[clang-repl] Implement code undo.

In interactive C++ it is convenient to roll back to a previous state of the
compiler. For example:
clang-repl> int x = 42;
clang-repl> %undo
clang-repl> float x = 24 // not an error

To support this, the patch extends the functionality used to recover from
errors and adds functionality to recover the low-level execution infrastructure.

The current implementation is based on watermarks. It exploits the fact that
at each incremental input the underlying compiler infrastructure is in a valid
state. We can only go N incremental inputs back to a previous valid state. We do
not need and do not do any further dependency tracking.

This patch was co-developed with V. Vassilev, relies on the past work of Purva
Chaudhari in clang-repl and is inspired by the past work on the same feature
in the Cling interpreter.

Co-authored-by: Purva-Chaudhari <purva.chaudhari02@gmail.com>
Co-authored-by: Vassil Vassilev <v.g.vassilev@gmail.com>
Signed-off-by: Jun Zhang <jun@junz.org>

1 files changed, 45 insertions, 0 deletions

diff --git a/clang/unittests/Interpreter/InterpreterTest.cpp b/clang/unittests/Interpreter/InterpreterTest.cpp
index 280c6d7f..720e30f 100644
--- a/clang/unittests/Interpreter/InterpreterTest.cpp
+++ b/clang/unittests/Interpreter/InterpreterTest.cpp
@@ -128,6 +128,51 @@ TEST(InterpreterTest, DeclsAndStatements) {
   EXPECT_EQ("Parsing failed.", llvm::toString(std::move(Err)));
 }
 
+TEST(InterpreterTest, UndoCommand) {
+  Args ExtraArgs = {"-Xclang", "-diagnostic-log-file", "-Xclang", "-"};
+
+  // Create the diagnostic engine with unowned consumer.
+  std::string DiagnosticOutput;
+  llvm::raw_string_ostream DiagnosticsOS(DiagnosticOutput);
+  auto DiagPrinter = std::make_unique<TextDiagnosticPrinter>(
+      DiagnosticsOS, new DiagnosticOptions());
+
+  auto Interp = createInterpreter(ExtraArgs, DiagPrinter.get());
+
+  // Fail to undo.
+  auto Err1 = Interp->Undo();
+  EXPECT_EQ("Operation failed. Too many undos",
+            llvm::toString(std::move(Err1)));
+  auto Err2 = Interp->Parse("int foo = 42;");
+  EXPECT_TRUE(!!Err2);
+  auto Err3 = Interp->Undo(2);
+  EXPECT_EQ("Operation failed. Too many undos",
+            llvm::toString(std::move(Err3)));
+
+  // Succeed to undo.
+  auto Err4 = Interp->Parse("int x = 42;");
+  EXPECT_TRUE(!!Err4);
+  auto Err5 = Interp->Undo();
+  EXPECT_FALSE(Err5);
+  auto Err6 = Interp->Parse("int x = 24;");
+  EXPECT_TRUE(!!Err6);
+  auto Err7 = Interp->Parse("#define X 42");
+  EXPECT_TRUE(!!Err7);
+  auto Err8 = Interp->Undo();
+  EXPECT_FALSE(Err8);
+  auto Err9 = Interp->Parse("#define X 24");
+  EXPECT_TRUE(!!Err9);
+
+  // Undo input contains errors.
+  auto Err10 = Interp->Parse("int y = ;");
+  EXPECT_FALSE(!!Err10);
+  EXPECT_EQ("Parsing failed.", llvm::toString(Err10.takeError()));
+  auto Err11 = Interp->Parse("int y = 42;");
+  EXPECT_TRUE(!!Err11);
+  auto Err12 = Interp->Undo();
+  EXPECT_FALSE(Err12);
+}
+
 static std::string MangleName(NamedDecl *ND) {
   ASTContext &C = ND->getASTContext();
   std::unique_ptr<MangleContext> MangleC(C.createMangleContext());