Revisiting Dynamic Dispatch for Modern Architectures Dave Mason dmason@torontomu.ca Toronto Metropolitan University Toronto, Canada Abstract Since the 1980s, Deutsch-Schifman dispatch has been the standard method dispatch mechanism for languages like Smalltalk, Ruby, and Python. While it is a huge improvement over the simple, semantic execution model, it has some sig- nifcant drawbacks for modern hardware and applications. This paper proposes an alternative dispatch mechanism that addresses these concerns, with only memory space as a trade-of, that should demonstrate dynamic performance only slightly worse than the best possible with full type information for the program. CCS Concepts: · Software and its engineering → Run- time environments; Dynamic compilers; Just-in-time compilers. Keywords: method dispatch, dynamic types ACM Reference Format: Dave Mason. 2023. Revisiting Dynamic Dispatch for Modern Ar- chitectures. In Proceedings of the 15th ACM SIGPLAN International Workshop on Virtual Machines and Intermediate Languages (VMIL ’23), October 23, 2023, Cascais, Portugal. ACM, New York, NY, USA, 7 pages. htps://doi.org/10.1145/3623507.3623551 1 Introduction Since the 1980s, Deutsch-Schifman dispatch[4, 8, 25] has been the standard method dispatch mechanism for dynamically- typed, object-oriented, languages like Smalltalk, Ruby, and Python. This dispatch is much more efcient than the sim- ple algorithm derived from the semantics of dispatch, and was originally created in order to make Smalltalk useable on commodity hardware, rather than the custom machines upon which it had originally been implemented. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proft or commercial advantage and that copies bear this notice and the full citation on the frst page. Copyrights for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specifc permission and/or a fee. Request permissions from permissions@acm.org. VMIL ’23, October 23, 2023, Cascais, Portugal © 2023 Copyright held by the owner/author(s). Publication rights licensed to ACM. ACM ISBN 979-8-4007-0401-7/23/10. . . $15.00 htps://doi.org/10.1145/3623507.3623551 This paper describes dispatch for class-based, object-oriented systems like Smalltalk, Ruby, and Python. Prototype-based object-oriented languages like Javascript and Self should beneft from the same approach, but in the interests of simpler exposition the relevant details have been omitted. While this paper specifcally uses Smalltalk as the language for discussion purposes, most of the same consid- erations apply to Ruby and Python. Improving the performance of dynamic dispatch is impor- tant because there is considerable evidence of interest[15, 21, 22] in dynamic and OO languages. Most particularly, accord- ing to the TIOBE Index[20], the most popular language in the world as of this writing (and in the top 3 for most of the last 4 years), is Python. At least 4 other of the top 20 (Javascript, R, Lua, and Ruby) have similar dispatch characteristics, and 4 others (Visual Basic, Delphi, Swift, Objective-C) probably have some similarities. If full type information is available at compile time, the- oretically all dispatch could be done by directly indexing into a virtual dispatch table (a so-called vtable), but even in languages such as Java this is not fully exploited. 1.1 Problems Smalltalk is an interesting performance challenge for several reasons. 1. it is dynamically-typed to a fault - everything is an ob- ject including methods, classes, and even stack frames; 2. there is no control-fow syntax - all control fow is im- plemented via message-sending with blocks (lambda functions) as parameters, and the blocks often contain non-local returns; 3. there are no primitive operators as found in most lan- guages (arithmetic operators, comparisons, and index- ing are all messages); 4. methods are typically very short (1-5 lines of code) and all expressions are literals, variable references, assignments, or message sends. In order to obtain adequate performance for Smalltalk on commodity hardware, there are two main techniques: 1. compilers assume the meaning of some particular mes- sages (such as ifTrue:ifFalse: and whileTrue:) and compile them as if they were syn- tax in a more conventional language; 11