org.jetbrains.kotlin.backend.wasm.lower.UnitToVoidLowering.kt Maven / Gradle / Ivy

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/*
 * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
 * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
 */

package org.jetbrains.kotlin.backend.wasm.lower

import org.jetbrains.kotlin.backend.common.FileLoweringPass
import org.jetbrains.kotlin.backend.common.lower.AbstractValueUsageTransformer
import org.jetbrains.kotlin.backend.wasm.WasmBackendContext
import org.jetbrains.kotlin.ir.IrStatement
import org.jetbrains.kotlin.ir.declarations.IrFile
import org.jetbrains.kotlin.ir.expressions.*
import org.jetbrains.kotlin.ir.expressions.impl.IrCallImpl
import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid

/**
 * Replaces some `Unit`s with `Void`s. **NOTE**: This is an optional lowering.
 *
 * In the Wasm backend we treat `Unit` type as a regular object (except for the function return values). This means that all expressions
 * with `Unit` return type will actually produce a real `Unit` instance. However, sometimes `Unit` type signifies that the result of the
 * expression will never be used, and for such cases we want to be able to avoid generating redundant `Unit` objects.
 *
 * In order to simplify the reasoning we introduce a special type `Void`. It means absence of any value, so there are never any real
 * objects of type `Void`. The only way to produce a `Void` type is by calling intrinsic `consumeAnyIntoVoid`. In the Wasm code it will turn
 * into a single drop instruction.
 *
 * This lowering visits all statements with `Unit` return type and tries to replace it with `Void` type. In the result of
 * such statements it emits `consumeAnyIntoVoid(previous_result)`. In other words, it tries to propagate `consumeAnyIntoVoid` as
 * deep as possible, changing statement return type to `Void` on each nesting level it crosses. For example, from this code:
 *
 *    block_body {
 *        block: Unit {
 *            if (some_condition): Unit {
 *                foo(): Unit
 *            } else {
 *                IMPLICIT_COERCION_TO_UNIT(bar(): Int)
 *            }
 *            foo.bar = 10
 *        }
 *        return 1
 *    }
 *
 * We will get this code:
 *
 *    block_body {
 *        block: Void {
 *            if (some_condition): Void {
 *                consumeAnyIntoVoid(foo(): Unit): Void
 *            } else {
 *                consumeAnyIntoVoid(bar(): Int): Void
 *            }
 *            foo.bar = 10
 *        }
 *        return 1
 *    }
 *
 * **NOTE**: In order to reduce the amount of new nodes in the IR, we only do this transformation if it involves `when` or `try`/`catch`
 * statements because Wasm backend handles other cases well enough.
 *
 * As a further optimization, we can directly mark `call`, `set_value` and `set_field` with `Void` return type and handle them as special
 * cases in the backend.
 */
class UnitToVoidLowering(val context: WasmBackendContext) : FileLoweringPass, AbstractValueUsageTransformer(context.irBuiltIns) {
    val builtIns = context.irBuiltIns
    val symbols = context.wasmSymbols

    override fun lower(irFile: IrFile) {
        irFile.transformChildrenVoid(this)
    }

    override fun IrExpression.useAsStatement(): IrExpression {
        if (type != builtIns.unitType)
            return this

        if (shouldVoidify(this))
            return voidify(this) as IrExpression
        return this
    }

    // Checks if this expression is an interesting target for the voidification.
    private fun shouldVoidify(expr: IrStatement): Boolean {
        if (expr !is IrExpression)
            return true

        if (expr.type == symbols.voidType || expr.type == builtIns.nothingType)
            return false

        return when (expr) {
            is IrContainerExpression ->
                expr.statements.isNotEmpty() && expr !is IrReturnableBlock && shouldVoidify(expr.statements.last())

            is IrWhen, is IrTry ->
                true

            is IrTypeOperatorCall ->
                expr.operator == IrTypeOperator.IMPLICIT_COERCION_TO_UNIT

            else -> false
        }
    }

    // Takes expression of any type and unconditionally turns it into an expression of a void type.
    private fun voidify(expr: IrStatement): IrStatement {
        // Declarations are implicitly void
        if (expr !is IrExpression)
            return expr

        // Don't voidify 'Nothing' because it can be casted to anything, including void.
        if (expr.type == symbols.voidType || expr.type == builtIns.nothingType)
            return expr

        // Try to look through one of the known expression types.
        when (expr) {
            is IrContainerExpression -> {
                expr.type = symbols.voidType
                expr.statements.lastOrNull()?.let { last ->
                    expr.statements[expr.statements.lastIndex] = voidify(last)
                }
                return expr
            }
            is IrWhen -> {
                expr.type = symbols.voidType
                expr.branches.forEachIndexed { i, branch ->
                    expr.branches[i].result = voidify(branch.result) as IrExpression
                }
                return expr
            }
            is IrTry -> {
                expr.type = symbols.voidType
                expr.tryResult = voidify(expr.tryResult) as IrExpression
                expr.catches.forEachIndexed { i, catch ->
                    expr.catches[i].result = voidify(catch.result) as IrExpression
                }
                return expr
            }
            is IrTypeOperatorCall -> {
                if (expr.operator == IrTypeOperator.IMPLICIT_COERCION_TO_UNIT)
                    return voidify(expr.argument)
            }
        }

        return IrCallImpl(
            expr.startOffset, expr.endOffset, symbols.voidType, symbols.findVoidConsumer(expr.type),
            typeArgumentsCount = 0,
        ).apply {
            putValueArgument(0, expr)
        }
    }
}