IRGen: Factor out LegacyLayoutFormat.h and add a descriptive comment

Author: slavapestov

lib/IRGen/LegacyLayoutFormat.h

@@ -0,0 +1,104 @@
+//===--- LegacyLayoutFormat.h - YAML format for legacy layout ---*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the YAML format for the backward deployment type layout
+// dump.
+//
+// When a class with @objc ancestry is statically visible to Clang code, older
+// Objective-C runtimes do not give us the opportunity to run any code to
+// initialize the class metadata when it is realized.
+//
+// This creates a problem if the class has resiliently-sized fields. Since the
+// standard library and overlays are now built with resilience enabled, this
+// creates a backward-compatibility issue where such class definitions would now
+// require singleton metadata initialization instead of idempotent metadata
+// initialization, and singleton metadata initializaiton precludes the class
+// from being statically visible to Clang.
+//
+// To support this case, we emit fixed metadata for any such class, and in place
+// of each resilient field type, we use previously-emitted fixed type info.
+//
+// This fixed type info must match the Swift standard library and overlays used
+// for backward deployment, since on older Objective-C runtimes these layouts
+// will be used at runtime.
+//
+// However, since these types are resilient, their layouts might change in the
+// future. Newer Objective-C runtimes will expose a hook allowing the Swift
+// runtime to re-compute the class layout when the class is realized.
+//
+// Note that except for metadata emission, field accesses and instance
+// allocation for such classes must proceed as if they use singleton metadata
+// initialization, loading the field offsets from global variables and loading
+// the size and alignment dynamically from metadata when allocating.
+//
+// Also, any Swift-side accesses of the metadata must call the metadata accessor
+// function, allowing the Swift runtime to re-initialize the layout if
+// necessary.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_IRGEN_LEGACY_LAYOUT_FORMAT_H
+#define SWIFT_IRGEN_LEGACY_LAYOUT_FORMAT_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/YAMLTraits.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace swift {
+namespace irgen {
+
+struct YAMLTypeInfoNode {
+  std::string Name;
+  uint64_t Size;
+  uint64_t Alignment;
+  uint64_t NumExtraInhabitants;
+
+  bool operator<(const YAMLTypeInfoNode &other) const {
+    return Name < other.Name;
+  }
+};
+
+struct YAMLModuleNode {
+  StringRef Name;
+  std::vector<YAMLTypeInfoNode> Decls;
+};
+
+} // namespace irgen
+} // namespace swift
+
+namespace llvm {
+namespace yaml {
+
+template <> struct MappingTraits<swift::irgen::YAMLTypeInfoNode> {
+  static void mapping(IO &io, swift::irgen::YAMLTypeInfoNode &node) {
+    io.mapRequired("Name", node.Name);
+    io.mapRequired("Size", node.Size);
+    io.mapRequired("Alignment", node.Alignment);
+    io.mapRequired("ExtraInhabitants", node.NumExtraInhabitants);
+  }
+};
+
+template <> struct MappingTraits<swift::irgen::YAMLModuleNode> {
+  static void mapping(IO &io, swift::irgen::YAMLModuleNode &node) {
+    io.mapRequired("Name", node.Name);
+    io.mapOptional("Decls", node.Decls);
+  }
+};
+
+} // namespace yaml
+} // namespace llvm
+
+LLVM_YAML_IS_SEQUENCE_VECTOR(swift::irgen::YAMLTypeInfoNode);
+
+#endif // SWIFT_IRGEN_LEGACY_LAYOUT_FORMAT_H

lib/IRGen/TypeLayoutDumper.cpp

@@ -20,6 +20,7 @@
 #include "GenType.h"
 #include "IRGen.h"
 #include "IRGenModule.h"
+#include "LegacyLayoutFormat.h"
 
 #include "swift/AST/ASTContext.h"
 #include "swift/AST/ASTMangler.h"
@@ -58,48 +59,8 @@ class NominalTypeWalker : public ASTWalker {
   }
 };
 
-struct YAMLTypeInfoNode {
-  std::string Name;
-  uint64_t Size;
-  uint64_t Alignment;
-  uint64_t NumExtraInhabitants;
-
-  bool operator<(const YAMLTypeInfoNode &other) const {
-    return Name < other.Name;
-  }
-};
-
-struct YAMLModuleNode {
-  StringRef Name;
-  std::vector<YAMLTypeInfoNode> Decls;
-};
-
 } // end anonymous namespace
 
-namespace llvm {
-namespace yaml {
-
-template <> struct MappingTraits<YAMLTypeInfoNode> {
-  static void mapping(IO &io, YAMLTypeInfoNode &node) {
-    io.mapRequired("Name", node.Name);
-    io.mapRequired("Size", node.Size);
-    io.mapRequired("Alignment", node.Alignment);
-    io.mapRequired("ExtraInhabitants", node.NumExtraInhabitants);
-  }
-};
-
-template <> struct MappingTraits<YAMLModuleNode> {
-  static void mapping(IO &io, YAMLModuleNode &node) {
-    io.mapRequired("Name", node.Name);
-    io.mapOptional("Decls", node.Decls);
-  }
-};
-
-} // namespace yaml
-} // namespace llvm
-
-LLVM_YAML_IS_SEQUENCE_VECTOR(YAMLTypeInfoNode);
-
 static std::string mangleTypeAsContext(const NominalTypeDecl *type) {
   Mangle::ASTMangler Mangler;
   return Mangler.mangleTypeAsContextUSR(type);