Merge pull request #42242 from eeckstein/escapeinfo

Swift Optimizer: add `EscapeInfo`, a new utility for escape analysis

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/Analysis/AliasAnalysis.swift

@@ -43,4 +43,21 @@ struct AliasAnalysis {
         return false
     }
   }
+
+  /// Returns the correct path for address-alias functions.
+  static func getPtrOrAddressPath(for value: Value) -> EscapeInfo.Path {
+    let ty = value.type
+    if ty.isAddress {
+      // This is the regular case: the path selects any sub-fields of an address.
+      return EscapeInfo.Path(.anyValueFields)
+    }
+    // Some optimizations use the address-alias APIs with non-address SIL values.
+    // TODO: this is non-intuitive and we should eliminate those API uses.
+    if ty.isClass {
+    // If the value is a (non-address) reference it means: all addresses within the class instance.
+      return EscapeInfo.Path(.anyValueFields).push(.anyClassField)
+    }
+    // Any other non-address value means: all addresses of any referenced class instances within the value.
+    return EscapeInfo.Path(.anyValueFields).push(.anyClassField).push(.anyValueFields)
+  }
 }

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/CMakeLists.txt

@@ -8,6 +8,8 @@
 
 swift_compiler_sources(Optimizer
   AssumeSingleThreaded.swift
+  ComputeEffects.swift
+  EscapeInfoDumper.swift
   SILPrinter.swift
   MergeCondFails.swift
   RangeDumper.swift

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/ComputeEffects.swift

@@ -0,0 +1,235 @@
+//===--- ComputeEffects.swift - Compute function effects ------------------===//
+//
+// 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 SIL
+
+fileprivate typealias Selection = ArgumentEffect.Selection
+fileprivate typealias Path = ArgumentEffect.Path
+
+/// Computes effects for function arguments.
+///
+/// For example, if an argument does not escape, adds a non-escaping effect,
+/// e.g. "[escapes !%0.**]":
+///
+///   sil [escapes !%0.**] @foo : $@convention(thin) (@guaranteed X) -> () {
+///   bb0(%0 : $X):
+///     %1 = tuple ()
+///     return %1 : $()
+///   }
+///
+/// The pass does not try to change or re-compute _defined_ effects.
+/// Currently, only escaping effects are handled.
+/// In future, this pass may also add other effects, like memory side effects.
+let computeEffects = FunctionPass(name: "compute-effects", {
+  (function: Function, context: PassContext) in
+
+  var argsWithDefinedEffects = getArgIndicesWithDefinedEffects(of: function)
+
+  var escapeInfo = EscapeInfo(calleeAnalysis: context.calleeAnalysis)
+  var newEffects = Stack<ArgumentEffect>(context)
+  let returnInst = function.returnInstruction
+
+  for arg in function.arguments {
+    // We are not interested in arguments with trivial types.
+    if !arg.type.isNonTrivialOrContainsRawPointer(in: function) { continue }
+    
+    // Also, we don't want to override defined effects.
+    if argsWithDefinedEffects.contains(arg.index) { continue }
+    
+    // First check: is the argument (or a projected value of it) escaping at all?
+    if !escapeInfo.isEscapingWhenWalkingDown(object: arg, path: Path(.anything),
+        visitUse: { op, _, _ in
+          isOperandOfRecursiveCall(op) ? .ignore : .continueWalking
+        }) {
+      let selectedArg = Selection(arg, pathPattern: Path(.anything))
+      newEffects.push(ArgumentEffect(.notEscaping, selectedArg: selectedArg))
+      continue
+    }
+  
+    // Now compute effects for two important cases:
+    //   * the argument itself + any value projections, and...
+    if addArgEffects(arg, argPath: Path(), to: &newEffects, returnInst, &escapeInfo) {
+      //   * single class indirections
+      _ = addArgEffects(arg, argPath: Path(.anyValueFields).push(.anyClassField),
+                        to: &newEffects, returnInst, &escapeInfo)
+    }
+  }
+
+  context.modifyEffects(in: function) { (effects: inout FunctionEffects) in
+    effects.removeDerivedEffects()
+    effects.argumentEffects.append(contentsOf: newEffects)
+  }
+  newEffects.removeAll()
+})
+
+/// Returns true if an argument effect was added.
+private
+func addArgEffects(_ arg: FunctionArgument, argPath ap: Path,
+                   to newEffects: inout Stack<ArgumentEffect>,
+                   _ returnInst: ReturnInst?,
+                   _ escapeInfo: inout EscapeInfo) -> Bool {
+
+  var toSelection: Selection?
+  // Correct the path if the argument is not a class reference itself, but a value type
+  // containing one or more references.
+  let argPath = arg.type.isClass ? ap : ap.push(.anyValueFields)
+
+  if escapeInfo.isEscapingWhenWalkingDown(object: arg, path: argPath,
+      visitUse: { op, path, followStores in
+        if op.instruction == returnInst {
+          // The argument escapes to the function return
+          if followStores {
+            // The escaping path must not introduce a followStores.
+            return .markEscaping
+          }
+          if let ta = toSelection {
+            if ta.value != .returnValue { return .markEscaping }
+            toSelection = Selection(.returnValue, pathPattern: path.merge(with: ta.pathPattern))
+          } else {
+            toSelection = Selection(.returnValue, pathPattern: path)
+          }
+          return .ignore
+        }
+        if isOperandOfRecursiveCall(op) {
+          return .ignore
+        }
+        return .continueWalking
+      },
+      visitDef: { def, path, followStores in
+        guard let destArg = def as? FunctionArgument else {
+          return .continueWalkingUp
+        }
+        // The argument escapes to another argument (e.g. an out or inout argument)
+        if followStores {
+          // The escaping path must not introduce a followStores.
+          return .markEscaping
+        }
+        let argIdx = destArg.index
+        if let ta = toSelection {
+          if ta.value != .argument(argIdx) { return .markEscaping }
+          toSelection = Selection(.argument(argIdx), pathPattern: path.merge(with: ta.pathPattern))
+        } else {
+          toSelection = Selection(.argument(argIdx), pathPattern: path)
+        }
+        return .continueWalkingDown
+      }) {
+    return false
+  }
+
+  let fromSelection = Selection(arg, pathPattern: argPath)
+
+  guard let toSelection = toSelection else {
+    newEffects.push(ArgumentEffect(.notEscaping, selectedArg: fromSelection))
+    return true
+  }
+  
+  // If the function never returns, the argument can not escape to another arg/return.
+  guard let returnInst = returnInst else {
+    return false
+  }
+
+  let exclusive = isExclusiveEscape(fromArgument: arg, fromPath: argPath, to: toSelection, returnInst, &escapeInfo)
+
+  newEffects.push(ArgumentEffect(.escaping(toSelection, exclusive), selectedArg: fromSelection))
+  return true
+}
+
+/// Returns a set of argument indices for which there are "defined" effects (as opposed to derived effects).
+private func getArgIndicesWithDefinedEffects(of function: Function) -> Set<Int> {
+  var argsWithDefinedEffects = Set<Int>()
+
+  for effect in function.effects.argumentEffects {
+    if effect.isDerived { continue }
+
+    if case .argument(let argIdx) = effect.selectedArg.value {
+      argsWithDefinedEffects.insert(argIdx)
+    }
+
+    switch effect.kind {
+    case .notEscaping:
+      break
+    case .escaping(let to, _):
+      if case .argument(let toArgIdx) = to.value {
+        argsWithDefinedEffects.insert(toArgIdx)
+      }
+    }
+  }
+  return argsWithDefinedEffects
+}
+
+/// Returns true if `op` is passed to a recursive call to the current function -
+/// at the same argument index.
+private func isOperandOfRecursiveCall(_ op: Operand) -> Bool {
+  let inst = op.instruction
+  if let applySite = inst as? FullApplySite,
+     let callee = applySite.referencedFunction,
+     callee == inst.function,
+     let argIdx = applySite.argumentIndex(of: op),
+     op.value == callee.arguments[argIdx] {
+    return true
+  }
+  return false
+}
+
+/// Returns true if when walking from the `toSelection` to the `fromArgument`,
+/// there are no other arguments or escape points than `fromArgument`. Also, the
+/// path at the `fromArgument` must match with `fromPath`.
+private
+func isExclusiveEscape(fromArgument: Argument, fromPath: Path, to toSelection: Selection,
+                       _ returnInst: ReturnInst, _ escapeInfo: inout EscapeInfo) -> Bool {
+  switch toSelection.value {
+  
+  // argument -> return
+  case .returnValue:
+    if escapeInfo.isEscaping(
+          object: returnInst.operand, path: toSelection.pathPattern,
+          visitUse: { op, path, followStores in
+            if op.instruction == returnInst {
+              if followStores { return .markEscaping }
+              if path.matches(pattern: toSelection.pathPattern) {
+                return .ignore
+              }
+              return .markEscaping
+            }
+            return .continueWalking
+          },
+          visitDef: { def, path, followStores in
+            guard let arg = def as? FunctionArgument else {
+              return .continueWalkingUp
+            }
+            if followStores { return .markEscaping }
+            if arg == fromArgument && path.matches(pattern: fromPath) {
+              return .continueWalkingDown
+            }
+            return .markEscaping
+          }) {
+      return false
+    }
+    
+  // argument -> argument
+  case .argument(let toArgIdx):
+    let toArg = returnInst.function.arguments[toArgIdx]
+    if escapeInfo.isEscaping(object: toArg, path: toSelection.pathPattern,
+        visitDef: { def, path, followStores in
+          guard let arg = def as? FunctionArgument else {
+            return .continueWalkingUp
+          }
+          if followStores { return .markEscaping }
+          if arg == fromArgument && path.matches(pattern: fromPath) { return .continueWalkingDown }
+          if arg == toArg && path.matches(pattern: toSelection.pathPattern) { return .continueWalkingDown }
+          return .markEscaping
+        }) {
+      return false
+    }
+  }
+  return true
+}