serviceability agent : problems when using gcc LTO (link time optimization)

[Baesken, Matthias]

Hi Magnus, thanks for the info , I already noticed yesterday the setting for arm-32 in the minimal build .
Do you think  we could set it too for the other Linux platforms  in the minimal build  ( serviceability agent is not supported there as well so the  observed issue wouldn’t be a problem).

Best regards, Matthias




On 2020-01-10 11:01, Baesken, Matthias wrote:

Hello,   I recently looked into  the  gcc  lto  optimization mode (see for some details https://gcc.gnu.org/onlinedocs/gccint/LTO-Overview.html  and  http://hubicka.blogspot.com/2019/05/gcc-9-link-time-and-inter-procedural.html  ).

This mode can lead to more compact binaries (~10% smaller)  , it also might bring  small performance improvements  but that wasn't my (main)  goal  .



The changes for this are rather small , one needs to use a recent gcc  , add  -flto   to the compile flags  , for example



--- a/make/autoconf/flags-cflags.m4      Wed Jan 01 03:08:45 2020 +0100

+++ b/make/autoconf/flags-cflags.m4   Wed Jan 08 17:39:10 2020 +0100

@@ -530,8 +530,13 @@

   fi

   if test "x$TOOLCHAIN_TYPE" = xgcc; then

-    TOOLCHAIN_CFLAGS_JVM="$TOOLCHAIN_CFLAGS_JVM -fcheck-new -fstack-protector"

-    TOOLCHAIN_CFLAGS_JDK="-pipe -fstack-protector"

+    TOOLCHAIN_CFLAGS_JVM="$TOOLCHAIN_CFLAGS_JVM -fcheck-new -fstack-protector -flto"

+    TOOLCHAIN_CFLAGS_JDK="-pipe -fstack-protector -flto"



  .... and you have to make sure  to use  gcc-ar  and  gcc-nm instead   of  ar / nm .

Build and test(s)  work,  however with  one exception.

The  serviceability   tests like  serviceability/sa   seems to rely   heavily  on the "normal"   structure  of   libjvm.so   (from what I   understand  e.g. in  LinuxVtblAccess  it is attempted to access  internal symbols  like  _ZTV ).



Errors in the sa  tests look like :





java.lang.InternalError: Metadata does not appear to be polymorphic

         at jdk.hotspot.agent/sun.jvm.hotspot.types.basic.BasicTypeDataBase.findDynamicTypeForAddress(BasicTypeDataBase.java:279)

         at jdk.hotspot.agent/sun.jvm.hotspot.runtime.VirtualBaseConstructor.instantiateWrapperFor(VirtualBaseConstructor.java:102)

         at jdk.hotspot.agent/sun.jvm.hotspot.oops.Metadata.instantiateWrapperFor(Metadata.java:74)

         at jdk.hotspot.agent/sun.jvm.hotspot.memory.SystemDictionary.getClassLoaderKlass(SystemDictionary.java:96)

         at jdk.hotspot.agent/sun.jvm.hotspot.tools.ClassLoaderStats.printClassLoaderStatistics(ClassLoaderStats.java:93)

         at jdk.hotspot.agent/sun.jvm.hotspot.tools.ClassLoaderStats.run(ClassLoaderStats.java:78)

         at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.run(JMap.java:115)

         at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.startInternal(Tool.java:262)

         at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.start(Tool.java:225)

         at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.execute(Tool.java:118)

         at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.main(JMap.java:176)

         at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.runJMAP(SALauncher.java:321)

         at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.main(SALauncher.java:406)



Has anyone experimented with LTO optimization ?

Hi Matthias,

We used to have LTO enabled on the old, closed-source Oracle arm-32 builds. There is still a "link-time-opt" JVM feature present; afaik it still works and adds the -flto flag. The main drawback of this is the *extremely* long link times of libjvm.so.

I don't think servicability was ever supported for that platform, so I'm not surprised this does not work.

/Magnus







And to the  serviceability   agent experts -  any idea  how to make the  jdk.hotspot.agent   more independent from  optimization settings ?





Best regards, Matthias

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