backend: treat kotlin.Unit type specially only when used as return type

also remove some hacks related to Unit.

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/descriptors/DescriptorUtils.kt

@@ -51,7 +51,6 @@ internal val FunctionDescriptor.isIntrinsic: Boolean
     get() = this.annotations.findAnnotation(intrinsicAnnotation) != null
 
 private val intrinsicTypes = setOf(
-        "kotlin.Unit",
         "kotlin.Boolean", "kotlin.Char",
         "kotlin.Byte", "kotlin.Short",
         "kotlin.Int", "kotlin.Long",

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

@@ -4,6 +4,7 @@ import llvm.*
 import org.jetbrains.kotlin.backend.konan.descriptors.isUnboundCallableReference
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.types.KotlinType
+import org.jetbrains.kotlin.types.typeUtil.isUnit
 
 internal fun ContextUtils.getLLVMType(type: KotlinType): LLVMTypeRef {
     return when {
@@ -15,12 +16,18 @@ internal fun ContextUtils.getLLVMType(type: KotlinType): LLVMTypeRef {
         KotlinBuiltIns.isShort(type) || KotlinBuiltIns.isChar(type) -> LLVMInt16Type()
         KotlinBuiltIns.isInt(type) -> LLVMInt32Type()
         KotlinBuiltIns.isLong(type) -> LLVMInt64Type()
-        KotlinBuiltIns.isUnit(type) -> LLVMVoidType() // TODO: handle Unit parameter case
-        // TODO: stdlib have methods taking Nothing, such as kotlin.collections.EmptySet.contains().
-        // KotlinBuiltIns.isNothing(type) -> LLVMVoidType()
         KotlinBuiltIns.isFloat(type) -> LLVMFloatType()
         KotlinBuiltIns.isDouble(type) -> LLVMDoubleType()
         !KotlinBuiltIns.isPrimitiveType(type) -> this.kObjHeaderPtr
         else -> throw NotImplementedError(type.toString() + " is not supported")
     }!!
 }
+
+internal fun ContextUtils.getLLVMReturnType(type: KotlinType): LLVMTypeRef {
+    return when {
+        type.isUnit() -> LLVMVoidType()!!
+        // TODO: stdlib have methods taking Nothing, such as kotlin.collections.EmptySet.contains().
+        // KotlinBuiltIns.isNothing(type) -> LLVMVoidType()
+        else -> getLLVMType(type)
+    }
+}

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

@@ -614,12 +614,6 @@ internal class CodeGeneratorVisitor(val context: Context) : IrElementVisitorVoid
             return
         }
 
-        if (KotlinBuiltIns.isUnit(declaration.descriptor.defaultType)) {
-            // Do not generate any code for Unit.
-            // TODO: Unit has toString() operation, which we may want to support.
-            return
-        }
-
         super.visitClass(declaration)
     }
 
@@ -1543,17 +1537,12 @@ internal class CodeGeneratorVisitor(val context: Context) : IrElementVisitorVoid
         context.log("evaluateReturn             : ${ir2string(expression)}")
         val value = expression.value
 
-        val evaluated = if (value is IrGetObjectValue && value.type.isUnit()) {
-            // hack to make "return without value" work
-            null
-        } else {
-            evaluateExpression(value)
-        }
+        val evaluated = evaluateExpression(value)
 
         val ret = if (!value.type.isUnit()) {
             evaluated
         } else {
-            null // hack for bad Unit type handling
+            null // `Unit` return type is generated as `void`.
         }
 
         currentCodeContext.genReturn(expression.returnTarget, ret)

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

@@ -143,7 +143,7 @@ internal fun structType(types: List<LLVMTypeRef>): LLVMTypeRef = memScoped {
 }
 
 internal fun ContextUtils.getLlvmFunctionType(function: FunctionDescriptor): LLVMTypeRef {
-    val returnType = if (function is ConstructorDescriptor) voidType else getLLVMType(function.returnType!!)
+    val returnType = if (function is ConstructorDescriptor) voidType else getLLVMReturnType(function.returnType!!)
     val paramTypes = ArrayList(function.allValueParameters.map { getLLVMType(it.type) })
     if (isObjectType(returnType)) paramTypes.add(kObjHeaderPtrPtr)