[K/N] Turn on mimalloc by default for experimental MM

kotlin-native/backend.native/cli.bc/src/org/jetbrains/kotlin/cli/bc/K2Native.kt

@@ -205,7 +205,6 @@ class K2Native : CLICompiler<K2NativeCompilerArguments>() {
                 })
                 put(STATIC_FRAMEWORK, selectFrameworkType(configuration, arguments, outputKind))
                 put(OVERRIDE_CLANG_OPTIONS, arguments.clangOptions.toNonNullList())
-                put(ALLOCATION_MODE, arguments.allocator)
 
                 put(EXPORT_KDOC, arguments.exportKDoc)
 
@@ -340,6 +339,20 @@ class K2Native : CLICompiler<K2NativeCompilerArguments>() {
                         false
                     }
                 })
+                put(ALLOCATION_MODE, when (arguments.allocator) {
+                    null -> {
+                        when (memoryModel) {
+                            MemoryModel.EXPERIMENTAL -> "mimalloc"
+                            else -> "std"
+                        }
+                    }
+                    "std" -> arguments.allocator!!
+                    "mimalloc" -> arguments.allocator!!
+                    else -> {
+                        configuration.report(ERROR, "Expected 'std' or 'mimalloc' for allocator")
+                        "std"
+                    }
+                })
                 put(WORKER_EXCEPTION_HANDLING, when (arguments.workerExceptionHandling) {
                     null -> if (memoryModel == MemoryModel.EXPERIMENTAL) WorkerExceptionHandling.USE_HOOK else WorkerExceptionHandling.LEGACY
                     "legacy" -> WorkerExceptionHandling.LEGACY

kotlin-native/backend.native/cli.bc/src/org/jetbrains/kotlin/cli/bc/K2NativeCompilerArguments.kt

@@ -289,7 +289,7 @@ class K2NativeCompilerArguments : CommonCompilerArguments() {
     var clangOptions: Array<String>? = null
 
     @Argument(value="-Xallocator", valueDescription = "std | mimalloc", description = "Allocator used in runtime")
-    var allocator: String = "std"
+    var allocator: String? = null
 
     @Argument(value = "-Xmetadata-klib", description = "Produce a klib that only contains the declarations metadata")
     var metadataKlib: Boolean = false