IR: an option to automatically select the number of lowering threads

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JVMCompilerArguments.kt

@@ -115,11 +115,12 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
     var doNotClearBindingContext: Boolean by FreezableVar(false)
 
     @Argument(
-        value = "-Xir-threads-for-file-lowerings",
+        value = "-Xparallel-backend-threads",
         description = "When using the IR backend, run lowerings by file in N parallel threads.\n" +
-                      "Default value is 1"
+                "0 means use a thread per processor core.\n" +
+                "Default value is 1"
     )
-    var threadsForFileLowerings: String by FreezableVar("1")
+    var parallelBackendThreads: String by FreezableVar("1")
 
     @Argument(value = "-Xmodule-path", valueDescription = "<path>", description = "Paths where to find Java 9+ modules")
     var javaModulePath: String? by NullableStringFreezableVar(null)

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/jvmArguments.kt

@@ -292,7 +292,9 @@ fun CompilerConfiguration.configureAdvancedJvmOptions(arguments: K2JVMCompilerAr
 
     arguments.declarationsOutputPath?.let { put(JVMConfigurationKeys.DECLARATIONS_JSON_PATH, it) }
 
-    put(CommonConfigurationKeys.THREADS_FOR_FILE_LOWERINGS, arguments.threadsForFileLowerings.toIntOrNull() ?: 1)
+    val nThreadsRaw = arguments.parallelBackendThreads.toIntOrNull() ?: 1
+    val nThreads = if (nThreadsRaw == 0) Runtime.getRuntime().availableProcessors() else nThreadsRaw
+    put(CommonConfigurationKeys.PARALLEL_BACKEND_THREADS, nThreads)
 }
 
 fun CompilerConfiguration.configureKlibPaths(arguments: K2JVMCompilerArguments) {