cds is also disabled for minimalVM so testing of cds with LTO probably
has not been done. There are a number of features that minimalVM
excludes such as jvmti, cds and SA (which I think falls under
"services"), and there was very little testing done with these features
individually disabled. They would all at least build (if any one was
disabled) and I think heartbeat testing was done, but probably no more
than that. Also various combinations were not tested, other than the one
combination that minimalVM used. Search for NON_MINIMAL_FEATURES in
hotspot.m4 to see which features are disabled for minimalVM.
Chris
On 1/11/20 5:38 AM, Volker Simonis wrote:
SA pretends to know the exact types of objects in the JVM and for
polymorphic objects it wants to read their vtable from the shared library.
If LTO de-virtulizes methods and thus changes polymorphic to
non-polymorphic types, this won't work. But if LTO can de-virtulizes a
type, maybe you can do that manually (and update the corresponding
representation in the SA), because it doesn't seem to be needed.
Notice that other places in the VM may also rely on this. E.g. CDS stores
Metadata objects in the CDS archive and restores their vtable pointers when
they are loaded. On the other hand, if the CDS tests have passed, this
doesn't seem to be a problem.
Baesken, Matthias <matthias.baes...@sap.com> schrieb am Fr., 10. Jan. 2020,
11:03:
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 ?
And to the serviceability agent experts - any idea how to make the
jdk.hotspot.agent more independent from optimization settings ?
Best regards, Matthias
