swift-mirror/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/ClosureSpecialization.swift

//===--- ClosureSpecialization.swift ---------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2024 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===-----------------------------------------------------------------------===//

import AST
import SIL

/// Closure Specialization
/// ----------------------
/// Specializes functions which take a closure (a `partial_apply` or `thin_to_thick_function` as argument.
/// The closure is created directly in the specialized function whereas the captured arguments are passed
/// as additional arguments to the specialized function. As a heuristic, this is only done if the closure
/// is actually called in the function because after specialization, the closure and its call can be
/// further optimized, e.g. the closure can be inlined.
///
/// ```
///   %3 = function_ref @closure
///   %4 = partial_apply %3(%1, %2) : (Float, Int, Bool) -> ()
///   %5 = function_ref @closure_user
///   apply %5(%4)
///   ...
///
/// sil @closure_user : ((Float) -> (), Float) -> () {
/// bb0(%0 : $(Float) -> (), %1 : $Float):
///   apply %0(%1)
///   ...
/// ```
/// ->
/// ```
///   %5 = function_ref @specialized_closure_user
///   apply %5(%1, %2)
///   ...
///
/// sil @specialized_closure_user : (Float, Int, Bool) -> () {
/// bb0(%0 : $Float, %1 : $Int, %2 : $Bool):
///   %3 = function_ref @closure
///   %4 = partial_apply %3(%1, %2)
///   apply %3(%0)
///   ...
/// ```
///
let closureSpecialization = FunctionPass(name: "closure-specialization") {
  (function: Function, context: FunctionPassContext) in

  guard function.hasOwnership else {
    return
  }

  for inst in function.instructions {
    if let apply = inst as? FullApplySite {
      _ = trySpecialize(apply: apply, context)
    }
  }
  if context.needFixStackNesting {
    context.fixStackNesting(in: function)
  }
}

/// AutoDiff Closure Specialization
/// -------------------------------
/// This optimization performs closure specialization tailored for the patterns seen in Swift Autodiff. In principle,
/// the optimization does the same thing as the general closure specialization pass. However, it is tailored to the
/// patterns of Swift Autodiff.
///
/// The compiler performs reverse-mode differentiation on functions marked with `@differentiable(reverse)`. In doing so,
/// it generates corresponding VJP and Pullback functions, which perform the forward and reverse pass respectively. You
/// can think of VJPs as functions that "differentiate" an original function and Pullbacks as the calculated
/// "derivative" of the original function.
///
/// VJPs always return a tuple of 2 values -- the original result and the Pullback. Pullbacks are essentially a chain
/// of closures, where the closure-contexts are implicitly used as the so-called "tape" during the reverse
/// differentiation process. It is this chain of closures contained within the Pullbacks that this optimization aims
/// to optimize via closure specialization.
///
/// The code patterns that this optimization targets, look similar to the one below:
/// ``` swift
///
/// // Since `foo` is marked with the `differentiable(reverse)` attribute the compiler
/// // will generate corresponding VJP and Pullback functions in SIL. Let's assume that
/// // these functions are called `vjp_foo` and `pb_foo` respectively.
/// @differentiable(reverse)
/// func foo(_ x: Float) -> Float {
///   return sin(x)
/// }
///
/// //============== Before closure specialization ==============//
/// // VJP of `foo`. Returns the original result and the Pullback of `foo`.
/// sil @vjp_foo: $(Float) -> (originalResult: Float, pullback: (Float) -> Float) {
/// bb0(%0: $Float):
///   // __Inlined__ `vjp_sin`: It is important for all intermediate VJPs to have
///   // been inlined in `vjp_foo`, otherwise `vjp_foo` will not be able to determine
///   // that `pb_foo` is closing over other closures and no specialization will happen.
///                                                                               \
///   %originalResult = apply @sin(%0): $(Float) -> Float                          \__ Inlined `vjp_sin`
///   %partially_applied_pb_sin = partial_apply pb_sin(%0): $(Float) -> Float      /
///                                                                               /
///
///   %pb_foo = function_ref @pb_foo: $@convention(thin) (Float, (Float) -> Float) -> Float
///   %partially_applied_pb_foo = partial_apply %pb_foo(%partially_applied_pb_sin): $(Float, (Float) -> Float) -> Float
///
///   return (%originalResult, %partially_applied_pb_foo)
/// }
///
/// // Pullback of `foo`.
/// //
/// // It receives what are called as intermediate closures that represent
/// // the calculations that the Pullback needs to perform to calculate a function's
/// // derivative.
/// //
/// // The intermediate closures may themselves contain intermediate closures and
/// // that is why the Pullback for a function differentiated at the "top" level
/// // may end up being a "chain" of closures.
/// sil @pb_foo: $(Float, (Float) -> Float) -> Float {
/// bb0(%0: $Float, %pb_sin: $(Float) -> Float):
///   %derivative_of_sin = apply %pb_sin(%0): $(Float) -> Float
///   return %derivative_of_sin: Float
/// }
///
/// //============== After closure specialization ==============//
/// sil @vjp_foo: $(Float) -> (originalResult: Float, pullback: (Float) -> Float) {
/// bb0(%0: $Float):
///   %originalResult = apply @sin(%0): $(Float) -> Float
///
///   // Before the optimization, pullback of `foo` used to take a closure for computing
///   // pullback of `sin`. Now, the specialized pullback of `foo` takes the arguments that
///   // pullback of `sin` used to close over and pullback of `sin` is instead copied over
///   // inside pullback of `foo`.
///   %specialized_pb_foo = function_ref @specialized_pb_foo: $@convention(thin) (Float, Float) -> Float
///   %partially_applied_pb_foo = partial_apply %specialized_pb_foo(%0): $(Float, Float) -> Float
///
///   return (%originalResult, %partially_applied_pb_foo)
/// }
///
/// sil @specialized_pb_foo: $(Float, Float) -> Float {
/// bb0(%0: $Float, %1: $Float):
///   %2 = partial_apply @pb_sin(%1): $(Float) -> Float
///   %3 = apply %2(): $() -> Float
///   return %3: $Float
/// }
/// ```
///
let autodiffClosureSpecialization = FunctionPass(name: "autodiff-closure-specialization") {
  (function: Function, context: FunctionPassContext) in

  guard function.hasOwnership else {
    return
  }

  guard !function.isDefinedExternally,
    function.isAutodiffVJP
  else {
    return
  }

  var remainingSpecializationRounds = 5

  repeat {
    var changed = false
    for inst in function.instructions {
      if let partialApply = inst as? PartialApplyInst,
         partialApply.isPullbackInResultOfAutodiffVJP
      {
        if trySpecialize(apply: partialApply, context) {
          changed = true
        }
      }
    }
    if context.needFixStackNesting {
      context.fixStackNesting(in: function)
    }
    if !changed {
      break
    }

    remainingSpecializationRounds -= 1
  } while remainingSpecializationRounds > 0
}

// ===================== Utility functions and extensions ===================== //

private func trySpecialize(apply: ApplySite, _ context: FunctionPassContext) -> Bool {
  guard isCalleeSpecializable(of: apply),
        let specialization = analyzeArguments(of: apply, context)
  else {
    return false
  }

  // We need to make each captured argument of all closures a unique Value. Otherwise the Cloner would
  // wrongly map added capture arguments to the re-generated closure in the specialized function:
  //
  //   %4 = partial_apply %3(%1, %1) : (Int, Int) -> ()  // %1 is captured twice
  //
  // sil @specialized_closure_user : (Int, Int) -> () {
  // bb0(%0 : $Int, %1 : $Int):
  //   %3 = function_ref @closure
  //   %4 = partial_apply %3(%0, %0)   <- instead of an argument list of `(%0, %1)`!
  //
  // This wouldn't be a problem per se - the code is correct. However, this is not reflected in the mangling.
  // And the same specialized function could be re-used at a call-site where not two identical values are
  // passed to the closure.
  //
  specialization.uniqueCaptureArguments(context)

  let specializedFunction = specialization.getOrCreateSpecializedFunction(context)

  specialization.unUniqueCaptureArguments(context)

  specialization.rewriteApply(for: specializedFunction, context)

  specialization.deleteDeadClosures(context)

  return true
}

private func isCalleeSpecializable(of apply: ApplySite) -> Bool {
  if let callee = apply.referencedFunction,
     callee.isDefinition,

     // We don't support generic functions (yet)
     !apply.hasSubstitutions,

     // Don't specialize non-fragile (read as non-serialized) callees if the caller is fragile; the
     // specialized callee will have shared linkage, and thus cannot be referenced from the fragile caller.
     !(apply.parentFunction.isSerialized && !callee.isSerialized),

     // If the callee uses a dynamic Self, we cannot specialize it, since the resulting specialization
     // might no longer have 'self' as the last parameter.
     //
     // TODO: Keep the self argument the last when appending arguments.
     !callee.mayBindDynamicSelf
  {
    return true
  }
  return false
}

private func analyzeArguments(of apply: ApplySite, _ context: FunctionPassContext) -> SpecializationInfo? {
  var argumentsToSpecialize = [(Operand, Closure)]()
  var rootClosures = [PartialApplyInst]()
  var rootClosuresAdded = InstructionSet(context)
  defer { rootClosuresAdded.deinitialize() }

  for argOp in apply.argumentOperands {
    var visited = ValueSet(context)
    defer { visited.deinitialize() }
    if let closure = findSpecializableClosure(of: argOp.value, &visited),
       // Ok, we know that we can perform the optimization but not whether or not the optimization
       // is profitable. Check if the closure is actually called in the callee (or in a function
       // called by the callee). This opens optimization opportunities, like inlining.
       isClosureApplied(apply.calleeArgument(of: argOp, in: apply.referencedFunction!)!)
    {
      argumentsToSpecialize.append((argOp, closure))
      if let partialApply = closure as? PartialApplyInst,
         rootClosuresAdded.insert(partialApply)
      {
        rootClosures.append(partialApply)
      }
    }
  }
  if argumentsToSpecialize.isEmpty {
    return nil
  }
  return SpecializationInfo(apply: apply, closureArguments: argumentsToSpecialize, rootClosures: rootClosures)
}

// Walks down the use-def chain of a function argument, recursively, to find a rootClosure.
private func findSpecializableClosure(of value: Value, _ visited: inout ValueSet) -> Closure? {
  visited.insert(value)

  let specializationLevelLimit = 2

  switch value {
  case is ConvertFunctionInst,
       is ConvertEscapeToNoEscapeInst,
       is MoveValueInst,
       is CopyValueInst:
    return findSpecializableClosure(of: (value as! UnaryInstruction).operand.value, &visited)

  case let mdi as MarkDependenceInst:
    guard mdi.value.type.isNoEscapeFunction, mdi.value.type.isThickFunction else {
      return nil
    }
    guard let operandClosure = findSpecializableClosure(of: mdi.value, &visited) else {
      return nil
    }
    // Make sure that the mark_dependence's base is part of the use-def chain and will therefore be cloned as well.
    if !visited.contains(mdi.base) {
      return nil
    }
    return operandClosure

  case let partialApply as PartialApplyInst:
    // Don't specialize for re-abstractions via a partial_apply, but treat such re-abstractions like
    // closure "conversions". E.g.
    // ```
    //   %1 = partial_apply             // root closure
    //   %2 = function_ref @thunk
    //   %3 = partial_apply %2(%1)      // re-abstraction
    //   apply %f(%3)
    // ```
    if partialApply.isPartialApplyOfThunk,
       let argumentClosure = findSpecializableClosure(of: partialApply.arguments[0], &visited)
    {
      return argumentClosure
    }
    guard let callee = partialApply.referencedFunction,
          !partialApply.hasSubstitutions,

          // Avoid an infinite specialization loop caused by repeated runs of ClosureSpecialization and
          // ConstantCapturePropagation.
          // ConstantCapturePropagation propagates constant function-literals. Such function specializations
          // can then be optimized again by ClosureSpecialization and so on. This happens if a closure argument
          // is called _and_ referenced in another closure, which is passed to a recursive call. E.g.
          //
          // func foo(_ c: @escaping () -> ()) {
          //  c() foo({ c() })
          // }
          //
          // A limit of 2 is good enough and will not be exceed in "regular" optimization scenarios.
          callee.specializationLevel <= specializationLevelLimit,

          // Functions with a readnone, readonly or releasenone effect and a consumed captures cannot be
          // specialized because the captured arguments are passed as owned arguments to the specialized
          // function. Destroy for such arguments in the specialized function violates the effect.
          (partialApply.isOnStack || callee.effectAllowsSpecialization),

          // TODO: handle other kind of indirect arguments
          partialApply.hasOnlyInoutIndirectArguments,

          (partialApply.isOnStack || partialApply.allArgumentsCanBeCopied)
    else {
      return nil
    }
    return partialApply

  case let tttfi as ThinToThickFunctionInst:
    guard let callee = tttfi.referencedFunction,
          callee.specializationLevel <= specializationLevelLimit
    else {
      return nil
    }
    return tttfi

  default:
    return nil
  }
}

/// Either a `partial_apply` or a `thin_to_thick_function`
private typealias Closure = SingleValueInstruction

/// Information about the function to be specialized and for which closure arguments.
private struct SpecializationInfo {

  // The apply which we want to specialize
  let apply: ApplySite

  // All closure arguments of the apply which we want to replace (usually there is one, but there can be
  // multiple).
  // Note that the `Closure` is not necessarily the value of the `Operand`. There can be function conventions
  // and re-abstractions (via a thunk) in-between:
  //
  //   %4 = partial_apply %3(%1) : (Int) -> ()   // %4 = rootClosure
  //   %5 = convert_function %4
  //   %6 = copy_value %5
  //   apply %5(%5)                              // %5 = closureArgument
  //
  let closureArguments: [(closureArgument: Operand, rootClosure: Closure)]

  // All rootClosures of `closureArguments` which are `partial_apply`s, and uniqued: if a rootClosure
  // appears multiple times in `closureArguments`, it's only added a single time here.
  let rootClosures: [PartialApplyInst]

  // The function to specialize
  var callee: Function { apply.referencedFunction! }

  private typealias Cloner = SIL.Cloner<FunctionPassContext>

  func getOrCreateSpecializedFunction(_ context: FunctionPassContext) -> Function {
    let specializedFunctionName = getSpecializedFunctionName(context)

    if let existingSpecializedFunction = context.lookupFunction(name: specializedFunctionName) {
      return existingSpecializedFunction
    }

    let specializedParameters = getSpecializedParameters()

    let specializedFunction =
      context.createSpecializedFunctionDeclaration(
        from: callee, withName: specializedFunctionName,
        withParams: specializedParameters,
        // The specialized function is always a thin function. This is important because we add additional
        // parameters after the Self parameter of witness methods. In this case the new function is not a
        // method anymore.
        makeThin: true, makeBare: true)

    context.buildSpecializedFunction(
      specializedFunction: specializedFunction,
      buildFn: { (specializedFunction, specializedContext) in
        var cloner = Cloner(cloneToEmptyFunction: specializedFunction, specializedContext)
        defer { cloner.deinitialize() }

        cloneAndSpecializeFunctionBody(using: &cloner)
      })

    context.notifyNewFunction(function: specializedFunction, derivedFrom: callee)

    return specializedFunction
  }


  private func getSpecializedFunctionName(_ context: FunctionPassContext) -> String {
    var visited = Dictionary<Closure, Int>()

    let argumentManglings = closureArguments.map { (argOp, closure) in
      let argIdx = apply.calleeArgumentIndex(of: argOp)!
      if let prevArgIdx = visited[closure] {

        // If the same closure is passed multiple times to a function, we need to reflect this in the mangling:
        //
        //   %3 = function_ref @closure
        //   %4 = partial_apply %3(%1) : (Int) -> ()
        //   apply %6(%4, %4)
        //
        // sil @specialized_closure_user : (Int) -> () {
        //
        // is different than
        //
        //   %3 = function_ref @closure
        //   %4 = partial_apply %3(%1) : (Int) -> ()
        //   %5 = partial_apply %3(%1) : (Int) -> ()
        //   apply %6(%4, %5)
        //
        // sil @specialized_closure_user : (Int, Int) -> () {
        //
        return (argIdx, FunctionPassContext.ClosureArgumentMangling.previousArgumentIndex(prevArgIdx))
      } else {
        visited[closure] = argIdx
        return (argIdx, FunctionPassContext.ClosureArgumentMangling.closure(closure))
      }
    }
    return context.mangle(withClosureArguments: argumentManglings, from: callee)
  }

  private func getSpecializedParameters() -> [ParameterInfo] {
    var specializedParamInfoList: [ParameterInfo] = []

    // Start by adding all original parameters except for the closure parameters.
    let firstParamIndex = callee.argumentConventions.firstParameterIndex
    for (index, paramInfo) in callee.convention.parameters.enumerated() {
      let argIndex = index + firstParamIndex
      if !closureArguments.contains(where: { apply.calleeArgumentIndex(of: $0.0) == argIndex}) {
        specializedParamInfoList.append(paramInfo)
      }
    }

    // Now, append parameters captured by each of the root closures.
    for partialApply in rootClosures {
      let closureConvention = partialApply.functionConvention
      let unappliedArgumentCount = partialApply.unappliedArgumentCount - closureConvention.indirectSILResultCount

      for paramInfo in closureConvention.parameters[unappliedArgumentCount...] {
        let newParamInfo = paramInfo.withSpecializedConvention(for: partialApply, in: callee)
        specializedParamInfoList.append(newParamInfo)
      }
    }

    return specializedParamInfoList
  }

  private func cloneAndSpecializeFunctionBody(using cloner: inout Cloner) {
    addFunctionArgumentsWithoutClosures(using: &cloner)

    for rootClosure in rootClosures {
      addFunctionArgumentsForCaptures(of: rootClosure, using: &cloner)
    }

    let clonedClosureArguments = cloneClosures(using: &cloner)

    cloner.cloneFunctionBody(from: callee)

    addMissingDestroysAtFunctionExits(for: clonedClosureArguments, cloner.context)
  }

  private func addFunctionArgumentsWithoutClosures(using cloner: inout Cloner) {
    let clonedEntryBlock = cloner.getOrCreateEntryBlock()

    for originalArg in callee.arguments where !isClosureArgument(calleeArgument: originalArg) {
      let argType = originalArg.type.getLoweredType(in: cloner.targetFunction)
      let clonedArg = clonedEntryBlock.addFunctionArgument(type: argType, cloner.context)
      clonedArg.copyFlags(from: originalArg, cloner.context)
      cloner.recordFoldedValue(originalArg, mappedTo: clonedArg)
    }
  }

  private func addFunctionArgumentsForCaptures(of closure: PartialApplyInst, using cloner: inout Cloner) {
    for originalClosureArg in closure.arguments {
      let capturedArg = cloner.targetFunction.entryBlock.addFunctionArgument(
        type: originalClosureArg.type.getLoweredType(in: cloner.targetFunction),
        cloner.context)
      if !cloner.isCloned(value: originalClosureArg) {
        cloner.recordFoldedValue(originalClosureArg, mappedTo: capturedArg)
      }
    }
  }

  private func cloneClosures(using cloner: inout Cloner) -> [Value] {
    return closureArguments.map { (closureArgOp, _) in
      let clonedArg = cloner.cloneRecursively(value: closureArgOp.value)

      let originalArg = apply.calleeArgument(of: closureArgOp, in: callee)!
      cloner.recordFoldedValue(originalArg, mappedTo: clonedArg)

      return clonedArg
    }
  }

  func rewriteApply(for specializedFunction: Function, _ context: FunctionPassContext) {
    insertCompensatingDestroysForOwnedClosureArguments(context)

    let newApplyArgs = getNewApplyArguments(context)

    let builder = Builder(before: apply, context)
    let funcRef = builder.createFunctionRef(specializedFunction)

    switch apply {
    case let oldPartialApply as PartialApplyInst:
      let newPartialApply = builder.createPartialApply(
        function: funcRef, substitutionMap: SubstitutionMap(),
        capturedArguments: newApplyArgs, calleeConvention: oldPartialApply.calleeConvention,
        hasUnknownResultIsolation: oldPartialApply.hasUnknownResultIsolation,
        isOnStack: oldPartialApply.isOnStack)
      oldPartialApply.replace(with: newPartialApply, context)

    case let oldApply as ApplyInst:
      let newApply = builder.createApply(function: funcRef, SubstitutionMap(), arguments: newApplyArgs,
                                         isNonThrowing: oldApply.isNonThrowing,
                                         isNonAsync: oldApply.isNonAsync)
      oldApply.replace(with: newApply, context)

    case let oldTryApply as TryApplyInst:
      builder.createTryApply(function: funcRef, SubstitutionMap(), arguments: newApplyArgs,
                             normalBlock: oldTryApply.normalBlock, errorBlock: oldTryApply.errorBlock,
                             isNonAsync: oldTryApply.isNonAsync)
      context.erase(instruction: oldTryApply)

    case let oldBeginApply as BeginApplyInst:
      let newApply = builder.createBeginApply(function: funcRef, SubstitutionMap(), arguments: newApplyArgs,
                                              isNonThrowing: oldBeginApply.isNonThrowing,
                                              isNonAsync: oldBeginApply.isNonAsync)
      oldBeginApply.replace(with: newApply, context)
    default:
      fatalError("unknown apply")
    }
  }

  private func insertCompensatingDestroysForOwnedClosureArguments(_ context: FunctionPassContext) {
    let builder = Builder(before: apply, context)
    for (argOp, _) in closureArguments where argOp.endsLifetime {
      builder.createDestroyValue(operand: argOp.value)
    }
  }

  private func getNewApplyArguments(_ context: FunctionPassContext) -> [Value] {
    let newCapturedArguments = rootClosures.flatMap { partialApply in
      partialApply.arguments.map { capturedArg in
        if partialApply.isOnStack || capturedArg.ownership == .none {
          // Non-escaping closures don't consume their captures. Therefore we pass them also as "guaranteed"
          // arguments to the specialized function.
          // Note that because the non-escaping closure was passed to the original function, this guarantees
          // that the lifetime of the captured arguments also extend to at least the apply of the function.
          capturedArg
        } else {
          // Escaping closures consume their captures. Therefore we pass them as "owned" arguments to the
          // specialized function.
          capturedArg.copy(at: partialApply, andMakeAvailableIn: apply.parentBlock, context)
        }
      }
    }
    return nonClosureArguments.values + newCapturedArguments
  }

  func uniqueCaptureArguments(_ context: FunctionPassContext) {
    let builder = Builder(before: apply, context)

    // Insert identity cast instructions for all closure arguments to make them unique. We could use any kind
    // of forwarding instruction - we'll delete them afterwards, anyway.
    //
    //   %4 = partial_apply %3(%1, %1) : (Int, Int) -> ()  // %1 is captured twice
    // ->
    //   %2 = unchecked_value_cast %1
    //   %3 = unchecked_value_cast %1
    //   %4 = partial_apply %3(%2, %3) : (Int, Int) -> ()  // all arguments are unique values now!
    //
    for closure in rootClosures {
      for argOp in closure.argumentOperands {
        let cast = builder.createUncheckedValueCast(from: argOp.value, to: argOp.value.type)
        argOp.set(to: cast, context)
      }
    }
  }

  func unUniqueCaptureArguments(_ context: FunctionPassContext) {
    // Remove the inserted identity casts again.
    for closure in rootClosures {
      for argOp in closure.argumentOperands {
        let cast = argOp.value as! UncheckedValueCastInst
        cast.replace(with: cast.fromValue, context)
      }
    }
  }

  func deleteDeadClosures(_ context: FunctionPassContext) {
    for  (_, closure) in closureArguments where !closure.isDeleted {
      if context.tryDeleteDeadClosure(closure: closure) {
        context.notifyInvalidatedStackNesting()
      }
    }
  }

  private var nonClosureArguments: LazyFilterSequence<OperandArray> {
    apply.argumentOperands.lazy.filter{ argOp in !closureArguments.contains{ $0.0 == argOp } }
  }

  private func isClosureArgument(calleeArgument: FunctionArgument) -> Bool {
    closureArguments.contains { apply.calleeArgument(of: $0.0, in: callee) == calleeArgument }
  }
}

private func isClosureApplied(_ closure: Value) -> Bool {
  var handledFuncs: Set<Function> = []
  return checkRecursivelyIfClosureIsApplied(closure, &handledFuncs)
}

private func checkRecursivelyIfClosureIsApplied(_ closure: Value, _ handledFuncs: inout Set<Function>) -> Bool {
  for use in closure.uses {
    switch use.instruction {

    case let apply as FullApplySite:
      if apply.callee == closure {
        return true
      }
      let recursionBudget = 8

      // Recurse into called function
      if let callee = apply.referencedFunction,
         callee.isDefinition,
         handledFuncs.insert(callee).inserted,
         handledFuncs.count <= recursionBudget,
         let calleeArg = apply.calleeArgument(of: use, in: callee)
      {
        if checkRecursivelyIfClosureIsApplied(calleeArg, &handledFuncs) {
          return true
        }
      }

    case is CopyValueInst, is MoveValueInst:
      if checkRecursivelyIfClosureIsApplied(use.instruction as! SingleValueInstruction, &handledFuncs) {
        return true
      }

    default:
      break
    }
  }

  return false
}

/// Add destroys for values which are not consumed, yet.
/// There are two cases of values for which we need this:
/// 1. guaranteed closure arguments
/// 2. operands of copies at the call site
///
/// ```
///   %1 = partial_apply %closure   // is cloned to the specialized function
///   %2 = copy_value %1            // is cloned to the specialized function
///   apply %function(%2) : (@guaranteed () -> ()) -> ()
///   destroy_value %1
///
/// sil @specializedFunction() -> () {
/// bb0:
///   %1 = partial_apply %closure
///   %2 = copy_value %1
///   ... // body
///   destroy_value %2              // case 1: destroy for the guaranteed closure argument
///   destroy_value %1              // case 2: destroy for the operand of the copy
///   return
/// ```
private func addMissingDestroysAtFunctionExits(for clonedArguments: [Value], _ context: FunctionPassContext) {
  var needDestroy = ValueWorklist(context)
  defer { needDestroy.deinitialize() }

  for clonedArg in clonedArguments {
    findValuesWhichNeedDestroyRecursively(value: clonedArg, needDestroy: &needDestroy)
  }

  while let valueToDestroy = needDestroy.pop() {
    Builder.insertCleanupAtFunctionExits(of: valueToDestroy.parentFunction, context) { builder in
      builder.createDestroyValue(operand: valueToDestroy)
    }
  }
}

private func findValuesWhichNeedDestroyRecursively(value: Value, needDestroy: inout ValueWorklist) {
  if let svi = value as? SingleValueInstruction {
    for op in svi.operands {
      findValuesWhichNeedDestroyRecursively(value: op.value, needDestroy: &needDestroy)
    }
  }
  if value.ownership == .owned && value.uses.endingLifetime.isEmpty {
    needDestroy.pushIfNotVisited(value)
  }
}

private extension ParameterInfo {
  func withSpecializedConvention(for partialApply: PartialApplyInst, in callee: Function) -> Self {
    let argType = type.loweredType(in: partialApply.parentFunction)
    let specializedParamConvention = if self.convention.isIndirect {
      self.convention
    } else {
      if argType.isTrivial(in: callee) {
        ArgumentConvention.directUnowned
      } else {
        if partialApply.isOnStack {
          ArgumentConvention.directGuaranteed
        } else {
          ArgumentConvention.directOwned
        }
      }
    }

    return ParameterInfo(
      type: self.type, convention: specializedParamConvention, options: self.options,
      hasLoweredAddresses: self.hasLoweredAddresses)
  }
}

private extension PartialApplyInst {
  /// True, if the closure obtained from this partial_apply is the
  /// pullback returned from an autodiff VJP
  var isPullbackInResultOfAutodiffVJP: Bool {
    if self.parentFunction.isAutodiffVJP,
      let use = self.uses.singleUse,
      let tupleInst = use.instruction as? TupleInst,
      let returnInst = self.parentFunction.returnInstruction,
      tupleInst == returnInst.returnedValue
    {
      return true
    }

    return false
  }

  var isPartialApplyOfThunk: Bool {
    if self.numArguments == 1,
      let fun = self.referencedFunction,
      fun.thunkKind == .reabstractionThunk || fun.thunkKind == .thunk,
      self.arguments[0].type.isLoweredFunction,
      self.arguments[0].type.isReferenceCounted(in: self.parentFunction) || self.callee.type.isThickFunction
    {
      return true
    }
    return false
  }

  var hasOnlyInoutIndirectArguments: Bool {
    self.argumentOperands
      .filter { !$0.value.type.isObject }
      .allSatisfy { self.convention(of: $0)!.isInout }
  }

  var allArgumentsCanBeCopied: Bool {
    arguments.allSatisfy { !$0.type.isMoveOnly }
  }
}

private extension Function {
  var effectAllowsSpecialization: Bool {
    switch self.effectAttribute {
    case .readNone, .readOnly, .releaseNone: return false
    default: return true
    }
  }
}