KT-63264: Introduce a new ExportedBridge annotation

Swift Export Frontend generates a Kotlin file/library that contains a
set of simple bridging functions that connect Swift wrappers to their
Kotlin counterparts.
There are certain requirements for these wrappers:
1. They should not be DCEd when compiling a binary library.
In other words, these functions are roots.
2. They should provide a stable simple binary name.
3. Their signatures should be much simpler and restricted comparing to
other Kotlin functions.

Altogether, these requirements should be covered by introducing the new
ExportedBridge annotation.

Note: Frontend checks of ExportedBridge functions are not implemented
yet.

core/compiler.common.native/src/org/jetbrains/kotlin/name/NativeRuntimeNames.kt

@@ -11,6 +11,7 @@ object NativeRuntimeNames {
     object Annotations {
         val symbolNameClassId = ClassId(kotlinNativePackage, Name.identifier("SymbolName"))
         val cNameClassId = ClassId(kotlinNativePackage, Name.identifier("CName"))
+        val exportedBridgeClassId = ClassId(kotlinNativeInternalPackage, Name.identifier("ExportedBridge"))
         val exportForCppRuntimeClassId = ClassId(kotlinNativeInternalPackage, Name.identifier("ExportForCppRuntime"))
         val exportForCompilerClassId = ClassId(kotlinNativeInternalPackage, Name.identifier("ExportForCompiler"))
         val gcUnsafeCallClassId = ClassId(kotlinNativeInternalPackage, Name.identifier("GCUnsafeCall"))

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/RuntimeNames.kt

@@ -8,6 +8,7 @@ object RuntimeNames {
     val cnameAnnotation = NativeRuntimeNames.Annotations.cNameClassId.asSingleFqName()
     val frozenAnnotation = FqName("kotlin.native.internal.Frozen")
     val exportForCppRuntime = NativeRuntimeNames.Annotations.exportForCppRuntimeClassId.asSingleFqName()
+    val exportedBridge = NativeRuntimeNames.Annotations.exportedBridgeClassId.asSingleFqName()
     val exportForCompilerAnnotation = NativeRuntimeNames.Annotations.exportForCompilerClassId.asSingleFqName()
     val exportTypeInfoAnnotation = FqName("kotlin.native.internal.ExportTypeInfo")
     val cCall = FqName("kotlinx.cinterop.internal.CCall")

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/llvm/BinaryInterface.kt

@@ -6,19 +6,21 @@
 package org.jetbrains.kotlin.backend.konan.llvm
 
 import llvm.LLVMTypeRef
-import org.jetbrains.kotlin.backend.common.serialization.mangle.MangleConstant
 import org.jetbrains.kotlin.backend.common.serialization.mangle.SpecialDeclarationType
 import org.jetbrains.kotlin.backend.konan.RuntimeNames
 import org.jetbrains.kotlin.backend.konan.descriptors.externalSymbolOrThrow
-import org.jetbrains.kotlin.ir.util.getAnnotationStringValue
 import org.jetbrains.kotlin.backend.konan.descriptors.isAbstract
 import org.jetbrains.kotlin.backend.konan.ir.isUnit
+import org.jetbrains.kotlin.backend.konan.serialization.AbstractKonanIrMangler
+import org.jetbrains.kotlin.ir.declarations.IrClass
+import org.jetbrains.kotlin.ir.declarations.IrDeclaration
+import org.jetbrains.kotlin.ir.declarations.IrField
+import org.jetbrains.kotlin.ir.declarations.IrFunction
 import org.jetbrains.kotlin.ir.objcinterop.isExternalObjCClass
 import org.jetbrains.kotlin.ir.objcinterop.isKotlinObjCClass
-import org.jetbrains.kotlin.backend.konan.serialization.AbstractKonanIrMangler
-import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.util.findAnnotation
 import org.jetbrains.kotlin.ir.util.fqNameForIrSerialization
+import org.jetbrains.kotlin.ir.util.getAnnotationStringValue
 import org.jetbrains.kotlin.ir.util.render
 import org.jetbrains.kotlin.konan.library.KonanLibrary
 import org.jetbrains.kotlin.library.uniqueName
@@ -62,6 +64,10 @@ object KonanBinaryInterface {
 
     private fun withPrefix(prefix: String, mangle: String) = "$prefix:$mangle"
 
+    private fun IrFunction.findManglingAnnotation() =
+        this.annotations.findAnnotation(RuntimeNames.exportForCppRuntime)
+                ?: this.annotations.findAnnotation(RuntimeNames.exportedBridge)
+
     private fun IrFunction.funSymbolNameImpl(containerName: String?): String {
         if (isExternal) {
             this.externalSymbolOrThrow()?.let {
@@ -69,7 +75,7 @@ object KonanBinaryInterface {
             }
         }
 
-        this.annotations.findAnnotation(RuntimeNames.exportForCppRuntime)?.let {
+        this.findManglingAnnotation()?.let {
             val name = it.getAnnotationStringValue() ?: this.name.asString()
             return name // no wrapping currently required
         }

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/llvm/CodeGenerator.kt

@@ -114,6 +114,8 @@ internal inline fun generateFunction(
     val llvmFunction = codegen.llvmFunction(function)
 
     val isCToKotlinBridge = function.origin == CBridgeOrigin.C_TO_KOTLIN_BRIDGE
+            // TODO: Alternative approach: lowering that changes origin of such functions to C_TO_KOTLIN_BRIDGE?
+            || function.hasAnnotation(RuntimeNames.exportedBridge)
 
     val functionGenerationContext = DefaultFunctionGenerationContext(
             llvmFunction,

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/llvm/RetainAnnotation.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.backend.konan.llvm
 
+import org.jetbrains.kotlin.backend.konan.RuntimeNames
 import org.jetbrains.kotlin.ir.util.getAnnotationStringValue
 import org.jetbrains.kotlin.ir.declarations.IrFunction
 import org.jetbrains.kotlin.ir.util.findAnnotation
@@ -18,5 +19,6 @@ internal fun IrFunction.retainAnnotation(target: KonanTarget): Boolean {
     if (this.annotations.findAnnotation(retainAnnotationName) != null) return true
     val forTarget = this.annotations.findAnnotation(retainForTargetAnnotationName)
     if (forTarget != null && forTarget.getAnnotationStringValue() == target.name) return true
+    if (this.annotations.findAnnotation(RuntimeNames.exportedBridge) != null) return true
     return false
 }

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/optimizations/DataFlowIR.kt

@@ -603,6 +603,7 @@ internal object DataFlowIR {
             if (returnsNothing)
                 attributes = attributes or FunctionAttributes.RETURNS_NOTHING
             if (it.hasAnnotation(RuntimeNames.exportForCppRuntime)
+                    || it.hasAnnotation(RuntimeNames.exportedBridge)
                     || it.getExternalObjCMethodInfo() != null // TODO-DCE-OBJC-INIT
                     || it.hasAnnotation(RuntimeNames.objCMethodImp)) {
                 attributes = attributes or FunctionAttributes.EXPLICITLY_EXPORTED

kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/Annotations.kt

@@ -5,6 +5,8 @@
 
 package kotlin.native.internal
 
+import kotlin.experimental.ExperimentalNativeApi
+
 /**
  * Makes this function to be possible to call by given name from C++ part of runtime using C ABI.
  * The parameters are mapped in an implementation-dependent manner.
@@ -180,3 +182,13 @@ internal annotation class InternalForKotlinNativeTests
 @InternalForKotlinNativeTests
 @Target(AnnotationTarget.FILE)
 public annotation class ReflectionPackageName(val name: String)
+
+/**
+ * Indicates that the marked function is an exported bridge between Kotlin and the platform.
+ * This annotation prevents the function from being removed by DCE
+ * and specifies a stable [bridgeName] for the function symbol.
+ */
+@Target(AnnotationTarget.FUNCTION)
+@Retention(value = AnnotationRetention.BINARY)
+@ExperimentalNativeApi
+public annotation class ExportedBridge(val bridgeName: String)