Code Generation Performance optimization required in YangTemplate (Xtend bug?)

I have finally found a moment to performance profile the yangtools/mdsal code generation which occurs during a Maven build by attaching Java Mission Control (JMC) from Oracle JDK (not available on OpenJDK) to a local Maven build.

This is similar to earlier YANGTOOLS-629 but here focusing on looking at Maven itself, Maven plugins, code generation and build - so intentionally excluding the SingleFeatureTest and integration tests etc. using my new -Pq quick profile which local developers are increasingly using now.

Here are the exact steps I've used, if anyone wants to reproduce:

$ export JAVA_HOME=/home/vorburger/bin/jdk1.8.0_92-ORACLE
$ export PATH=$JAVA_HOME/bin:$PATH
$ export MAVEN_OPTS="-XX:+UnlockCommercialFeatures -XX:+FlightRecorder -XX:FlightRecorderOptions=defaultrecording=true,dumponexit=true,dumponexitpath=/home/vorburger/dev/ODL/git/netvirt/mvn.jfr"
$ mvno -o -Pq clean install

The result, attached as netvirt-mvn-o-Pq-clean-install.jfr.7z, is interesting IMHO - unless JMC is totally off, it would seem in [2] that we are spending ca. 20% of total Maven build time of e.g. netvirt in Xtend's org.eclipse.xtend2.lib.StringConcatenation called by YangTemplate, just to build those "verboseClassComments" for 'generated classes will include javadoc comments which re useful for users' with the "This class represents the following YANG schema fragment defined in module ...". The 20% just for that seems a lot - that's just a bunch of String concatenations, through the Xtend helper, which is supposed to "allow for efficient [, indentation aware] concatenation of character sequences" - I think either something is wrong with, or we're using it wrong. But the latter seems unlikely, as [1] YangTemplate looks like a pretty standard plain Xtend template class, to me...

As a first step, I'll try to raise this with the Xmen (Xtend community), to see what they have to say about this.

[1] https://github.com/opendaylight/mdsal/blob/32d45af3bfde08f4e8741632ecab43241831d1fe/binding/mdsal-binding-generator-impl/src/main/java/org/opendaylight/yangtools/sal/binding/generator/impl/YangTemplate.xtend

[2] JMC Code top Hot Method TXT copy/paste:

Stack Trace Sample Count Percentage(%)
org.eclipse.xtend2.lib.StringConcatenation.appendSegments(String, int, List, String) 2,407 19.344
org.eclipse.xtend2.lib.StringConcatenation.append(Object, String, int) 2,407 19.344
org.eclipse.xtend2.lib.StringConcatenation.append(Object, String) 2,407 19.344
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeListSchemaNode(ListSchemaNode) 674 5.417
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeDataSchemaNode(DataSchemaNode) 674 5.417
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeDataSchemaNodes(Collection) 609 4.894
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeContSchemaNode(ContainerSchemaNode) 423 3.4
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeDataSchemaNode(DataSchemaNode) 423 3.4
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeDataSchemaNodes(Collection) 223 1.792
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeListSchemaNode(ListSchemaNode) 128 1.029
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeDataSchemaNode(DataSchemaNode) 128 1.029
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeDataSchemaNodes(Collection) 105 0.844
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeListSchemaNode(ListSchemaNode) 34 0.273
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeDataSchemaNode(DataSchemaNode) 34 0.273
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeDataSchemaNodes(Collection) 31 0.249
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeContSchemaNode(ContainerSchemaNode) 11 0.088
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.writeDataSchemaNode(DataSchemaNode) 11 0.088
org.opendaylight.yangtools.sal.binding.generator.impl.YangTemplate.generateYangSnipet(SchemaNode) 8 0.064
org.opendaylight.yangtools.sal.binding.generator.impl.BindingGeneratorImpl.createDescription(SchemaNode, String) 8 0.064