On the Naturalness of Fuzzer-Generated Code

Compiler fuzzing tools such as Csmith have uncovered many bugs in compilers by randomly sampling programs from a generative model. The success of these tools is often attributed to their ability to generate unexpected corner case inputs that developers tend to overlook during manual testing. At the same time, their chaotic nature makes fuzzer-generated test cases notoriously hard to interpret, which has lead to the creation of input simplification tools such as C-Reduce (for C compiler bugs). In until now unrelated work, researchers have also shown that human-written software tends to be rather repetitive and predictable to language models. Studies show that developers deliberately write more predictable code, whereas code with bugs is relatively unpredictable. In this study, we ask the natural questions of whether this high predictability property of code also, and perhaps counter-intuitively, applies to fuzzer-generated code. That is, we investigate whether fuzzer-generated compiler inputs are deemed unpredictable by a language model built on human-written code and surprisingly conclude that it is not. To the contrary, Csmith fuzzer-generated programs are more predictable on a per-token basis than human-written C programs. Furthermore, bug-triggering tended to be more predictable still than random inputs, and the C-Reduce minimization tool did not substantially increase this predictability. Rather, we find that bug-triggering inputs are unpredictable relative to Csmith's own generative model. This is encouraging; our results suggest promising research directions on incorporating predictability metrics in the fuzzing and reduction tools themselves.