Recovering from Errors in Clang-Repl and Code Undo

Developers: Jun Zhang (Software Engineering, Anhui Normal University, WuHu, China) and Purva Chaudhari (California State University Northridge, Northridge CA, USA)

Mentor: Vassil Vassilev (Princeton University/CERN)

Contact us!

Jun: jun@junz.org

GitHub username: junaire

Purva: Webpage

GitHub username: Purva-Chaudhari

Overview of the Project

Incremental C++ enables exploratory programming by considering the translation unit to be an ever-growing entity. This allows implementation of interpreter-like tools such as Cling and Clang-Repl, which consume C++ code piece by piece and use the JIT infrastructure to run each piecewise. One of the challenges of Incremental C++ is the reliable recovery from errors which allows the session to continue after faulty user code.

Supporting reliable error recovery requires splitting the translation unit into a sequence of Partial Translation Units (PTUs). Each declaration is associated with a unique PTU that owns it. Owning PTU isn’t always the “active” (most recent) PTU and it isn’t always the PTU that the declaration “comes from”. Even a new declaration that isn’t a declaration or or specialization of anything belongs to the active PTU. However, in case of a template specialization, it can be pulled into a more recent PTU by its template arguments. Additionally, processing a PTU might extend an earlier PTU. Rolling back the later PTU does not throw that extension away. Most declarations/definitions will only refer to entities from the same or earlier PTUs. Clang-Repl recovers from errors by disconnecting the most recent PTU and updating the primary PTU lookup tables.

The work on error recovery in clang repl is directed in making the Clang-Repl tool more robust to encountered errors. For example :

clang-repl> template<class T> T f() { return T(); }
clang-repl> auto ptu2 = f<float>(); err;
In file included from <<< inputs >>>:1:
input_line_1:1:25: error: C++ requires a type specifier for all declarations
auto ptu2 = f<float>(); err;
                        ^
clang-repl: /home/purva/llvm-project/clang/include/clang/Sema/Sema.h:9406: clang::Sema::GlobalEagerInstantiationScope::~GlobalEagerInstantiationScope(): Assertion `S.PendingInstantiations.empty() && "PendingInstantiations should be empty before it is discarded."' failed.
Aborted
 (core dumped)

and

clang-repl> template<class T> T f() { return T(); }
clang-repl> auto ptu2 = f<float>(); err;
In file included from <<< inputs >>>:1:
input_line_1:1:25: error: C++ requires a type specifier for all declarations
auto ptu2 = f<float>(); err;
                        ^
clang-repl: /home/purva/llvm-project/clang/include/clang/Sema/Sema.h:9406: clang::Sema::GlobalEagerInstantiationScope::~GlobalEagerInstantiationScope(): Assertion `S.PendingInstantiations.empty() && "PendingInstantiations should be empty before it is discarded."' failed.
Aborted
 (core dumped)
clang-repl> int x = 42;
clang-repl> %undo
clang-repl> float x = 24 // not an error

Contributions

The main contributions to this project are listed here.

Pull Requests:

Results

We implemented the initial code undo for Clang-Repl, the patch we submitted extends the functionality used to recover from errors and adds functionality to recover the low-level execution infrastructure. Now you can do below in clang-repl:
```
clang-repl> int x = 42;
clang-repl> %undo
clang-repl> float x = 24; // not an error
```
We fixed a bunch of bugs in Clang-Repl, by upstreamed ready-made patches in cling: Fix inline function in Clang-Repl. Take the example below:
```
inline int foo() { return 42; }
int r3 = foo(); // This fails before my fix.
```
More context: cd64a427

Partially fix incorrect return code in Clang-Repl. Take the example below:

clang-repl> BOOM!
clang-repl> int x = 42;
// This previously passed in the LLVM lit tests incorrectly

Partially fix weak attribute usage in Clang-Repl. Take the example below:

int __attribute__((weak)) bar() { return 42; }
auto r4 = printf("bar() = %d\n", bar()); // This fails before my patch. Note this is not supported in Windows yet.

We fixed some issues in lambda usage in Clang-Repl.

Conclusion

During this summer, I not only improved my technical skills but also enhanced my ability to work with others and appreciate the charm of open source. I would like to thank all the people who helped me, especially my mentor Vassil, who is not only an experienced programmer but also a respected life teacher. I’m also pretty happy working with my partner Purva, who made a great effort when preparing our LLVM Dev lightning talk this year.

In the future, I’ll continue my journey into the world of open source, and bring the code and love to all!