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

[Erik Joelsson]

Given the discussion regarding lto on hotspot and the extreme increased 
build time, have you noticed any difference in build times with this patch?

/Erik

On 2020-01-15 00:27, Baesken, Matthias wrote:
> Hi Erik, thanks for the review and for forwarding ,  you are correct  corelibs-dev is probably  interested in this as well .
>
> Best regards, Matthias
>
>
>> (adding core-libs-dev)
>>
>> Change looks good to me, but would like input from at least someone in
>> core-libs.
>>
>> /Erik
>>
>> On 2020-01-14 06:07, 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