Validate -Xbackend-threads CLI argument value

#KT-51846 Fixed

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

@@ -124,7 +124,7 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
                 "0 means use a thread per processor core.\n" +
                 "Default value is 1"
     )
-    var parallelBackendThreads: String by FreezableVar("1")
+    var backendThreads: String by FreezableVar("1")
 
     @Argument(value = "-Xmodule-path", valueDescription = "<path>", description = "Paths where to find Java 9+ modules")
     var javaModulePath: String? by NullableStringFreezableVar(null)
@@ -505,7 +505,7 @@ Also sets `-jvm-target` value equal to the selected JDK version"""
 
     @Argument(
         value = "-Xlink-via-signatures",
-        description = "Link JVM IR symbols via signatures, instead of descriptors. \n" +
+        description = "Link JVM IR symbols via signatures, instead of descriptors.\n" +
                 "This mode is slower, but can be useful in troubleshooting problems with the JVM IR backend"
     )
     var linkViaSignatures: Boolean by FreezableVar(false)

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

@@ -321,11 +321,27 @@ fun CompilerConfiguration.configureAdvancedJvmOptions(arguments: K2JVMCompilerAr
 
     arguments.declarationsOutputPath?.let { put(JVMConfigurationKeys.DECLARATIONS_JSON_PATH, it) }
 
-    val nThreadsRaw = arguments.parallelBackendThreads.toIntOrNull() ?: 1
+    val nThreadsRaw = parseBackendThreads(arguments.backendThreads, messageCollector)
     val nThreads = if (nThreadsRaw == 0) Runtime.getRuntime().availableProcessors() else nThreadsRaw
+    if (nThreads > 1) {
+        messageCollector.report(LOGGING, "Running backend in parallel with $nThreads threads")
+    }
     put(CommonConfigurationKeys.PARALLEL_BACKEND_THREADS, nThreads)
 }
 
+private fun parseBackendThreads(stringValue: String, messageCollector: MessageCollector): Int {
+    val value = stringValue.toIntOrNull()
+    if (value == null) {
+        messageCollector.report(ERROR, "Cannot parse -Xbackend-threads value: \"$stringValue\". Please use an integer number")
+        return 1
+    }
+    if (value < 0) {
+        messageCollector.report(ERROR, "-Xbackend-threads value cannot be negative")
+        return 1
+    }
+    return value
+}
+
 fun CompilerConfiguration.configureKlibPaths(arguments: K2JVMCompilerArguments) {
     val libraries = arguments.klibLibraries ?: return
     assert(arguments.useIR && !arguments.useOldBackend) { "Klib libraries can only be used with IR backend" }