StackPromotion.swift

//===--- StackPromotion.swift - Stack promotion optimization --------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2022 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

/// Promotes heap allocated objects to the stack.
///
/// It handles `alloc_ref` and `alloc_ref_dynamic` instructions of native swift
/// classes: if promoted, the `[stack]` attribute is set in the allocation
/// instruction and a `dealloc_stack_ref` is inserted at the end of the object's
/// lifetime.

/// The main criteria for stack promotion is that the allocated object must not
/// escape its function.
///
/// Example:
///   %k = alloc_ref $Klass
///   // .. some uses of %k
///   destroy_value %k  // The end of %k's lifetime
///
/// is transformed to:
///
///   %k = alloc_ref [stack] $Klass
///   // .. some uses of %k
///   destroy_value %k
///   dealloc_stack_ref %k
///
/// The destroy/release of the promoted object remains in the SIL, but is effectively
/// a no-op, because a stack promoted object is initialized with an "immortal"
/// reference count.
/// Later optimizations can clean that up.
let stackPromotion = FunctionPass(name: "stack-promotion") {
  (function: Function, context: FunctionPassContext) in
  
  let deadEndBlocks = context.deadEndBlocks

  var needFixStackNesting = false
  for inst in function.instructions {
    if let allocRef = inst as? AllocRefInstBase {
      if !context.continueWithNextSubpassRun(for: allocRef) {
        break
      }
      if tryPromoteAlloc(allocRef, deadEndBlocks, context) {
        needFixStackNesting = true
      }
    }
  }
  if needFixStackNesting {
    // Make sure that all stack allocating instructions are nested correctly.
    function.fixStackNesting(context)
  }
}

// Returns true if the allocation is promoted.
private func tryPromoteAlloc(_ allocRef: AllocRefInstBase,
                             _ deadEndBlocks: DeadEndBlocksAnalysis,
                             _ context: FunctionPassContext) -> Bool {
  if allocRef.isObjC || allocRef.canAllocOnStack {
    return false
  }

  // Usually resilient classes cannot be promoted anyway, because their initializers are
  // not visible and let the object appear to escape.
  if allocRef.type.nominal!.isResilient(in: allocRef.parentFunction) {
    return false
  }

  if let dtor = (allocRef.type.nominal as? ClassDecl)?.destructor {
    if dtor.isIsolated {
      // Classes (including actors) with isolated deinit can escape implicitly.
      //
      // We could optimize this further and allow promotion if we can prove that
      // deinit will take fast path (i.e. it will not schedule a job).
      // But for now, let's keep things simple and disable promotion conservatively.
      return false
    }
  }

  // The most important check: does the object escape the current function?
  if allocRef.isEscaping(context) {
    return false
  }

  if deadEndBlocks.isDeadEnd(allocRef.parentBlock) {

    // Allocations inside a code region which ends up in a no-return block may missing their
    // final release. Therefore we extend their lifetime indefinitely, e.g.
    //
    //  %k = alloc_ref $Klass
    //  ...
    //  unreachable  // The end of %k's lifetime
    //
    // There is one exception: if it's in a loop (within the dead-end region) we must not
    // extend its lifetime. In this case we can be sure that its final release is not
    // missing, because otherwise the object would be leaking. For example:
    //
    //  bb1:
    //    %k = alloc_ref $Klass
    //    ...                    // %k's lifetime must end somewhere here
    //    cond_br %c, bb1, bb2
    //  bb2:
    //    unreachable
    //
    // Therefore, if the allocation is inside a loop, we can treat it like allocations in
    // non dead-end regions.
    if !isInLoop(block: allocRef.parentBlock, context) {
      allocRef.setIsStackAllocatable(context)
      return true
    }
  }

  // Try to find the top most dominator block which dominates all use points.
  // * This block can be located "earlier" than the actual allocation block, in case the
  //   promoted object is stored into an "outer" object, e.g.
  //
  //   bb0:           // outerDominatingBlock                 _
  //     %o = alloc_ref $Outer                                 |
  //     ...                                                   |
  //   bb1:           // allocation block      _               |
  //     %k = alloc_ref $Klass                  |              | "outer"
  //     %f = ref_element_addr %o, #Outer.f     | "inner"      | liverange
  //     store %k to %f                         | liverange    |
  //     ...                                    |              |
  //     destroy_value %o                      _|             _|
  //
  // * Finding the `outerDominatingBlock` is not guaranteed to work.
  //   In this example, the top most dominator block is `bb0`, but `bb0` has no
  //   use points in the outer liverange. We'll get `bb3` as outerDominatingBlock.
  //   This is no problem because 1. it's an unusual case and 2. the `outerBlockRange`
  //   is invalid in this case and we'll bail later.
  //
  //   bb0:                    // real top most dominating block
  //     cond_br %c, bb1, bb2
  //   bb1:
  //     %o1 = alloc_ref $Outer
  //     br bb3(%o1)
  //   bb2:
  //     %o2 = alloc_ref $Outer
  //     br bb3(%o1)
  //   bb3(%o):                // resulting outerDominatingBlock: wrong!
  //     %k = alloc_ref $Klass
  //     %f = ref_element_addr %o, #Outer.f
  //     store %k to %f
  //     destroy_value %o
  //
  let domTree = context.dominatorTree
  let outerDominatingBlock = getDominatingBlockOfAllUsePoints(context: context, allocRef, domTree: domTree)

  // The "inner" liverange contains all use points which are dominated by the allocation block.
  // Note that this `visit` cannot fail because otherwise our initial `isEscaping` check would have failed already.
  var innerRange = allocRef.visit(using: ComputeInnerLiverange(of: allocRef, domTree, context), context)!
  defer { innerRange.deinitialize() }

  // The "outer" liverange contains all use points.
  // Same here: this `visit` cannot fail.
  var outerBlockRange = allocRef.visit(using: ComputeOuterBlockrange(dominatedBy: outerDominatingBlock, context), context)!
  defer { outerBlockRange.deinitialize() }

  assert(innerRange.blockRange.isValid, "inner range should be valid because we did a dominance check")

  if !outerBlockRange.isValid {
    // This happens if we fail to find a correct outerDominatingBlock.
    return false
  }

  // Check if there is a control flow edge from the inner to the outer liverange, which
  // would mean that the promoted object can escape to the outer liverange.
  // This can e.g. be the case if the inner liverange does not post dominate the outer range:
  //                                                              _
  //     %o = alloc_ref $Outer                                     |
  //     cond_br %c, bb1, bb2                                      |
  //   bb1:                                            _           |
  //     %k = alloc_ref $Klass                          |          | outer
  //     %f = ref_element_addr %o, #Outer.f             | inner    | range
  //     store %k to %f                                 | range    |
  //     br bb2       // branch from inner to outer    _|          |
  //   bb2:                                                        |
  //     destroy_value %o                                         _|
  //
  // Or if there is a loop with a back-edge from the inner to the outer range:
  //                                                              _
  //     %o = alloc_ref $Outer                                     |
  //     br bb1                                                    |
  //   bb1:                                            _           |
  //     %k = alloc_ref $Klass                          |          | outer
  //     %f = ref_element_addr %o, #Outer.f             | inner    | range
  //     store %k to %f                                 | range    |
  //     cond_br %c, bb1, bb2   // inner -> outer      _|          |
  //   bb2:                                                        |
  //     destroy_value %o                                         _|
  //
  if innerRange.blockRange.hasControlFlowEdge(to: outerBlockRange) {
    return false
  }

  // There shouldn't be any critical exit edges from the liverange, because that would mean
  // that the promoted allocation is leaking.
  // Just to be on the safe side, do a check and bail if we find critical exit edges: we
  // cannot insert instructions on critical edges.
  if innerRange.blockRange.containsCriticalExitEdges(deadEndBlocks: deadEndBlocks) {
    return false
  }

  // Do the transformation!
  // Insert `dealloc_stack_ref` instructions at the exit- and end-points of the inner liverange.
  for exitInst in innerRange.exits {
    if !deadEndBlocks.isDeadEnd(exitInst.parentBlock) {
      let builder = Builder(before: exitInst, context)
      builder.createDeallocStackRef(allocRef)
    }
  }

  for endInst in innerRange.ends {
    Builder.insert(after: endInst, location: allocRef.location, context) {
      (builder) in builder.createDeallocStackRef(allocRef)
    }
  }

  allocRef.setIsStackAllocatable(context)
  return true
}

private func getDominatingBlockOfAllUsePoints(context: FunctionPassContext,
                                              _ value: SingleValueInstruction,
                                              domTree: DominatorTree) -> BasicBlock {
  struct FindDominatingBlock : EscapeVisitorWithResult {
    var result: BasicBlock
    let domTree: DominatorTree
    mutating func visitUse(operand: Operand, path: EscapePath) -> UseResult {
      let defBlock = operand.value.parentBlock
      if defBlock.dominates(result, domTree) {
        result = defBlock
      }
      return .continueWalk
    }
  }
  
  return value.visit(using: FindDominatingBlock(result: value.parentBlock, domTree: domTree), context)!
}

private struct ComputeInnerLiverange : EscapeVisitorWithResult {
  var result: InstructionRange
  let domTree: DominatorTree

  init(of instruction: Instruction, _ domTree: DominatorTree, _ context: FunctionPassContext) {
    result = InstructionRange(begin: instruction, context)
    self.domTree = domTree
  }

  mutating func visitUse(operand: Operand, path: EscapePath) -> UseResult {
    let user = operand.instruction
    let beginBlockOfRange = result.blockRange.begin
    if beginBlockOfRange.dominates(user.parentBlock, domTree) {
      result.insert(user)
    }
    return .continueWalk
  }
}

private struct ComputeOuterBlockrange : EscapeVisitorWithResult {
  var result: BasicBlockRange

  init(dominatedBy: BasicBlock, _ context: FunctionPassContext) {
    result = BasicBlockRange(begin: dominatedBy, context)
  }

  mutating func visitUse(operand: Operand, path: EscapePath) -> UseResult {
    let user = operand.instruction
    result.insert(user.parentBlock)

    let value = operand.value
    let operandsDefinitionBlock = value.parentBlock

    // Also insert the operand's definition. Otherwise we would miss allocation
    // instructions (for which the `visitUse` closure is not called).
    result.insert(operandsDefinitionBlock)

    // We need to explicitly add predecessor blocks of phis because they
    // are not necesesarily visited during the down-walk in `isEscaping()`.
    // This is important for the special case where there is a back-edge from the
    // inner range to the inner rage's begin-block:
    //
    //   bb0:                      // <- need to be in the outer range
    //     br bb1(%some_init_val)
    //   bb1(%arg):
    //     %k = alloc_ref $Klass   // innerInstRange.begin
    //     cond_br bb2, bb1(%k)    // back-edge to bb1 == innerInstRange.blockRange.begin
    //
    if let phi = Phi(value) {
      result.insert(contentsOf: phi.predecessors)
    }
    return .continueWalk
  }
}

private extension BasicBlockRange {
  /// Returns true if there is a direct edge connecting this range with the `otherRange`.
  func hasControlFlowEdge(to otherRange: BasicBlockRange) -> Bool {
    func isOnlyInOtherRange(_ block: BasicBlock) -> Bool {
      return !inclusiveRangeContains(block) && otherRange.inclusiveRangeContains(block)
    }

    for lifeBlock in inclusiveRange {
      assert(otherRange.inclusiveRangeContains(lifeBlock), "range must be a subset of other range")
      for succ in lifeBlock.successors {
        if isOnlyInOtherRange(succ) && succ != otherRange.begin {
          return true
        }
        // The entry of the begin-block is conceptually not part of the range. We can check if
        // it's part of the `otherRange` by checking the begin-block's predecessors.
        if succ == begin && begin.predecessors.contains(where: { isOnlyInOtherRange($0) }) {
          return true
        }
      }
    }
    return false
  }

  func containsCriticalExitEdges(deadEndBlocks: DeadEndBlocksAnalysis) -> Bool {
    exits.contains { !deadEndBlocks.isDeadEnd($0) && !$0.hasSinglePredecessor }
  }
}

private func isInLoop(block startBlock: BasicBlock, _ context: FunctionPassContext) -> Bool {
  var worklist = BasicBlockWorklist(context)
  defer { worklist.deinitialize() }

  worklist.pushIfNotVisited(contentsOf: startBlock.successors)
  while let block = worklist.pop() {
    if block == startBlock {
      return true
    }
    worklist.pushIfNotVisited(contentsOf: block.successors)
  }
  return false
}