JVM IR: Implement -Xjvm-default=enable

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InterfaceDelegationLowering.kt

@@ -78,7 +78,7 @@ private class InterfaceDelegationLowering(val context: JvmBackendContext) : IrEl
         for (function in actualClass.functions.toList()) { // Copy the list, because we are adding new declarations from the loop
             if (function.origin !== IrDeclarationOrigin.FAKE_OVERRIDE) continue
 
-            // In classes, only generate interface delegation for functions immediately inherited from am interface.
+            // In classes, only generate interface delegation for functions immediately inherited from an interface.
             // (Otherwise, delegation will be present in the parent class)
             if (!isDefaultImplsGeneration &&
                 function.overriddenSymbols.any {
@@ -92,7 +92,9 @@ private class InterfaceDelegationLowering(val context: JvmBackendContext) : IrEl
             val implementation = function.resolveFakeOverride() ?: continue
             if (!implementation.hasInterfaceParent() ||
                 Visibilities.isPrivate(implementation.visibility) ||
-                implementation.isDefinitelyNotDefaultImplsMethod() || implementation.isMethodOfAny()
+                implementation.isDefinitelyNotDefaultImplsMethod() ||
+                implementation.isMethodOfAny() ||
+                (!context.state.jvmDefaultMode.isCompatibility && implementation.hasJvmDefault())
             ) {
                 continue
             }
@@ -205,8 +207,9 @@ private class InterfaceSuperCallsLowering(val context: JvmBackendContext) : IrEl
         if (expression.superQualifierSymbol?.owner?.isInterface != true) {
             return super.visitCall(expression)
         }
+
         val superCallee = (expression.symbol.owner as IrSimpleFunction).resolveFakeOverride()!!
-        if (superCallee.isDefinitelyNotDefaultImplsMethod()) return super.visitCall(expression)
+        if (superCallee.isDefinitelyNotDefaultImplsMethod() || superCallee.hasJvmDefault()) return super.visitCall(expression)
 
         val redirectTarget = context.declarationFactory.getDefaultImplsFunction(superCallee)
         val newCall = irCall(expression, redirectTarget, receiversAsArguments = true)
@@ -222,7 +225,7 @@ internal val interfaceDefaultCallsPhase = makeIrFilePhase(
 )
 
 private class InterfaceDefaultCallsLowering(val context: JvmBackendContext) : IrElementTransformerVoid(), FileLoweringPass {
-
+    // TODO If there are no default _implementations_ we can avoid generating defaultImpls class entirely by moving default arg dispatchers to the interface class
     override fun lower(irFile: IrFile) {
         irFile.transformChildrenVoid(this)
     }

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InterfaceLowering.kt

@@ -45,6 +45,7 @@ private class InterfaceLowering(val context: JvmBackendContext) : IrElementTrans
         if (!irClass.isInterface) return
 
         val defaultImplsIrClass = context.declarationFactory.getDefaultImplsClass(irClass)
+        //TODO: Don't add DefaultImpls if it's empty. Just like the old backend...?
         irClass.declarations.add(defaultImplsIrClass)
         val members = defaultImplsIrClass.declarations
 
@@ -52,8 +53,11 @@ private class InterfaceLowering(val context: JvmBackendContext) : IrElementTrans
             if (function !is IrSimpleFunction) continue
 
             if (function.modality != Modality.ABSTRACT && function.origin != IrDeclarationOrigin.FAKE_OVERRIDE) {
+
+                if(function.hasJvmDefault() && !context.state.jvmDefaultMode.isCompatibility && !mustMoveToDefaultImpls(function)) continue
+
                 val element = context.declarationFactory.getDefaultImplsFunction(function).also {
-                    if (shouldRemoveFunction(function))
+                    if (mustMoveToDefaultImpls(function))
                         removedFunctions[function.symbol] = it.symbol
                 }
                 members.add(element)
@@ -64,7 +68,6 @@ private class InterfaceLowering(val context: JvmBackendContext) : IrElementTrans
                 if (function.hasJvmDefault() &&
                     function.origin != JvmLoweredDeclarationOrigin.SYNTHETIC_METHOD_FOR_PROPERTY_ANNOTATIONS
                 ) {
-                    // TODO: don't touch function and only generate element / DefaultImpls when needed.
                     function.body = IrExpressionBodyImpl(callDefaultImpls(element, function))
                 } else {
                     function.body = null
@@ -98,7 +101,7 @@ private class InterfaceLowering(val context: JvmBackendContext) : IrElementTrans
         }
     }
 
-    private fun shouldRemoveFunction(function: IrFunction): Boolean =
+    private fun mustMoveToDefaultImpls(function: IrFunction): Boolean =
         Visibilities.isPrivate(function.visibility) && function.name != clinitName ||
                 function.origin == IrDeclarationOrigin.FUNCTION_FOR_DEFAULT_PARAMETER ||
                 function.origin == JvmLoweredDeclarationOrigin.SYNTHETIC_METHOD_FOR_PROPERTY_ANNOTATIONS

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrFunctionImpl.kt

@@ -16,6 +16,7 @@ import org.jetbrains.kotlin.ir.symbols.IrPropertySymbol
 import org.jetbrains.kotlin.ir.symbols.IrSimpleFunctionSymbol
 import org.jetbrains.kotlin.ir.symbols.impl.IrSimpleFunctionSymbolImpl
 import org.jetbrains.kotlin.ir.types.IrType
+import org.jetbrains.kotlin.ir.util.name
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitor
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.utils.SmartList

compiler/testData/writeFlags/jvm8/defaults/defaultMethod.kt

@@ -1,5 +1,4 @@
 // !JVM_DEFAULT_MODE: enable
-// IGNORE_BACKEND: JVM_IR
 // JVM_TARGET: 1.8
 // WITH_RUNTIME