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) {

compiler/config/src/org/jetbrains/kotlin/config/CommonConfigurationKeys.kt

@@ -52,7 +52,7 @@ object CommonConfigurationKeys {
     val USE_FIR_EXTENDED_CHECKERS = CompilerConfigurationKey.create<Boolean>("fir extended checkers")
 
     @JvmField
-    val THREADS_FOR_FILE_LOWERINGS =
+    val PARALLEL_BACKEND_THREADS =
         CompilerConfigurationKey.create<Int>("When using the IR backend, run lowerings by file in N parallel threads")
 }
 

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/phaser/PhaseBuilders.kt

@@ -122,7 +122,7 @@ private class PerformByIrFilePhase<Context : CommonBackendContext>(
         context: Context,
         input: IrModuleFragment
     ): IrModuleFragment {
-        val nThreads = context.configuration.get(CommonConfigurationKeys.THREADS_FOR_FILE_LOWERINGS) ?: 1
+        val nThreads = context.configuration.get(CommonConfigurationKeys.PARALLEL_BACKEND_THREADS) ?: 1
         return if (nThreads > 1)
             invokeParallel(phaseConfig, phaserState, context, input, nThreads)
         else

compiler/testData/cli/jvm/extraHelp.out

@@ -90,6 +90,9 @@ where advanced options include:
   -Xno-receiver-assertions   Don't generate not-null assertion for extension receiver arguments of platform types
   -Xno-reset-jar-timestamps  Do not reset jar entry timestamps to a fixed date
   -Xno-unified-null-checks   Use pre-1.4 exception types in null checks instead of java.lang.NPE. See KT-22275 for more details
+  -Xparallel-backend-threads When using the IR backend, run lowerings by file in N parallel threads.
+                             0 means use a thread per processor core.
+                             Default value is 1
   -Xprofile=<profilerPath:command:outputDir>
                              Debug option: Run compiler with async profiler, save snapshots to outputDir, command is passed to async-profiler on start
                              You'll have to provide async-profiler.jar on classpath to use this
@@ -122,9 +125,6 @@ where advanced options include:
                              Suppress deprecation warning about deprecated JVM target versions
   -Xsuppress-missing-builtins-error
                              Suppress the "cannot access built-in declaration" error (useful with -no-stdlib)
-  -Xir-threads-for-file-lowerings
-                             When using the IR backend, run lowerings by file in N parallel threads.
-                             Default value is 1
   -Xuse-ir                   Use the IR backend
   -Xuse-javac                Use javac for Java source and class files analysis
   -Xuse-old-backend          Use the old JVM backend
@@ -193,3 +193,4 @@ where advanced options include:
 
 Advanced options are non-standard and may be changed or removed without any notice.
 OK
+