SIL: add two utility functions to AccessPath

Author: eeckstein

include/swift/SIL/MemAccessUtils.h

@@ -821,6 +821,10 @@ class AccessPath {
   /// access, thus not within an access scope.
   static AccessPath computeInScope(SILValue address);
 
+  /// Creates an AccessPass, which identifies the first tail-element of the
+  /// object \p rootReference.
+  static AccessPath forTailStorage(SILValue rootReference);
+
   // Encode a dynamic index_addr as an UnknownOffset.
   static constexpr int UnknownOffset = std::numeric_limits<int>::min() >> 1;
 
@@ -984,6 +988,12 @@ class AccessPath {
               SILFunction *function,
               unsigned useLimit = std::numeric_limits<unsigned>::max()) const;
 
+  /// Returns a new AccessPass, identical to this AccessPath, except that the
+  /// offset is replaced with \p newOffset.
+  AccessPath withOffset(int newOffset) const {
+    return AccessPath(storage, pathNode, newOffset);
+  }
+
   void printPath(raw_ostream &os) const;
   void print(raw_ostream &os) const;
   void dump() const;

lib/SIL/Utils/MemAccessUtils.cpp

@@ -763,6 +763,13 @@ AccessedStorage AccessedStorage::computeInScope(SILValue sourceAddress) {
 //                              MARK: AccessPath
 //===----------------------------------------------------------------------===//
 
+AccessPath AccessPath::forTailStorage(SILValue rootReference) {
+  return AccessPath(
+    AccessedStorage::forClass(rootReference, AccessedStorage::TailIndex),
+    PathNode(rootReference->getModule()->getIndexTrieRoot()),
+    /*offset*/ 0);
+}
+
 bool AccessPath::contains(AccessPath subPath) const {
   if (!isValid() || !subPath.isValid()) {
     return false;