swift-mirror/SwiftCompilerSources/Sources/Optimizer/ModulePasses/MandatoryPerformanceOptimizations.swift

//===--- MandatoryPerformanceOptimizations.swift --------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2023 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 SIL

/// Performs mandatory optimizations for performance-annotated functions, and global
/// variable initializers that are required to be statically initialized.
///
/// Optimizations include:
/// * de-virtualization
/// * mandatory inlining
/// * generic specialization
/// * mandatory memory optimizations
/// * dead alloc elimination
/// * instruction simplification
///
/// The pass starts with performance-annotated functions / globals and transitively handles
/// called functions.
///
let mandatoryPerformanceOptimizations = ModulePass(name: "mandatory-performance-optimizations") {
  (moduleContext: ModulePassContext) in

  var worklist = FunctionWorklist()
  worklist.addAllPerformanceAnnotatedFunctions(of: moduleContext)
  worklist.addAllAnnotatedGlobalInitOnceFunctions(of: moduleContext)

  optimizeFunctionsTopDown(using: &worklist, moduleContext)
}

private func optimizeFunctionsTopDown(using worklist: inout FunctionWorklist,
                                      _ moduleContext: ModulePassContext) {
  while let f = worklist.pop() {
    moduleContext.transform(function: f) { context in
      if !context.loadFunction(function: f, loadCalleesRecursively: true) {
        return
      }
      optimize(function: f, context)
      worklist.add(calleesOf: f)
    }
  }
}

fileprivate struct PathFunctionTuple: Hashable {
  var path: SmallProjectionPath
  var function: Function
}

private func optimize(function: Function, _ context: FunctionPassContext) {
  var alreadyInlinedFunctions: Set<PathFunctionTuple> = Set()
  
  var changed = true
  while changed {
    changed = runSimplification(on: function, context, preserveDebugInfo: true) { instruction, simplifyCtxt in
      if let i = instruction as? OnoneSimplifyable {
        i.simplify(simplifyCtxt)
        if instruction.isDeleted {
          return
        }
      }
      switch instruction {
      case let apply as FullApplySite:
        inlineAndDevirtualize(apply: apply, alreadyInlinedFunctions: &alreadyInlinedFunctions, context, simplifyCtxt)
      default:
        break
      }
    }

    _ = context.specializeApplies(in: function, isMandatory: true)

    removeUnusedMetatypeInstructions(in: function, context)

    // If this is a just specialized function, try to optimize copy_addr, etc.
    changed = context.optimizeMemoryAccesses(in: function) || changed
    _ = context.eliminateDeadAllocations(in: function)
  }
}

private func inlineAndDevirtualize(apply: FullApplySite, alreadyInlinedFunctions: inout Set<PathFunctionTuple>,
                                   _ context: FunctionPassContext, _ simplifyCtxt: SimplifyContext) {
  if simplifyCtxt.tryDevirtualize(apply: apply, isMandatory: true) != nil {
    return
  }

  guard let callee = apply.referencedFunction else {
    return
  }

  if !context.loadFunction(function: callee, loadCalleesRecursively: true) {
    // We don't have the funcion body of the callee.
    return
  }

  if shouldInline(apply: apply, callee: callee, alreadyInlinedFunctions: &alreadyInlinedFunctions) {
    simplifyCtxt.inlineFunction(apply: apply, mandatoryInline: true)

    // In OSSA `partial_apply [on_stack]`s are represented as owned values rather than stack locations.
    // It is possible for their destroys to violate stack discipline.
    // When inlining into non-OSSA, those destroys are lowered to dealloc_stacks.
    // This can result in invalid stack nesting.
    if callee.hasOwnership && !apply.parentFunction.hasOwnership  {
      simplifyCtxt.notifyInvalidatedStackNesting()
    }
  }
}

private func removeUnusedMetatypeInstructions(in function: Function, _ context: FunctionPassContext) {
  for inst in function.instructions {
    if let mt = inst as? MetatypeInst,
       mt.isTriviallyDeadIgnoringDebugUses {
      context.erase(instructionIncludingDebugUses: mt)
    }
  }
}

private func shouldInline(apply: FullApplySite, callee: Function, alreadyInlinedFunctions: inout Set<PathFunctionTuple>) -> Bool {
  if callee.isTransparent {
    return true
  }
  if apply is BeginApplyInst {
    // Avoid co-routines because they might allocate (their context).
    return true
  }
  if apply.parentFunction.isGlobalInitOnceFunction && callee.inlineStrategy == .always {
    // Some arithmetic operations, like integer conversions, are not transparent but `inline(__always)`.
    // Force inlining them in global initializers so that it's possible to statically initialize the global.
    return true
  }
  if apply.parentFunction.isGlobalInitOnceFunction,
      let global = apply.parentFunction.getInitializedGlobal(),
      global.mustBeInitializedStatically,
      let applyInst = apply as? ApplyInst,
      let projectionPath = applyInst.isStored(to: global),
      alreadyInlinedFunctions.insert(PathFunctionTuple(path: projectionPath, function: callee)).inserted {
    return true
  }
  return false
}

private extension Value {
  /// Analyzes the def-use chain of an apply instruction, and looks for a single chain that leads to a store instruction
  /// that initializes a part of a global variable or the entire variable:
  ///
  /// Example:
  ///   %g = global_addr @global
  ///   ...
  ///   %f = function_ref @func
  ///   %apply = apply %f(...)
  ///   store %apply to %g   <--- is a store to the global trivially (the apply result is immediately going into a store)
  ///
  /// Example:
  ///   %apply = apply %f(...)
  ///   %apply2 = apply %f2(%apply)
  ///   store %apply2 to %g   <--- is a store to the global (the apply result has a single chain into the store)
  ///
  /// Example:
  ///   %a = apply %f(...)
  ///   %s = struct $MyStruct (%a, %b)
  ///   store %s to %g   <--- is a partial store to the global (returned SmallProjectionPath is MyStruct.s0)
  ///
  /// Example:
  ///   %a = apply %f(...)
  ///   %as = struct $AStruct (%other, %a)
  ///   %bs = struct $BStruct (%as, %bother)
  ///   store %bs to %g   <--- is a partial store to the global (returned SmallProjectionPath is MyStruct.s0.s1)
  ///
  /// Returns nil if we cannot find a singular def-use use chain (e.g. because a value has more than one user)
  /// leading to a store to the specified global variable.
  func isStored(to global: GlobalVariable) -> SmallProjectionPath? {
    var singleUseValue: any Value = self
    var path = SmallProjectionPath()
    while true {
      guard let use = singleUseValue.uses.singleNonDebugUse else {
        return nil
      }
      
      switch use.instruction {
      case is StructInst:
        path = path.push(.structField, index: use.index)
        break
      case is TupleInst:
        path = path.push(.tupleField, index: use.index)
        break
      case let ei as EnumInst:
        path = path.push(.enumCase, index: ei.caseIndex)
        break
      case let si as StoreInst:
        guard let storeDestination = si.destination as? GlobalAddrInst else {
          return nil
        }

        guard storeDestination.global == global else {
          return nil
        }
        
        return path
      default:
        return nil
      }

      guard let nextInstruction = use.instruction as? SingleValueInstruction else {
        return nil
      }

      singleUseValue = nextInstruction
    }
  }
}

private extension Function {
  /// Analyzes the global initializer function and returns global it initializes (from `alloc_global` instruction).
  func getInitializedGlobal() -> GlobalVariable? {
    for inst in self.entryBlock.instructions {
      switch inst {
      case let agi as AllocGlobalInst:
        return agi.global
      default:
        break
      }
    }

    return nil
  }
}

fileprivate struct FunctionWorklist {
  private(set) var functions = Array<Function>()
  private var pushedFunctions = Set<Function>()
  private var currentIndex = 0

  mutating func pop() -> Function? {
    if currentIndex < functions.count {
      let f = functions[currentIndex]
      currentIndex += 1
      return f
    }
    return nil
  }

  mutating func addAllPerformanceAnnotatedFunctions(of moduleContext: ModulePassContext) {
    for f in moduleContext.functions where f.performanceConstraints != .none {
      pushIfNotVisited(f)
    }
  }

  mutating func addAllAnnotatedGlobalInitOnceFunctions(of moduleContext: ModulePassContext) {
    for f in moduleContext.functions where f.isGlobalInitOnceFunction {
      if let global = f.getInitializedGlobal(),
         global.mustBeInitializedStatically {
        pushIfNotVisited(f)
      }
    }
  }

  mutating func add(calleesOf function: Function) {
    for inst in function.instructions {
      switch inst {
      case let apply as ApplySite:
        if let callee = apply.referencedFunction {
          pushIfNotVisited(callee)
        }
      case let bi as BuiltinInst:
        switch bi.id {
        case .Once, .OnceWithContext:
          if let fri = bi.operands[1].value as? FunctionRefInst {
            pushIfNotVisited(fri.referencedFunction)
          }
          break;
        default:
          break
        }
      default:
        break
      }
    }
  }

  mutating func pushIfNotVisited(_ element: Function) {
    if pushedFunctions.insert(element).inserted {
      functions.append(element)
    }
  }
}