1 files changed, 32 insertions, 0 deletions
diff --git a/lib/Lex/PPCaching.cpp b/lib/Lex/PPCaching.cpp
index 794e9c4e76..e69eac65ff 100644
--- a/lib/Lex/PPCaching.cpp
+++ b/lib/Lex/PPCaching.cpp
@@ -15,6 +15,38 @@
 #include "clang/Lex/Preprocessor.h"
 using namespace clang;
 
+/// EnableBacktrackAtThisPos - From the point that this method is called, and
+/// until DisableBacktrack() or Backtrack() is called, the Preprocessor keeps
+/// track of the lexed tokens so that a subsequent Backtrack() call will make
+/// the Preprocessor re-lex the same tokens.
+///
+/// Nested backtracks are allowed, meaning that EnableBacktrackAtThisPos can
+/// be called multiple times and DisableBacktrack/Backtrack calls will be
+/// combined with the EnableBacktrackAtThisPos calls in reverse order.
+void Preprocessor::EnableBacktrackAtThisPos() {
+  CacheTokens = true;
+  BacktrackPositions.push_back(CachedLexPos);
+  EnterCachingLexMode();
+}
+
+/// DisableBacktrack - Disable the last EnableBacktrackAtThisPos() call.
+void Preprocessor::DisableBacktrack() {
+  assert(!BacktrackPositions.empty()
+         && "EnableBacktrackAtThisPos was not called!");
+  BacktrackPositions.pop_back();
+  CacheTokens = !BacktrackPositions.empty();
+}
+
+/// Backtrack - Make Preprocessor re-lex the tokens that were lexed since
+/// EnableBacktrackAtThisPos() was previously called. 
+void Preprocessor::Backtrack() {
+  assert(!BacktrackPositions.empty()
+         && "EnableBacktrackAtThisPos was not called!");
+  CachedLexPos = BacktrackPositions.back();
+  BacktrackPositions.pop_back();
+  CacheTokens = !BacktrackPositions.empty();
+}
+
 void Preprocessor::CachingLex(Token &Result) {
   if (CachedLexPos < CachedTokens.size()) {
     Result = CachedTokens[CachedLexPos++];