Separate javac benchmark to a subproject to sidestep JPMS issue

If a recent JDK is used to run SBT, the JMH source generator
task fails when generating the benchmark sources based on
JavacBenchmark with this failure:

```
Annotation generator had thrown the exception.
java.lang.NoClassDefFoundError: javax/tools/JavaCompiler
	at java.base/java.lang.Class.getDeclaredFields0(Native Method)
	at java.base/java.lang.Class.privateGetDeclaredFields(Class.java:3229)
	at java.base/java.lang.Class.getDeclaredFields(Class.java:2335)
	at org.openjdk.jmh.generators.reflection.RFClassInfo.getFields(RFClassInfo.java:81)
	at org.openjdk.jmh.generators.core.BenchmarkGeneratorUtils.getAllFields(BenchmarkGeneratorUtils.java:128)
	at org.openjdk.jmh.generators.core.StateObjectHandler.validateState(StateObjectHandler.java:126)
	at org.openjdk.jmh.generators.core.BenchmarkGenerator.validateBenchmark(BenchmarkGenerator.java:246)
	at org.openjdk.jmh.generators.core.BenchmarkGenerator.generate(BenchmarkGenerator.java:83)
	at org.openjdk.jmh.generators.bytecode.JmhBytecodeGenerator.main(JmhBytecodeGenerator.java:100)
  | => jat java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:78)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:567)
	at sbt.Run.invokeMain(Run.scala:115)
	at sbt.Run.execute$1(Run.scala:79)
	at sbt.Run.$anonfun$runWithLoader$4(Run.scala:92)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at sbt.util.InterfaceUtil$$anon$1.get(InterfaceUtil.scala:10)
	at sbt.TrapExit$App.run(TrapExit.scala:257)
	at java.base/java.lang.Thread.run(Thread.java:831)
Caused by: java.lang.ClassNotFoundException: javax.tools.JavaCompiler
	at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:433)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:586)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:519)
	... 20 more
```

The workaround is to use an older JDK to build the project and only
use the new JDK as the `-jvm` parameter to the `jmh:run` task.

However, for casual use it is convenient to just switch the JDK in the
shell before starting SBT, so this commit makes that possible for the
common case of running ScalacBenchmark.

Author: retronym

build.sbt

@@ -64,6 +64,15 @@ lazy val compilation = addJmh(project).settings(
   fork in (Test, test) := true // jmh scoped tasks run with fork := true.
 ).settings(addJavaOptions).dependsOn(infrastructure)
 
+lazy val javaCompilation = addJmh(project).settings(
+  description := "Black box benchmark of the java compiler",
+  crossPaths := false,
+  mainClass in (Jmh, run) := Some("scala.bench.ScalacBenchmarkRunner"),
+  libraryDependencies += "com.novocode" % "junit-interface" % "0.11" % Test,
+  testOptions in Test += Tests.Argument(TestFrameworks.JUnit),
+  fork in (Test, test) := true // jmh scoped tasks run with fork := true.
+).settings(addJavaOptions).dependsOn(infrastructure)
+
 lazy val micro = addJmh(project).settings(
   description := "Finer grained benchmarks of compiler internals",
   libraryDependencies += scalaOrganization.value % "scala-compiler" % scalaVersion.value

compilation/src/main/scala/scala/tools/nsc/HotSbtBenchmark.scala

@@ -3,10 +3,11 @@ package scala.tools.nsc
 import java.io._
 import java.nio.file._
 import java.util.concurrent.TimeUnit
-
 import org.openjdk.jmh.annotations.Mode
 import org.openjdk.jmh.annotations._
 
+import scala.bench.IOUtils
+
 @State(Scope.Benchmark)
 @BenchmarkMode(Array(org.openjdk.jmh.annotations.Mode.SampleTime))
 @OutputTimeUnit(TimeUnit.MILLISECONDS)
@@ -166,7 +167,7 @@ class HotSbtBenchmark {
   @TearDown(Level.Trial) def terminate(): Unit = {
     processOutputReader.close()
     sbtProcess.destroyForcibly()
-    BenchmarkUtils.deleteRecursive(tempDir)
-    BenchmarkUtils.deleteRecursive(scalaHome)
+    IOUtils.deleteRecursive(tempDir)
+    IOUtils.deleteRecursive(scalaHome)
   }
 }

compilation/src/main/scala/scala/tools/nsc/ScalacBenchmark.scala

@@ -3,12 +3,13 @@ package scala.tools.nsc
 import java.io.File
 import java.nio.file._
 import java.util.concurrent.TimeUnit
-
 import com.typesafe.config.ConfigFactory
 import org.openjdk.jmh.annotations.Mode._
 import org.openjdk.jmh.annotations._
 import org.openjdk.jmh.profile.AsyncProfiler
+import org.openjdk.jmh.runner.Runner
 
+import scala.bench.IOUtils
 import scala.tools.benchmark.BenchmarkDriver
 
 trait BaseBenchmarkDriver {
@@ -73,7 +74,7 @@ class ScalacBenchmark extends BenchmarkDriver {
     else {
       val sourceDir = findSourceDir
       val sourceAssemblyDir = Paths.get(ConfigFactory.load.getString("sourceAssembly.localdir"))
-      BenchmarkUtils.deleteRecursive(sourceAssemblyDir)
+      IOUtils.deleteRecursive(sourceAssemblyDir)
       BenchmarkUtils.prepareSources(sourceDir, sourceAssemblyDir, scalaVersion)
     }
 
@@ -160,3 +161,15 @@ class HotScalacBenchmark extends ScalacBenchmark {
   @Benchmark
   def compile(): Unit = compileProfiled()
 }
+
+object BakeOff {
+  def main(args: Array[String]): Unit = {
+    import org.openjdk.jmh.runner.options.Options
+    import org.openjdk.jmh.runner.options.OptionsBuilder
+    import org.openjdk.jmh.runner.options.TimeValue
+    import org.openjdk.jmh.runner.options.VerboseMode
+    val baseOpts = new OptionsBuilder().include(classOf[HotScalacBenchmark].getName).warmupTime(TimeValue.milliseconds(200)).measurementTime(TimeValue.milliseconds(200)).warmupIterations(5).measurementIterations(5).forks(2).verbosity(VerboseMode.SILENT).build
+    new OptionsBuilder().parent(baseOpts).jvmArgsPrepend("-classpath", "-")
+    new Runner(baseOpts)
+  }
+}

infrastructure/src/main/java/scala/bench/IOUtils.java

@@ -0,0 +1,28 @@
+package scala.bench;
+
+import java.io.IOException;
+import java.nio.file.FileVisitResult;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.SimpleFileVisitor;
+import java.nio.file.attribute.BasicFileAttributes;
+
+public abstract class IOUtils {
+    private IOUtils() {}
+
+    public static void deleteRecursive(Path directory) throws IOException {
+        if (Files.exists(directory)) {
+            Files.walkFileTree(directory, new SimpleFileVisitor<Path>() {
+                public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
+                    Files.delete(file);
+                    return FileVisitResult.CONTINUE;
+                }
+
+                public FileVisitResult postVisitDirectory(Path dir, IOException exc) throws IOException {
+                    Files.delete(dir);
+                    return FileVisitResult.CONTINUE;
+                }
+            });
+        }
+    }
+}

compilation/src/main/scala/scala/tools/nsc/JavacBenchmark.java renamed to javaCompilation/src/main/scala/scala/tools/nsc/JavacBenchmark.java

@@ -97,8 +97,7 @@ public int bench() {
         tempFile.mkdir();
         tempDir = tempFile;
     }
-    @TearDown(Level.Trial) public void clearTemp() {
-        BenchmarkUtils.deleteRecursive(tempDir.toPath());
+    @TearDown(Level.Trial) public void clearTemp() throws IOException {
+        scala.bench.IOUtils.deleteRecursive(tempDir.toPath());
     }
-
 }