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

[David Holmes]

Hi Matthias,

I have reservations about turning this on by default and with no way to 
control it. This seems like it should be something you have to opt-in to 
initially while we gain some experience with it and ensure there are no 
unexpected side-effects. After that it could be enabled by default.

David

On 15/01/2020 12:07 am, Baesken, Matthias wrote:
> 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
>