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

[Baesken, Matthias]

Hi Erik, 
   yes it makes the build more chatty -
our   linux s390   product   build  log  of  jdk/jdk   goes from  ~ 60.000  lines to  ~ 123.000  lines with the patch. 

However the change  is  linux s390 only so I guess it will not disturb other people ( in case we bring it to more platforms later on, we could remove the printing or make it configurable ).

Additionally the "chatty" output  is used currently for eliminating   unused  functions + data    cross-platform  
(see for example   https://bugs.openjdk.java.net/browse/JDK-8234629  ).

Best regards, Matthias



> 
> 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
> >
> >