Type freezing: exploiting attribute type monomorphism in tracing JIT compilers

Dynamic programming languages continue to increase in popularity. While just-in-time (JIT) compilation can improve the performance of dynamic programming languages, a significant performance gap remains with respect to ahead-of-time compiled languages. Existing JIT compilers exploit type monomorphism through type specialization, and use runtime checks to ensure correctness. Unfortunately, these checks can introduce non-negligible overhead. In this paper, we present type freezing, a novel software solution for exploiting attribute type monomorphism. Type freezing "freezes" type monomorphic attributes of user-defined types, and eliminates the necessity of runtime type checks when performing reads from these attributes. Instead, runtime type checks are done when writing these attributes to validate type monomorphism. We implement type freezing as an extension to PyPy, a state-of-the-art tracing JIT compiler for Python. Our evaluation shows type freezing can improve performance and reduce dynamic instruction count for those applications with a significant number of attribute accesses.