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