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

[Doerr, Martin]

Hi Matthias and Erik,

I also think this is an interesting option.

I like the idea to generate smaller libraries. In addition to that, I could also imagine building with -Os (size optimized) by default and only select -O3 for performance critical files (e.g. C2's register allocation, some gc code, ...).

If we want to go into such a direction for all linux platforms and want to use this s390 only change as some kind of pipe cleaner, I think this change is fine and can get pushed.
Otherwise, I think building s390 differently and not intending to do the same for other linux platforms would be not so good.

We should only make sure the exported symbols are set up properly to avoid that this optimization throws out too much.

My 50 Cents.

Best regards,
Martin



> -----Original Message-----
> From: build-dev <build-dev-bounces at openjdk.java.net> On Behalf Of
> Baesken, Matthias
> Sent: Freitag, 22. November 2019 16:01
> To: Erik Joelsson <erik.joelsson at oracle.com>; 'build-dev at openjdk.java.net'
> <build-dev at openjdk.java.net>; core-libs-dev at openjdk.java.net
> Subject: RE: RFR: 8234525: enable link-time section-gc for linux
> s390x to remove unused code
> 
> 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
> > >
> > >