Writing Your First Kotlin Compiler Plugin

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Writing Your First Kotlin Compiler Plugin Kevin Most

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A brief intro

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Are these basically annotation processors?

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Are these basically annotation processors? • Annotation Processors:

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Are these basically annotation processors? • Annotation Processors: • Your code runs at compile-time

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Are these basically annotation processors? • Annotation Processors: • Your code runs at compile-time • Public, documented API

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Are these basically annotation processors? • Annotation Processors: • Your code runs at compile-time • Public, documented API • Emit Java source code

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Are these basically annotation processors? • Annotation Processors: • Your code runs at compile-time • Public, documented API • Emit Java source code • Works on Kotlin/Java source code

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Are these basically annotation processors? • Annotation Processors: • Your code runs at compile-time • Public, documented API • Emit Java source code • Works on Kotlin/Java source code • Multiplatform not supported • Compiler Plugins: • Your code runs at compile-time • Private, undocumented API

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Why write compiler plugins?

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Why write compiler plugins? • Incredibly powerful API; you can modify function/class internals

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Why write compiler plugins? • Incredibly powerful API; you can modify function/class internals • Enables you to solve new classes of metaprogramming problems

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Why write compiler plugins? • Incredibly powerful API; you can modify function/class internals • Enables you to solve new classes of metaprogramming problems • Annotation processors are JVM-only, while compiler plugins aren't

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Why NOT write compiler plugins?

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Why NOT write compiler plugins? • Annotation processors are much easier to write (if you only care about JVM)

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Why NOT write compiler plugins? • Annotation processors are much easier to write (if you only care about JVM) • Compiler plugins are a lot of work. You need to write:

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Why NOT write compiler plugins? • Annotation processors are much easier to write (if you only care about JVM) • Compiler plugins are a lot of work. You need to write: • An IntelliJ plugin (if creating synthetic members) • A Gradle (or Maven, or other build tool) plugin

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Examples of compiler plugins

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Examples of compiler plugins • allopen: Modiﬁes annotated class to be open

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Examples of compiler plugins • allopen: Modiﬁes annotated class to be open • noarg: Modiﬁes annotated class to have a zero-argument constructor

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Examples of compiler plugins • allopen: Modiﬁes annotated class to be open • noarg: Modiﬁes annotated class to have a zero-argument constructor • android-extensions: findViewById(R.id.foo) aliased, and automatic Parcelable impl generation via @Parcelize • kotlin-serialization: Automatic generation of Serializable impl

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Examples of compiler plugins

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• All existing compiler plugins are 1st party (github.com/JetBrains/kotlin) Examples of compiler plugins

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• All existing compiler plugins are 1st party (github.com/JetBrains/kotlin) • plugins/{name}/ ... for the actual plugin business logic Examples of compiler plugins

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• All existing compiler plugins are 1st party (github.com/JetBrains/kotlin) • plugins/{name}/ ... for the actual plugin business logic • libraries/tools/kotlin-{name}/ ... for the Gradle wrappers Examples of compiler plugins

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Plugin Architecture s Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Gradle API (totally unrelated to Kotlin)

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Gradle API (totally unrelated to Kotlin) • Provides an entry point from a build.gradle script

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Gradle API (totally unrelated to Kotlin) • Provides an entry point from a build.gradle script • Allows conﬁguration via Gradle extensions

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • The interface between the Gradle and Kotlin APIs

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • The interface between the Gradle and Kotlin APIs • Read Gradle extension options

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • The interface between the Gradle and Kotlin APIs • Read Gradle extension options • Write out Kotlin SubpluginOptions • Deﬁne the compiler plugin's ID (internal unique key)

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Reads kotlinc -Xplugin args

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Reads kotlinc -Xplugin args • Subplugin options actually get passed through this pipeline

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Reads kotlinc -Xplugin args • Subplugin options actually get passed through this pipeline • Write CompilerConfigurationKeys

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Read CompilerConfigurationKeys

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Read CompilerConfigurationKeys • Register Extensions

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Generates code (ﬁnally!)

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Generates code (ﬁnally!) • Multiple types of extensions, such as:

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Plugin Architecture Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension • Generates code (ﬁnally!) • Multiple types of extensions, such as: • ExpressionCodegenExtension • ClassBuilderInterceptorExtension • StorageComponentContainerContributor

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Let's build our own!

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Let's build our own!

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Let's build our own! • We'll build a compiler plugin that traces method calls

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Let's build our own! • We'll build a compiler plugin that traces method calls • A method annotated with a debug-log annotation will have its method body modiﬁed to include logging

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Let's build our own! • We'll build a compiler plugin that traces method calls • A method annotated with a debug-log annotation will have its method body modified to include logging • Could not be an annotation processor; modifies the function body • Prior art: • Hugo: github.com/jakewharton/hugo • Firebase Performance Monitoring: firebase.google.com/docs/perf-mon

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fun prime(n: Int): Long { println("⇢ prime(n=$n)") val startTime = System.currentTimeMillis() val result = primeNumberSequence.take(n).last() val timeToRun = System.currentTimeMillis() - startTime println("⇠ prime [ran in $timeToRun ms]") return result } The goal

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@DebugLog fun prime(n: Int): Long = primeNumberSequence.take(n).last() fun prime(n: Int): Long { println("⇢ prime(n=$n)") val startTime = System.currentTimeMillis() val result = primeNumberSequence.take(n).last() val timeToRun = System.currentTimeMillis() - startTime println("⇠ prime [ran in $timeToRun ms]") return result } The goal

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Let's build our own!

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Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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gradle-plugin/build.gradle apply plugin: "java-gradle-plugin" apply plugin: "org.jetbrains.kotlin.jvm" apply plugin: "kotlin-kapt" gradlePlugin { plugins { simplePlugin { id = "debuglog.plugin" implementationClass = "debuglog.DebugLogGradlePlugin" } } } dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib:$ktVersion" implementation "org.jetbrains.kotlin:kotlin-gradle-plugin-api:$ktVersion" compileOnly "com.google.auto.service:auto-service:1.0-rc4" kapt "com.google.auto.service:auto-service:1.0-rc4" }

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apply plugin: "java-gradle-plugin" apply plugin: "org.jetbrains.kotlin.jvm" apply plugin: "kotlin-kapt" gradlePlugin { plugins { simplePlugin { id = "debuglog.plugin" implementationClass = "debuglog.DebugLogGradlePlugin" } } } dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib:$ktVersion" implementation "org.jetbrains.kotlin:kotlin-gradle-plugin-api:$ktVersion" compileOnly "com.google.auto.service:auto-service:1.0-rc4" kapt "com.google.auto.service:auto-service:1.0-rc4" } gradle-plugin/build.gradle

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apply plugin: "java-gradle-plugin" apply plugin: "org.jetbrains.kotlin.jvm" apply plugin: "kotlin-kapt" gradlePlugin { plugins { simplePlugin { id = "debuglog.plugin" // `apply plugin: "debuglog.plugin"` implementationClass = "debuglog.DebugLogGradlePlugin" } } } dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib:$ktVersion" implementation "org.jetbrains.kotlin:kotlin-gradle-plugin-api:$ktVersion" compileOnly "com.google.auto.service:auto-service:1.0-rc4" kapt "com.google.auto.service:auto-service:1.0-rc4" } gradle-plugin/build.gradle

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apply plugin: "java-gradle-plugin" apply plugin: "org.jetbrains.kotlin.jvm" apply plugin: "kotlin-kapt" gradlePlugin { plugins { simplePlugin { id = "debuglog.plugin" // `apply plugin: "debuglog.plugin"` implementationClass = "debuglog.DebugLogGradlePlugin" // entry-point class } } } dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib:$ktVersion" implementation "org.jetbrains.kotlin:kotlin-gradle-plugin-api:$ktVersion" compileOnly "com.google.auto.service:auto-service:1.0-rc4" kapt "com.google.auto.service:auto-service:1.0-rc4" } gradle-plugin/build.gradle

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gradle-plugin/build.gradle apply plugin: "java-gradle-plugin" apply plugin: "org.jetbrains.kotlin.jvm" apply plugin: "kotlin-kapt" gradlePlugin { plugins { simplePlugin { id = "debuglog.plugin" // `apply plugin: "debuglog.plugin"` implementationClass = "debuglog.DebugLogGradlePlugin" // entry-point class } } } dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib:$ktVersion" implementation "org.jetbrains.kotlin:kotlin-gradle-plugin-api:$ktVersion" compileOnly "com.google.auto.service:auto-service:1.0-rc4" kapt "com.google.auto.service:auto-service:1.0-rc4" }

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gradle-plugin/build.gradle apply plugin: "java-gradle-plugin" apply plugin: "org.jetbrains.kotlin.jvm" apply plugin: "kotlin-kapt" gradlePlugin { plugins { simplePlugin { id = "debuglog.plugin" // `apply plugin: "debuglog.plugin"` implementationClass = "debuglog.DebugLogGradlePlugin" // entry-point class } } } dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib:$ktVersion" implementation "org.jetbrains.kotlin:kotlin-gradle-plugin-api:$ktVersion" compileOnly "com.google.auto.service:auto-service:1.0-rc4" kapt "com.google.auto.service:auto-service:1.0-rc4" }

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Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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class DebugLogGradlePlugin : org.gradle.api.Plugin { override fun apply(project: Project) { project.extensions.create( "debugLog", DebugLogGradleExtension ::class.java ) } } open class DebugLogGradleExtension { var enabled: Boolean = true var annotations: List = emptyList() }

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Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin : KotlinGradleSubplugin { override fun isApplicable( project: Project, task: AbstractCompile ): Boolean = TODO() override fun getCompilerPluginId(): String = TODO() override fun getPluginArtifact(): SubpluginArtifact = TODO() override fun apply(project: Project, /* ... */): List { TODO() } }

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin : KotlinGradleSubplugin { override fun isApplicable( project: Project, task: AbstractCompile ): Boolean = project.plugins.hasPlugin(DebugLogGradlePlugin ::class.java) override fun getCompilerPluginId(): String = TODO() override fun getPluginArtifact(): SubpluginArtifact = TODO() override fun apply(project: Project, /* ... */): List { TODO() } }

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin : KotlinGradleSubplugin { override fun isApplicable( project: Project, task: AbstractCompile ): Boolean = project.plugins.hasPlugin(DebugLogGradlePlugin ::class.java) override fun getCompilerPluginId(): String = TODO() override fun getPluginArtifact(): SubpluginArtifact = TODO() override fun apply(project: Project, /* ... */): List { TODO() } }

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin : KotlinGradleSubplugin { override fun isApplicable( project: Project, task: AbstractCompile ): Boolean = project.plugins.hasPlugin(DebugLogGradlePlugin ::class.java) override fun getCompilerPluginId(): String = TODO() override fun getPluginArtifact(): SubpluginArtifact = TODO() override fun apply(project: Project, /* ... */): List { TODO() } }

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin : KotlinGradleSubplugin { override fun isApplicable( project: Project, task: AbstractCompile ): Boolean = project.plugins.hasPlugin(DebugLogGradlePlugin ::class.java) override fun getCompilerPluginId(): String = "debuglog" override fun getPluginArtifact(): SubpluginArtifact = TODO() override fun apply(project: Project, /* ... */): List { TODO() } }

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin : KotlinGradleSubplugin { override fun isApplicable( project: Project, task: AbstractCompile ): Boolean = project.plugins.hasPlugin(DebugLogGradlePlugin ::class.java) override fun getCompilerPluginId(): String = "debuglog" override fun getPluginArtifact(): SubpluginArtifact = TODO() override fun apply(project: Project, /* ... */): List { TODO() } }

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin : KotlinGradleSubplugin { override fun isApplicable( project: Project, task: AbstractCompile ): Boolean = project.plugins.hasPlugin(DebugLogGradlePlugin ::class.java) override fun getCompilerPluginId(): String = "debuglog" override fun getPluginArtifact(): SubpluginArtifact = TODO() override fun apply(project: Project, /* ... */): List { TODO() }2 }1

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin : KotlinGradleSubplugin { override fun isApplicable( project: Project, task: AbstractCompile ): Boolean = project.plugins.hasPlugin(DebugLogGradlePlugin ::class.java) override fun getCompilerPluginId(): String = "debuglog" override fun getPluginArtifact(): SubpluginArtifact = SubpluginArtifact( groupId = "debuglog", artifactId = "kotlin-plugin", version = "0.0.1" ) override fun apply(project: Project, /* ... */): List { TODO() }2 }1

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin : KotlinGradleSubplugin { override fun isApplicable( project: Project, task: AbstractCompile ): Boolean = project.plugins.hasPlugin(DebugLogGradlePlugin ::class.java) override fun getCompilerPluginId(): String = "debuglog" override fun getPluginArtifact(): SubpluginArtifact = SubpluginArtifact( groupId = "debuglog", artifactId = "kotlin-plugin", version = "0.0.1" ) override fun apply(project: Project, /* ... */): List { TODO() } }

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin1:1KotlinGradleSubplugin1{1 override fun isApplicable( project: Project, task: AbstractCompile ): Boolean = project.plugins.hasPlugin(DebugLogGradlePlugin ::class.java) override fun getCompilerPluginId(): String = "debuglog" override fun getPluginArtifact(): SubpluginArtifact = SubpluginArtifact( groupId = "debuglog", artifactId = "kotlin-plugin", version = "0.0.1" ) override fun apply(project: Project, /* ... */): List1{ TODO() }2 }1

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@AutoService(KotlinGradleSubplugin ::class) // don't forget! class DebugLogGradleSubplugin1:1KotlinGradleSubplugin1{1 // other method impls override fun apply(project: Project, /* ... */): List1{ val extension = project.extensions.findByType() ?: DebugLogGradleExtension() if (extension.enabled && extension.annotations.isEmpty()) error("DebugLog is enabled, but no annotations were set") val annotationOptions = extension.annotations .map { SubpluginOption(key = "debugLogAnnotation", value = it) } val enabledOption = SubpluginOption( key = "enabled", value = extension.enabled.toString()) return annotationOptions + enabledOption }2 }1

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Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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kotlin-plugin/build.gradle apply plugin: "org.jetbrains.kotlin.jvm" apply plugin: "kotlin-kapt" dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib:$ktVersion" compileOnly "org.jetbrains.kotlin:kotlin-compiler-embeddable:$ktVersion" compileOnly "com.google.auto.service:auto-service:1.0-rc4" kapt "com.google.auto.service:auto-service:1.0-rc4" }

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Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = TODO() override val pluginOptions: Collection = listOf( ) override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) { TODO() } }

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = "debuglog" // same as ID from subplugin override val pluginOptions: Collection = listOf( ) override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) { TODO() } }

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = "debuglog" // same as ID from subplugin override val pluginOptions: Collection = listOf( ) override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) { TODO() } }

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = "debuglog" // same as ID from subplugin override val pluginOptions: Collection = listOf( )P override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) { TODO() }2 }1

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = "debuglog" // same as ID from subplugin override val pluginOptions: Collection = listOf( CliOption("enabled", "", "whether plugin is enabled"), CliOption( "debugLogAnnotation", "", "debug-log annotation names", required = true, allowMultipleOccurrences = true))P override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) { TODO() }2 }1

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = "debuglog" // same as ID from subplugin override val pluginOptions: Collection = listOf( CliOption("enabled", "", "whether plugin is enabled"), CliOption( "debugLogAnnotation", "", "debug-log annotation names", required = true, allowMultipleOccurrences = true))P override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) { TODO() } }

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = "debuglog" // same as ID from subplugin override val pluginOptions: Collection = listOf( CliOption("enabled", "", "whether plugin is enabled"), CliOption( "debugLogAnnotation", "", "debug-log annotation names", required = true, allowMultipleOccurrences = true))P override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) { TODO() } }

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = "debuglog" // same as ID from subplugin override val pluginOptions: Collection = listOf( CliOption("enabled", "", "whether plugin is enabled"), CliOption( "debugLogAnnotation", "", "debug-log annotation names", required = true, allowMultipleOccurrences = true))P override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) = when (option.name) { } }

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = "debuglog" // same as ID from subplugin override val pluginOptions: Collection = listOf( CliOption("enabled", "", "whether plugin is enabled"), CliOption( "debugLogAnnotation", "", "debug-log annotation names", required = true, allowMultipleOccurrences = true))P override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) = when (option.name) { "enabled" -> configuration.put(KEY_ENABLED, value.toBoolean()) }2 }1

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = "debuglog" // same as ID from subplugin override val pluginOptions: Collection = listOf( CliOption("enabled", "", "whether plugin is enabled"), CliOption( "debugLogAnnotation", "", "debug-log annotation names", required = true, allowMultipleOccurrences = true))P override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) = when (option.name) { "enabled" -> configuration.put(KEY_ENABLED, value.toBoolean()) "debugLogAnnotation" -> configuration.appendList(KEY_ANNOTATIONS, value) }2 }1

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@AutoService(CommandLineProcessor ::class) class DebugLogCommandLineProcessor : CommandLineProcessor { override val pluginId: String = "debuglog" // same as ID from subplugin override val pluginOptions: Collection = listOf( CliOption("enabled", "", "whether plugin is enabled"), CliOption( "debugLogAnnotation", "", "debug-log annotation names", required = true, allowMultipleOccurrences = true))P override fun processOption( option: CliOption, value: String, configuration: CompilerConfiguration ) = when (option.name) { "enabled" -> configuration.put(KEY_ENABLED, value.toBoolean()) "debugLogAnnotation" -> configuration.appendList(KEY_ANNOTATIONS, value) else -> error("Unexpected config option ${option.name}") }2

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Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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@AutoService(ComponentRegistrar ::class) class DebugLogComponentRegistrar : ComponentRegistrar { override fun registerProjectComponents( project: MockProject, configuration: CompilerConfiguration ) { if (configuration[KEY_ENABLED] == false) { return } ClassBuilderInterceptorExtension.registerExtension( project, DebugLogClassGenerationInterceptor( debugLogAnnotations = configuration[KEY_ANNOTATIONS] ?: error("debuglog plugin requires at least one annotation class option passed to it") ) ) } }

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Plugin Subplugin CommandLineProcessor ComponentRegistrar Extension Extension

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class DebugLogClassGenerationInterceptor( val debugLogAnnotations: List ) : ClassBuilderInterceptorExtension { override fun interceptClassBuilderFactory( interceptedFactory: ClassBuilderFactory, bindingContext: BindingContext, diagnostics: DiagnosticSink ): ClassBuilderFactory = TODO() } kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassGenerationInterceptor.kt

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassGenerationInterceptor.kt class DebugLogClassGenerationInterceptor( val debugLogAnnotations: List ) : ClassBuilderInterceptorExtension { override fun interceptClassBuilderFactory( interceptedFactory: ClassBuilderFactory, bindingContext: BindingContext, diagnostics: DiagnosticSink ): ClassBuilderFactory = object: ClassBuilderFactory by interceptedFactory { override fun newClassBuilder(origin: JvmDeclarationOrigin) = DebugLogClassBuilder( annotations = debugLogAnnotations, delegateBuilder = interceptedFactory.newClassBuilder(origin)) } }1

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt private class DebugLogClassBuilder( val annotations: List, delegateBuilder: ClassBuilder ) : DelegatingClassBuilder(delegateBuilder) { override fun newMethod( origin: JvmDeclarationOrigin, ... ): MethodVisitor { val original = super.newMethod(origin, ...) val function = origin.descriptor as? FunctionDescriptor ?: return original if (annotations.none { descriptor.annotations.hasAnnotation(it) }) { return original } }1 }2

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt return object : MethodVisitor(Opcodes.ASM5, original) { }3

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt return object : MethodVisitor(Opcodes.ASM5, original) { override fun visitCode() { super.visitCode() InstructionAdapter(this).apply { TODO("on method entry") } }4 override fun visitInsn(opcode: Int) { when (opcode) { RETURN /* void */, ARETURN /* object */, IRETURN /* int */ -> { InstructionAdapter(this).apply { TODO("on method exit") } } } super.visitInsn(opcode) } }3

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What now?

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What now? • You write bytecode

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What now? • You write bytecode • Uses the ObjectWeb ASM API

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What now? • You write bytecode • Uses the ObjectWeb ASM API • Neither related to ASM (assembly) or Web ASM (wasm) in any way

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What now? • You write bytecode • Uses the ObjectWeb ASM API • Neither related to ASM (assembly) or Web ASM (wasm) in any way • An API for modifying JVM bytecode

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What now? • You write bytecode • Uses the ObjectWeb ASM API • Neither related to ASM (assembly) or Web ASM (wasm) in any way • An API for modifying JVM bytecode • The JVM is a stack machine

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What now? • You write bytecode • Uses the ObjectWeb ASM API • Neither related to ASM (assembly) or Web ASM (wasm) in any way • An API for modifying JVM bytecode • The JVM is a stack machine • One stack that methods operate upon

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What does bytecode look like? fun printSimpleSum() { val sum = v1() + v2() println("sum of values was $sum") }

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What does bytecode look like? fun printSimpleSum() { val sum = v1() + v2() println("sum of values was $sum") } INVOKESTATIC myapp/RunnerKt.v1 ()I INVOKESTATIC myapp/RunnerKt.v2 ()I IADD ISTORE 1 GETSTATIC j/l/System.out : Lj/io/PrintStream; NEW j/l/StringBuilder DUP INVOKESPECIAL j/l/StringBuilder. ()V LDC "sum of values was " INVOKEVIRTUAL j/l/StringBuilder.append (Lj/l/String;)Lj/l/StringBuilder; ILOAD 1 INVOKEVIRTUAL j/l/StringBuilder.append (I)Lj/l/StringBuilder; INVOKEVIRTUAL j/l/StringBuilder.toString ()Lj/l/String; INVOKEVIRTUAL j/io/PrintStream.println (Lj/l/String;)V

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Back to our MethodVisitor!

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt return object : MethodVisitor(Opcodes.ASM5, original) { override fun visitCode() { super.visitCode() InstructionAdapter(this).apply { TODO("on method entry") } }4 override fun visitInsn(opcode: Int) { when (opcode) { RETURN , ARETURN, IRETURN -> { InstructionAdapter(this).apply { TODO("on method exit") } } } super.visitInsn(opcode) } }3

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt return object : MethodVisitor(Opcodes.ASM5, original) { override fun visitCode() { super.visitCode() InstructionAdapter(this).apply { TODO("on method entry") } } override fun visitInsn(opcode: Int) { when (opcode) { RETURN , ARETURN, IRETURN -> { InstructionAdapter(this).apply { TODO("on method exit") } } } super.visitInsn(opcode) } }

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).apply { TODO("on method entry") }1

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).apply { getstatic("j/l/System", "out", "Ljava/io/PrintStream;") }1

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).apply { getstatic("j/l/System", "out", "Ljava/io/PrintStream;") }1 stdout PrintStream

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).apply { getstatic("j/l/System", "out", "Ljava/io/PrintStream;") anew("j/l/StringBuilder") }1 stdout PrintStream

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).apply { getstatic("j/l/System", "out", "Ljava/io/PrintStream;") anew("j/l/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇢ ${function.name}(") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/Object;)Lj/l/StringBuilder;") function.valueParameters.forEachIndexed { i, param -> visitLdcInsn(" ${param.name}=") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") visitVarInsn(ALOAD, i + 1) invokevirtual("j/l/StringBuilder", "append", "(Lj/l/Object;)Lj/l/SB;") } }1 stdout PrintStream StringBuilder

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).apply {L getstatic("j/l/System", "out", "Ljava/io/PrintStream;") anew("j/l/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇢ ${function.name}(") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/Object;)Lj/l/StringBuilder;") function.valueParameters.forEachIndexed { i, param -> visitLdcInsn(" ${param.name}=") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") visitVarInsn(ALOAD, i + 1) invokevirtual("j/l/StringBuilder", "append", "(Lj/l/Object;)Lj/l/SB;") }R invokevirtual("j/l/StringBuilder", "toString", "()Lj/l/String;") invokevirtual("j/io/PrintStream", "println", "(Lj/l/String;)V") }1

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).apply {L // ... method-trace-printing code }1

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).apply {L // ... method-trace-printing code invokestatic("j/l/System", "currentTimeMillis", "()J") }1

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).apply {L // ... method-trace-printing code invokestatic("j/l/System", "currentTimeMillis", "()J") }1 currentTimeMillis

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyL{L // ... method-trace-printing code invokestatic("j/l/System", "currentTimeMillis", "()J") store(9001, LONG_TYPE) }1

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyL{L // ... method-trace-printing code // ... timestamp-storing code }W

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt return object : MethodVisitor(Opcodes.ASM5, original) { override fun visitCode() { super.visitCode() InstructionAdapter(this).applyL{L // ... method-trace-printing code // ... timestamp-storing code }W }4 override fun visitInsn(opcode: Int) { when (opcode) { RETURN , ARETURN, IRETURN -> { InstructionAdapter(this).apply { TODO("on method exit") } } } super.visitInsn(opcode) } }3

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt return object : MethodVisitor(Opcodes.ASM5, original) { override fun visitCode() { super.visitCode() InstructionAdapter(this).applyL{L // ... method-trace-printing code // ... timestamp-storing code }W }4 override fun visitInsn(opcode: Int) { when (opcode) { RETURN , ARETURN, IRETURN -> { InstructionAdapter(this).apply { TODO("on method exit") } } } super.visitInsn(opcode) } }3

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt return object : MethodVisitor(Opcodes.ASM5, original) { override fun visitCode() { super.visitCode() InstructionAdapter(this).apply { // ... method-trace-printing code // ... timestamp-storing code }W }4 override fun visitInsn(opcode: Int) { when (opcode) { RETURN , ARETURN, IRETURN -> { InstructionAdapter(this).applyM{MTODO("on method exit")M}M } } super.visitInsn(opcode) } }3

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M TODO("on method exit") }M

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") }M

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") }M stdout PrintStream

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") }M stdout PrintStream StringBuilder (current - start) time

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") }M stdout PrintStream StringBuilder

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") }M stdout PrintStream StringBuilder " ms]"

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") }M stdout PrintStream StringBuilder " ms]"

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") }M stdout PrintStream

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") }M stdout PrintStream

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") }M stdout PrintStream StringBuilder

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") invokevirtual("j/l/StringBuilder", "toString", "()Lj/l/String;") }M stdout PrintStream StringBuilder

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") invokevirtual("j/l/StringBuilder", "toString", "()Lj/l/String;") }M stdout PrintStream

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") invokevirtual("j/l/StringBuilder", "toString", "()Lj/l/String;") }M stdout PrintStream String

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") invokevirtual("j/l/StringBuilder", "toString", "()Lj/l/String;") invokevirtual("j/io/PrintStream", "println", "(Lj/l/String;)V") }M stdout PrintStream String

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") invokevirtual("j/l/StringBuilder", "toString", "()Lj/l/String;") invokevirtual("j/io/PrintStream", "println", "(Lj/l/String;)V") }M

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M getstatic("j/l/System", "out", "Lj/io/PrintStream;") anew("java/lang/StringBuilder") dup() invokespecial("j/l/StringBuilder", "", "()V") visitLdcInsn("⇠ ${function.name} [ran in ") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/StringBuilder;") invokestatic("j/l/System", "currentTimeMillis", "()J") load(9001, LONG_TYPE) sub(LONG_TYPE) invokevirtual("j/l/StringBuilder", "append", "(J)Lj/l/StringBuilder;") visitLdcInsn(" ms]") invokevirtual("j/l/StringBuilder", "append", "(Lj/l/String;)Lj/l/SB;") invokevirtual("j/l/StringBuilder", "toString", "()Lj/l/String;") invokevirtual("j/io/PrintStream", "println", "(Lj/l/String;)V") }M

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt InstructionAdapter(this).applyM{M //G ...Gbenchmark-printingGcode }M

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt return object : MethodVisitor(Opcodes.ASM5, original) { override fun visitCode() { super.visitCode() InstructionAdapter(this).applyL{L // ... method-trace-printing code // ... timestamp-storing code }W }4 override fun visitInsn(opcode: Int) { when (opcode) { RETURN , ARETURN, IRETURN -> { InstructionAdapter(this).applyM{M //G ...Gbenchmark-printingGcode }M } } super.visitInsn(opcode) } }3

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kotlin-plugin/src/main/kotlin/debuglog/DebugLogClassBuilder.kt return object : MethodVisitor(Opcodes.ASM5, original) { override fun visitCode() { super.visitCode() InstructionAdapter(this).applyL{L // ... method-trace-printing code // ... timestamp-storing code }W }4 override fun visitInsn(opcode: Int) { when (opcode) { RETURN , ARETURN, IRETURN -> { InstructionAdapter(this).applyM{M //G ...Gbenchmark-printingGcode }M } } super.visitInsn(opcode) } }3

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Done! Demo time!!!

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Grab this code at: github.com/kevinmost/debuglog

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Resources • https://github.com/JetBrains/kotlin/tree/master/plugins. Speciﬁcally: • noarg: One of the simplest ones • android-extensions: Good prior art for many of the extension types • kotlin-serialization: Newest one, documented well, generates LLVM • https://github.com/JetBrains/kotlin/tree/master/libraries/tools • All look pretty similar; noarg and allopen are the simplest to grok

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Thank you! Kevin Most