Random test case generation, or fuzzing, is a viable means for uncovering compiler bugs. Unfortunately, compiler fuzzing can be time-consuming and inefficient with purely randomly generated test cases due to the complexity of modern compilers. We present COMFUZZ, a focused compiler fuzzing framework. COMFUZZ aims to improve compiler fuzzing efficiency by focusing on testing components and language features that are likely to trigger compiler bugs. Our key insight is human developers tend to make common and repeat errors across compiler implementations; hence, we can leverage the previously reported buggy-exposing test cases of a programming language to test a new compiler implementation. To this end, COMFUZZ employs deep learning to learn a test program generator from open-source projects hosted on GitHub. With the machine-generated test programs in place, COMFUZZ then leverages a set of carefully designed mutation rules to improve the coverage and bug-exposing capabilities of the test cases. We evaluate COMFUZZ on 11 compilers for JS and Java programming languages. Within 260 hours of automated testing runs, we discovered 33 unique bugs across nine compilers, of which 29 have been confirmed and 22, including an API documentation defect, have already been fixed by the developers. We also compared COMFUZZ to eight prior fuzzers on four evaluation metrics. In a 24-hour comparative test, COMFUZZ uncovers at least 1.5× more bugs than the state-of-the-art baselines.