RFR: 8236714: enable link-time section-gc for linux to remove unused code

[Baesken, Matthias]

Hello,  the following change enables the  link-time section-gc for linux .

gcc and ld support enabling "garbage collection" of unused input sections.
This can be used to eliminate unused coding from native libraries (especially when already compiling the objects with compiler flags -ffunction-sections -fdata-sections .
See for details the --gc-sections and --print-gc-sections parts of the ld documentation :

https://linux.die.net/man/1/ld


We had this enabled already  for  linux s390x ,  with https://bugs.openjdk.java.net/browse/JDK-8234525
8234525: enable link-time section-gc for linux s390x to remove unused code .

This  time we enable it too for  the other linux platforms .

For the other platforms I do not enable it for JVM, just for the JDK libs.  The reason is that the serviceability agent  (not supported on linux s390x )    is not  (yet) ready for the optimization .
Below you see the results , for some libraries  a significant  size reduction can be achieved .


Results from linux x86_64 product builds :

without / with ltgc

320K / 300K    /images/jdk/lib/libsunec.so       <-------------------------
36K  / 36K     /images/jdk/lib/libdt_socket.so
280K / 276K   /images/jdk/lib/libjdwp.so
23M  / 23M    /images/jdk/lib/server/libjvm.so    <---- not set for libjvm.so for x86_64
16K  / 16K    /images/jdk/lib/server/libjsig.so
72K  / 72M    /images/jdk/lib/libverify.so
84K  / 84M   /images/jdk/lib/libjli.so
16K  / 16K    /images/jdk/lib/libjsig.so
196K / 196K   /images/jdk/lib/libjava.so
44K  / 44K    /images/jdk/lib/libzip.so
144K / 136K   /images/jdk/lib/libjimage.so
112K / 112K   /images/jdk/lib/libnet.so
100K / 100K   /images/jdk/lib/libnio.so
36K  / 36K    /images/jdk/lib/libsctp.so
576K / 556K   /images/jdk/lib/libmlib_image.so
752K / 752K   /images/jdk/lib/libawt.so
260K / 252K   /images/jdk/lib/libjavajpeg.so
784K / 784K   /images/jdk/lib/libfreetype.so
368K / 236K /images/jdk/lib/libsplashscreen.so   <-------------------------
88K / 88K    /images/jdk/lib/libjsound.so
472K / 468K    /images/jdk/lib/libawt_xawt.so
564K / 404K   /images/jdk/lib/liblcms.so         <--------------------------
48K / 48K    /images/jdk/lib/libawt_headless.so
12K / 12K    /images/jdk/lib/libjawt.so
1.5M / 900K   /images/jdk/lib/libfontmanager.so  <------------------------------
12K / 12K    /images/jdk/lib/libjaas.so
92K / 92K    /images/jdk/lib/libj2pkcs11.so
16K / 16K    /images/jdk/lib/libattach.so
8.0K / 8.0K   /images/jdk/lib/librmi.so
56K / 56K    /images/jdk/lib/libinstrument.so
16K / 16K    /images/jdk/lib/libprefs.so
52K / 52K    /images/jdk/lib/libj2gss.so
12K / 12K    /images/jdk/lib/libmanagement_agent.so
36K / 32K    /images/jdk/lib/libmanagement.so
16K / 16K    /images/jdk/lib/libextnet.so
20K / 20K    /images/jdk/lib/libj2pcsc.so
40K / 40K    /images/jdk/lib/libmanagement_ext.so
60K / 60K    /images/jdk/lib/libsaproc.so


Bug/webrev :

https://bugs.openjdk.java.net/browse/JDK-8236714

http://cr.openjdk.java.net/~mbaesken/webrevs/8236714.2/


Thanks, Matthias