[SILOptimizer] Generalize optimization of static keypaths

We have an optimization in SILCombiner that "inlines" the use of compile-time constant key paths by performing the property access directly instead of calling a runtime function (leading to huge performance gains e.g. for heavy use of @dynamicMemberLookup). However, this optimization previously only supported key paths which solely access stored properties, so computed properties, optional chaining, etc. still had to call a runtime function. This commit generalizes the optimization to support all types of key paths.

include/swift/SILOptimizer/Utils/KeyPathProjector.h

@@ -0,0 +1,81 @@
+//===-- KeyPathProjector.h - Project a static key path ----------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Utility class to project a statically known key path
+/// expression to a direct property access sequence.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_SILOPTIMIZER_UTILS_KEYPATHPROJECTOR_H
+#define SWIFT_SILOPTIMIZER_UTILS_KEYPATHPROJECTOR_H
+
+#include "swift/SIL/SILBuilder.h"
+#include <memory>
+
+namespace swift {
+
+/// Projects a statically known key path expression to
+/// a direct property access.
+class KeyPathProjector {
+public:
+  /// The type of a key path access.
+  enum class AccessType {
+    /// A get-only access (i.e. swift_getAtKeyPath).
+    Get,
+    
+    /// A set-only access (i.e. swift_setAtWritableKeyPath).
+    Set,
+    
+    /// A modification (i.e. swift_modifyAtWritableKeyPath).
+    Modify
+  };
+  
+  /// Creates a key path projector for a key path.
+  ///
+  /// Returns nullptr if \p keyPath is not a keypath instruction or if there is
+  /// any other reason why the optimization cannot be done.
+  ///
+  /// \param keyPath The key path to project. Must be the result of either
+  ///    a keypath instruction or an upcast of a key path instruction.
+  /// \param root The address of the object the key path is applied to.
+  /// \param loc The location of the key path application.
+  /// \param builder The SILBuilder to use.
+  static std::unique_ptr<KeyPathProjector>
+  create(SILValue keyPath, SILValue root, SILLocation loc, SILBuilder &builder);
+  
+  /// Projects the key path to an address. Sets up the projection,
+  /// invokes the callback, then tears down the projection.
+  /// \param accessType The access type of the projected address.
+  /// \param callback A callback to invoke with the projected adddress.
+  ///     The projected address is only valid from within \p callback.
+  virtual void project(AccessType accessType,
+               std::function<void(SILValue addr)> callback) = 0;
+    
+  virtual ~KeyPathProjector() {};
+  
+  /// Whether this projection returns a struct.
+  virtual bool isStruct() = 0;
+protected:
+  KeyPathProjector(SILLocation loc, SILBuilder &builder)
+      : loc(loc), builder(builder) {}
+  
+  /// The location of the key path application.
+  SILLocation loc;
+  
+  /// The SILBuilder to use.
+  SILBuilder &builder;
+};
+
+}  // end namespace swift
+
+#endif /* KeyPathProjector_h */