[Syntax] Perf: Optimize 'root' and 'ancestorOrSelf'

Author: rintaro

Diff for: Sources/SwiftSyntax/Syntax.swift

@@ -81,10 +81,13 @@ public struct Syntax: SyntaxProtocol, SyntaxHashable {
     }
   }
 
-  private var root: Syntax {
-    switch info.info! {
-    case .root(_): return self
-    case .nonRoot(let info): return info.parent.root
+  public var root: Syntax {
+    return self.withUnownedSyntax {
+      var node = $0
+      while let parent = node.parent {
+        node = parent
+      }
+      return node.value
     }
   }
 
@@ -129,7 +132,10 @@ public struct Syntax: SyntaxProtocol, SyntaxHashable {
   }
 
   /// "designated" memberwise initializer of `Syntax`.
-  init(_ raw: RawSyntax, info: Info) {
+  // transparent because normal inlining is too late for eliminating ARC traffic for Info.
+  // FIXME: Remove @_transparent after OSSA enabled.
+  @_transparent
+  init(_ raw: RawSyntax, info: __shared Info) {
     self.raw = raw
     self.info = info
   }
@@ -309,7 +315,7 @@ public struct Syntax: SyntaxProtocol, SyntaxHashable {
   /// Create a ``Syntax`` node from a specialized syntax node.
   // Inline always so the optimizer can optimize this to a member access on `syntax` without having to go through
   // generics.
-  @inline(__always)
+  @_transparent
   public init(_ syntax: __shared some SyntaxProtocol) {
     self = syntax._syntaxNode
   }
@@ -380,6 +386,59 @@ extension Syntax {
   }
 }
 
+/// Temporary non-owning Syntax.
+///
+/// This can be used for handling Syntax node without ARC traffic.
+struct UnownedSyntax {
+  private let raw: RawSyntax
+  private let info: Unmanaged<Syntax.Info>
+
+  @_transparent
+  init(_ node: __shared Syntax) {
+    self.raw = node.raw
+    self.info = .passUnretained(node.info.unsafelyUnwrapped)
+  }
+
+  /// Extract the Syntax value.
+  @inline(__always)
+  var value: Syntax {
+    Syntax(raw, info: info.takeUnretainedValue())
+  }
+
+  /// Get the parent of the Syntax value, but without retaining it.
+  @inline(__always)
+  var parent: UnownedSyntax? {
+    return info._withUnsafeGuaranteedRef {
+      switch $0.info.unsafelyUnwrapped {
+      case .nonRoot(let info):
+        return UnownedSyntax(info.parent)
+      case .root(_):
+        return nil
+      }
+    }
+  }
+
+  /// Temporarily use the Syntax value.
+  @inline(__always)
+  func withValue<T>(_ body: (Syntax) -> T) -> T {
+    info._withUnsafeGuaranteedRef {
+      body(Syntax(self.raw, info: $0))
+    }
+  }
+}
+
+extension SyntaxProtocol {
+  /// Execute the `body` with ``UnownedSyntax`` of `node`.
+  ///
+  /// This guarantees the life time of the `node` during the `body` is executed.
+  @inline(__always)
+  func withUnownedSyntax<T>(_ body: (UnownedSyntax) -> T) -> T {
+    return withExtendedLifetime(self) {
+      body(UnownedSyntax(Syntax($0)))
+    }
+  }
+}
+
 /// ``SyntaxNode`` used to be a pervasive type name in SwiftSyntax that has been
 /// replaced by the ``Syntax`` type.
 @available(*, unavailable, message: "use 'Syntax' instead")

Diff for: Sources/SwiftSyntax/SyntaxProtocol.swift

@@ -212,11 +212,7 @@ extension SyntaxProtocol {
 
   /// The root of the tree in which this node resides.
   public var root: Syntax {
-    var this = _syntaxNode
-    while let parent = this.parent {
-      this = parent
-    }
-    return this
+    return Syntax(self).root
   }
 
   /// Whether or not this node has a parent.
@@ -241,14 +237,18 @@ extension SyntaxProtocol {
 
   /// Returns this node or the first ancestor that satisfies `condition`.
   public func ancestorOrSelf<T>(mapping map: (Syntax) -> T?) -> T? {
-    var walk: Syntax? = Syntax(self)
-    while let unwrappedParent = walk {
-      if let mapped = map(unwrappedParent) {
-        return mapped
+    return self.withUnownedSyntax {
+      var node = $0
+      while true {
+        if let mapped = node.withValue(map) {
+          return mapped
+        }
+        guard let parent = node.parent else {
+          return nil
+        }
+        node = parent
       }
-      walk = unwrappedParent.parent
     }
-    return nil
   }
 }