NeurIPS 2019
Sun Dec 8th through Sat the 14th, 2019 at Vancouver Convention Center
Paper ID:6375
Title:Neural Attribution for Semantic Bug-Localization in Student Programs

Reviewer 1

This work targets the localization of bugs in student programs. This is accomplished by training an CNN to, given the input source code, predict whether that program passes given test cases. For failing tests, the Integrated Gradients technique is then used to try and attribute the prediction back to specific lines of code with the intention being that these lines are likely to be responsible (ie, buggy). One key trick here is an AST-based featurization that "spatializes" the data such that adjacent rows correspond to subtree structure (Section 3.1.1). Encoding the tree-based structure of the target code into a spatial-like matrix in this way has the advantage of allowing the use of an "off the shelf" CNN implementation, with the accompanying engineering advantages. The idea of using attribution or interpretability in this way is potentially novel and significant. However the submission could be improved with some clarifications, more detail, and a more explicit characterization of the dependencies on the particular properties of the student program problem domain data. MORE DETAILED COMMENTS Figure 3: this small worked example was essential for understanding Section 3.1.1. L159: if I'm understanding correctly, there are unique tokens for each of the node contents from Figure 3 ("IdentifierType:int", "BinaryOp:%", and so on), but then there is some special "normalization" done for variable names like "Decl:even" or "ID:num"? The mechanics of this normalization step were not clear to me. The mechanics of the initial embedding layer are not very clear from the paper either, but I'm guessing it is just using the Keras Embedding layer, which is ultimately jointly trained along with the full classifier network? So, the input to Supplement Figure 1 ends up being (24 x max_subtrees x max_nodes)?? Supplement Figure 1: this contains insufficient detail to easily understand the model. It would help to track the current dimensionality at each subcomponent, or use some other representation or notation to capture more granular details of what's going on here. Section 3.1.2: how was this architecture determined? Was it derived from some mixture of general intuition plus trial-and-error, based on other related works, or some other method? Section 3.1.2: wouldn't stride 3 cause us to "miss" subtrees such as the one rooted at Node 3 in Figure 3? That is, we'd have a convolution for the tree rooted at Node 1, then stride all the way to the subtree rooted at Node 5. L45: the summary of bug localization techniques as static vs dynamic is a helpful framing, and it was good to see both families of approaches represented in the experimental baselines.] L120: "Note that..." - this is a key aspect of the approach and useful to emphasize here. L194: the requirement for a "nearby" correct program might not be easy to meet in other application domains, is there any idea how robust the NeuralBugLocator would be to using less similar correct programs for the attribution? Section 4.3: it makes sense that the classification accuracy would be lower for the "nearly correct" programs in the evaluation set. It seems a little strange to restrict the evaluation against baselines to only cases which were classified correctly though, throwing away nearly half of the evaluation set? In a "real" system, these would simply be misses or failures for NeuralBugLocator. Also, the sense in which Tarantula and Ochiai use the passing test data is not very clear - maybe some more detail about how they are using the input data would clarify this. Table 1: it is confusing to mix the "test id" and "Lines" granularities with the "Programs" results in this table. UPDATE: I have read the author response and found the clarifications and additional details helpful. I believe the submission would be enhanced by the revisions proposed by the authors in the response.

Reviewer 2

This paper addresses finding bugs in short pieces of code. The dataset used in the paper is created from student's submissions/instructor's tests to a introductory programming class. The technique consists in two parts: (1) a component that predicts which programs are buggy (fail a test) without running the program by training a convolutional network that operates on flattened code ASTs. I found the encoding of the programs/ASTs to matrices and the use of CNN quite interesting. I'm guessing it might be faster to train such networks than the RNNs that are usually employed when representing/embedding code. The application of prediction attribution to bug localization is also interesting and most likely novel. The paper is generally well written and straightforward until the evaluation section. Please see comments below. ================ UPDATE: Thank you for your feedback. I encourage the authors to polish the experimental section which is hard to follow. The explanations in the feedback helped me better understand the work, thank you. In addition, I would state - even if informally - the experience with running RNNs on this problem. Thank you for confirming my intuition that the RNNs are super-slow to train for this type of problems. I think it's important to communicate this type of results. Currently we have few/no venues to communicate "negative" results, and, IMO "negative" results are as important (if not more important in some cases...) as "positive" results. (I'm using quotes because everything is relative). I'm increasing my overall score from 6 to 7 in the light of the clarifications during author feedback.

Reviewer 3

First, I should say that the paper is very novel in comparison to many of the works that apply deep learning for programming tasks. From methodological perspective this may be the best paper in the area for 2019. The idea to train a classifier on whether a test will succeed and then to use feature attribution techniques for localizing the errors in the program is new, original and quite interesting. I also expect that despite the limited use case shown in the current paper, the idea is much larger and should be tried in a broader set of scenarios. The paper is also well written, well motivated and should be of significant interest to most researchers working on learning for code. Furthermore, it provides good details both on the high-level idea, as well as on the detailed of the neural architecture. In terms of evaluation, while results are not completely conclusive that the technique is superior to the state-of-the-art in all scenarios, it shows good promise. One should keep in mind, however, that the comparison is not to naive neural baselines, but to complex specialized systems to perform similar tasks.