See JDK-8152842. In a post-modular world, javax.annotation.Generated is not easily accessible:

[WARNING] at org.openjdk.jmh.generators.core.BenchmarkGenerator.generateImport(BenchmarkGenerator.java:582)
   at org.openjdk.jmh.generators.core.BenchmarkGenerator.generateClass(BenchmarkGenerator.java:540)
   at org.openjdk.jmh.generators.core.BenchmarkGenerator.generate(BenchmarkGenerator.java:94)
   at org.openjdk.jmh.generators.BenchmarkProcessor.process(BenchmarkProcessor.java:58)
   at com.sun.tools.javac.processing.JavacProcessingEnvironment.callProcessor(jdk.compiler@9-ea/JavacProcessingEnvironment.java:958)
   at com.sun.tools.javac.processing.JavacProcessingEnvironment.discoverAndRunProcs(jdk.compiler@9-ea/JavacProcessingEnvironment.java:874)
   at com.sun.tools.javac.processing.JavacProcessingEnvironment.access$2100(jdk.compiler@9-ea/JavacProcessingEnvironment.java:106)
   at com.sun.tools.javac.processing.JavacProcessingEnvironment$Round.run(jdk.compiler@9-ea/JavacProcessingEnvironment.java:1181)
   at com.sun.tools.javac.processing.JavacProcessingEnvironment.doProcessing(jdk.compiler@9-ea/JavacProcessingEnvironment.java:1289)
   at com.sun.tools.javac.main.JavaCompiler.processAnnotations(jdk.compiler@9-ea/JavaCompiler.java:1229)
   at com.sun.tools.javac.main.JavaCompiler.compile(jdk.compiler@9-ea/JavaCompiler.java:903)
   at com.sun.tools.javac.main.Main.compile(jdk.compiler@9-ea/Main.java:261)
   at com.sun.tools.javac.main.Main.compile(jdk.compiler@9-ea/Main.java:143)
   at com.sun.tools.javac.Main.compile(jdk.compiler@9-ea/Main.java:55)
   at com.sun.tools.javac.Main.main(jdk.compiler@9-ea/Main.java:41)
  Caused by: java.lang.ClassNotFoundException: javax.annotation.Generated
   at java.net.URLClassLoader.findClass(java.base@9-ea/URLClassLoader.java:384)
   at java.lang.ClassLoader.loadClass(java.base@9-ea/ClassLoader.java:486)
   at java.lang.ClassLoader.loadClass(java.base@9-ea/ClassLoader.java:419)
   ... 15 more

The workaround is to avoid emitting @Generated altogether.