-gc-sections for the libnet-build (linux s390x) :
>
> /bin/ld: Removing unused section '.bss.my_gconf_init_func' in file '/builddir/support/native/java.base/libnet/DefaultProxySelector.o'  <--- seems to be dead
> /bin/ld: Removing unused section '.text.NET_ReadV' in file '/builddir/support/native/java.base/libnet/linux_close.o'                   <--- seems to be dead, I requested cross-platform removal with https://bugs.openjdk.java.net/browse/JDK-8234501
> /bin/ld: Removing unused section '.text.getInet6Address_scopeifname' in file '/builddir/support/native/java.base/libnet/net_util.o'    <--- seems to be dead
> /bin/ld: Removing unused section '.text.getInet6Address_scopeid_set' in file '/builddir/support/native/java.base/libnet/net_util.o'    <--- seems to be dead
> /bin/ld: Removing unused section '.text.getInetAddress_hostName' in file '/builddir/support/native/java.base/libnet/net_util.o'        <--- seems to be dead
> /bin/ld: Removing unused section '.text.setDefaultScopeID' in file '/builddir/support/native/java.base/libnet/net_util_md.o'           <--- seems to be dead indeed
> /bin/ld: Removing unused section '.text.getDefaultScopeID' in file '/builddir/support/native/java.base/libnet/net_util_md.o'           <--- seems to be dead indeed
> /bin/ld: Removing unused section '.text.kernelIsV24' in file '/builddir/support/native/java.base/libnet/net_util_md.o'                 <--- seems to be dead indeed
> /bin/ld: Removing unused section '.bss.ni_defaultIndexID.8722' in file '/builddir/support/native/java.base/libnet/net_util_md.o'       <--- only used in getDefaultScopeID , which is dead
> /bin/ld: Removing unused section '.bss.ni_class.8721' in file '/builddir/support/native/java.base/libnet/net_util_md.o'                <--- only used in getDefaultScopeID , which is dead
> /bin/ld: Removing unused section '.bss.vinit24' in file '/builddir/support/native/java.base/libnet/net_util_md.o'                      <--- only used in kernelIsV24 which is dead
> /bin/ld: Removing unused section '.bss.kernelV24' in file '/builddir/support/native/java.base/libnet/net_util_md.o'                    <--- only used in kernelIsV24 which is dead
>
> bug/webrev :
> https://bugs.openjdk.java.net/browse/JDK-8234525
>
> http://cr.openjdk.java.net/~mbaesken/webrevs/8234525.1/
>
> Thanks, Matthias
>
>