Hello , I can comment on the code size . This is what I get when comparing a build without and with -flto .
gcc7 linux x86_64 product build, normal / with -flto ---------------------------------------------------------------------------------- du -sh on the *.so files gives : 16K / 16K ./lib/libattach.so 48K / 44K ./lib/libawt_headless.so 752K / 760K ./lib/libawt.so <------------------ this one gets a bit larger with flto 472K / 456K ./lib/libawt_xawt.so <------------------ small gain 36K / 32K ./lib/libdt_socket.so 16K /16K ./lib/libextnet.so 1.5M / 824K ./lib/libfontmanager.so <------------------ HUGE gain 784K / 792K ./lib/libfreetype.so <------------------ this one gets a bit larger with flto 56K / 56K ./lib/libinstrument.so 52K / 52K ./lib/libj2gss.so 20K / 20K ./lib/libj2pcsc.so 92K / 84K ./lib/libj2pkcs11.so 12K / 12k ./lib/libjaas.so 260K / 244K ./lib/libjavajpeg.so <----------------- small gain 196K / 188K ./lib/libjava.so 12K / 12K ./lib/libjawt.so 280K / 256K ./lib/libjdwp.so <----------------- small gain 144K / 140K ./lib/libjimage.so 84K / 76K ./lib/libjli.so 16K / 16K ./lib/libjsig.so 88K / 80K ./lib/libjsound.so 564K / 420K ./lib/liblcms.so <----------------- large gain 12K / 12K ./lib/libmanagement_agent.so 40K / 36K ./lib/libmanagement_ext.so 36K / 32K ./lib/libmanagement.so 576K / 496K ./lib/libmlib_image.so <----------------- large gain 112K / 108K ./lib/libnet.so 100K / 100K ./lib/libnio.so 16K / 16K ./lib/libprefs.so 8.0K / 8.0K ./lib/librmi.so 60K / 60K ./lib/libsaproc.so 36K / 32K ./lib/libsctp.so 368K / 212K ./lib/libsplashscreen.so <----------------- large gain 320K / 296K ./lib/libsunec.so <----------------- medium gain 72K / 72K ./lib/libverify.so 44K / 44K ./lib/libzip.so 16K / 16K ./lib/server/libjsig.so 23M / 17M ./lib/server/libjvm.so <----------------- big gain maybe because it is C++ ? So for some libs you see 10% and more , but not for all . But most large libs like libjvm.so, libfontmanager.so or liblcms.so we see good results regarding reduced code size. I Cannot say much about performance improvements , probably it would be small . For SPEC you find something at http://hubicka.blogspot.com/2019/05/gcc-9-link-time-and-inter-procedural.html (not that these results would say too much about JVM performance ). Best regards, Matthias From: Volker Simonis <volker.simo...@gmail.com> Sent: Mittwoch, 15. Januar 2020 14:40 To: Aleksei Voitylov <aleksei.voity...@bell-sw.com> Cc: Baesken, Matthias <matthias.baes...@sap.com>; Magnus Ihse Bursie <magnus.ihse.bur...@oracle.com>; serviceability-dev@openjdk.java.net; build-dev <build-...@openjdk.java.net>; hotspot-...@openjdk.java.net Subject: Re: serviceability agent : problems when using gcc LTO (link time optimization) While we are speaking about all the drawbacks of LTO, it's still not clear what the benefits are? In the very first mail Matthias mentioned that there might be performance improvements but that performance is not the main driving factor behind this initiative. So is it the reduced code size (Matthias mentioned something around ~10%)? It would be nice to see some real numbers on various platform for both, the performance improvements for native parts like JIT/GC as well as for the size reduction. Aleksei Voitylov <aleksei.voity...@bell-sw.com<mailto:aleksei.voity...@bell-sw.com>> schrieb am Di., 14. Jan. 2020, 09:54: On 14/01/2020 19:57, Baesken, Matthias wrote: > Hello Magnus and Aleksei, thanks for the input . > > The times you provided really look like they make a big difference at least > for people often building minimal-vm . > Guess I have to measure myself a bit (maybe the difference is not that big > on our linux s390x / ppc64(le) ) . > >> If the change to enable lto by default is proposed, what would be the >> recommended strategy for development? >> > Probably we should a) do not enable it by default but just make sure it > can be enabled easily and works for the minimal-vm That would be welcome. I have high hopes to LTO the VM some time by default, and the tendency observed is that the compiler time overhead for GCC becomes smaller. At the same time there is no reason why vendors that invested in testing and can absorb the build time hit could provide binaries with LTO built VMs by passing an additional option flag. > or b) take it easy to disable it for local development. > > Best regards, Matthias > > > >> Magnus, Matthias, >> >> for me, lto is a little heavyweight for development. x86_64 build time >> with gcc 7: >> >> Server 1m32.484s >> Server+Minimal 1m42.166s >> Server+Minimal (--with-jvm-features="link-time-opt") 5m29.422s >> >> If the change to enable lto by default is proposed, what would be the >> recommended strategy for development? >> >> For ARM32 Minimal, please keep in mind that it's not uncommon to disable >> LTO plugin in commodity ARM32 gcc compiler distributions, so for some it >> does not matter what settings we have in OpenJDK. I believe there could >> be other reasons for that on top of build time (bugs?). >>
