org.jetbrains.kotlin.backend.common.lower.TailrecLowering.kt Maven / Gradle / Ivy
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/*
* Copyright 2010-2017 JetBrains s.r.o.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.jetbrains.kotlin.backend.common.lower
import org.jetbrains.kotlin.backend.common.BackendContext
import org.jetbrains.kotlin.backend.common.BodyLoweringPass
import org.jetbrains.kotlin.backend.common.collectTailRecursionCalls
import org.jetbrains.kotlin.ir.IrElement
import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
import org.jetbrains.kotlin.ir.builders.*
import org.jetbrains.kotlin.ir.declarations.*
import org.jetbrains.kotlin.ir.expressions.*
import org.jetbrains.kotlin.ir.expressions.impl.IrConstImpl
import org.jetbrains.kotlin.ir.symbols.IrValueParameterSymbol
import org.jetbrains.kotlin.ir.transformStatement
import org.jetbrains.kotlin.ir.types.IrType
import org.jetbrains.kotlin.ir.util.deepCopyWithSymbols
import org.jetbrains.kotlin.ir.util.explicitParameters
import org.jetbrains.kotlin.ir.util.getArgumentsWithIr
import org.jetbrains.kotlin.ir.util.patchDeclarationParents
import org.jetbrains.kotlin.ir.util.defaultValueForType
import org.jetbrains.kotlin.ir.visitors.IrElementVisitorVoid
import org.jetbrains.kotlin.ir.visitors.acceptChildrenVoid
import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
/**
* This pass lowers tail recursion calls in `tailrec` functions.
*
* Note: it currently can't handle local functions and classes declared in default arguments.
*/
open class TailrecLowering(val context: BackendContext) : BodyLoweringPass {
override fun lower(irBody: IrBody, container: IrDeclaration) {
if (container is IrFunction) {
// Lower local declarations
irBody.acceptChildrenVoid(object : IrElementVisitorVoid {
override fun visitElement(element: IrElement) {
element.acceptChildrenVoid(this)
}
override fun visitSimpleFunction(declaration: IrSimpleFunction) {
declaration.acceptChildrenVoid(this)
if (declaration.isTailrec) {
lowerTailRecursionCalls(declaration)
}
}
})
if (container is IrSimpleFunction && container.isTailrec) {
lowerTailRecursionCalls(container)
}
}
}
open val useProperComputationOrderOfTailrecDefaultParameters: Boolean
get() = true
open fun followFunctionReference(reference: IrFunctionReference): Boolean = false
open fun nullConst(startOffset: Int, endOffset: Int, type: IrType): IrExpression =
IrConstImpl.defaultValueForType(startOffset, endOffset, type)
}
private fun TailrecLowering.lowerTailRecursionCalls(irFunction: IrFunction) {
val (tailRecursionCalls, someCallsAreFromOtherFunctions) = collectTailRecursionCalls(irFunction, ::followFunctionReference)
if (tailRecursionCalls.isEmpty()) {
return
}
val oldBody = irFunction.body as? IrBlockBody ?: return
val oldBodyStatements = ArrayList(oldBody.statements)
val builder = context.createIrBuilder(irFunction.symbol).at(oldBody)
oldBody.statements.clear()
oldBody.statements += builder.irBlockBody {
// `return recursiveCall(...)` is rewritten into assignments to parameters followed by a jump to the start.
// While we may be able to write to the parameters directly, the recursive call may be inside an inline lambda,
// so the parameters are captured and assigning to them requires temporarily rewriting their types (see
// `SharedVariablesLowering`), and that we can't do. So we have to create new `var`s for this purpose.
// TODO: an optimization pass will rewrite the types of vars back since the lambdas are guaranteed to be inlined
// in place (otherwise they can't jump to the start of the function at all), so this is all a waste of CPU time.
val parameterToVariable = irFunction.explicitParameters.associateWith {
if (someCallsAreFromOtherFunctions || !it.isAssignable)
createTmpVariable(irGet(it), nameHint = it.symbol.suggestVariableName(), isMutable = true)
else
it
}
+irDoWhile().apply loop@{
body = irBlock(startOffset, endOffset, resultType = context.irBuiltIns.unitType) {
val transformer = BodyTransformer(
this@lowerTailRecursionCalls, builder, irFunction, this@loop, parameterToVariable, tailRecursionCalls
)
oldBodyStatements.forEach {
+it.transformStatement(transformer)
}
+irBreak(this@loop)
}
condition = irBlock {
// The problem with creating new `var`s is that they do not show up in the debugger, so stopping inside
// a nested call will still display the parameters from the outermost call. To fix this, we need to
// write the new values back even though the parameters are now otherwise unused.
for ((parameter, variable) in parameterToVariable.entries) {
if (parameter.isAssignable && parameter !== variable) {
+irSet(parameter, irGet(variable))
}
}
+irTrue()
}
}
}.statements
// TODO BodyTransformer creates temporary variables with wrong parents in nested functions
oldBody.patchDeclarationParents(irFunction)
}
private class BodyTransformer(
private val lowering: TailrecLowering,
private val builder: IrBuilderWithScope,
irFunction: IrFunction,
private val loop: IrLoop,
private val parameterToVariable: Map,
private val tailRecursionCalls: Set,
) : VariableRemapper(parameterToVariable) {
val parameters = irFunction.explicitParameters
override fun visitCall(expression: IrCall): IrExpression {
expression.transformChildrenVoid(this)
if (expression !in tailRecursionCalls) {
return expression
}
return builder.at(expression).genTailCall(expression)
}
override fun visitFunctionReference(expression: IrFunctionReference): IrExpression {
if (lowering.followFunctionReference(expression)) {
expression.symbol.owner.body?.transformChildrenVoid(this)
}
return super.visitFunctionReference(expression)
}
private fun IrBuilderWithScope.genTailCall(expression: IrCall) = this.irBlock(expression) {
// Get all specified arguments:
val parameterToArgument = expression.getArgumentsWithIr().associateTo(mutableMapOf()) { (parameter, argument) ->
// Note that we create `val`s for those parameters so that if some default value contains an object
// that captures another parameter, it won't capture it as a mutable ref.
parameter to irTemporary(argument)
}
// Create new null-initialized variables for all other values in case of forward references:
// fun f(x: () -> T = { y }, y: T = ...) // in `f()`, `x()` returns `null`
val defaultValuedParameters = parameters.filter { it !in parameterToArgument }
defaultValuedParameters.associateWithTo(parameterToArgument) {
// Note that we intentionally keep the original type of the parameter for the variable even though that violates type safety
// if it's non-null. This ensures that capture parameters have the same types for all copies of `x`.
irTemporary(lowering.nullConst(UNDEFINED_OFFSET, UNDEFINED_OFFSET, it.type))
}
// Now replace those variables with ones containing actual default values. Unused null-valued temporaries will hopefully
// be optimized out later.
val remapper = VariableRemapper(parameterToArgument)
defaultValuedParameters.let { if (lowering.useProperComputationOrderOfTailrecDefaultParameters) it else it.asReversed() }
.associateWithTo(parameterToArgument) { parameter ->
val originalDefaultValue = parameter.defaultValue?.expression ?: throw Error("no argument specified for $parameter")
irTemporary(originalDefaultValue.deepCopyWithSymbols(parent).transform(remapper, null))
}
// Copy the new `val`s into the `var`s declared outside the loop:
parameterToArgument.forEach { (parameter, argument) ->
at(argument)
+irSet(parameterToVariable[parameter]!!.symbol, irGet(argument))
}
// Jump to the entry:
+irContinue(loop)
}
}
private fun IrValueParameterSymbol.suggestVariableName(): String =
if (owner.name.isSpecial) {
val oldNameStr = owner.name.asString()
"$" + oldNameStr.substring(1, oldNameStr.length - 1)
} else {
owner.name.identifier
}