Superoptimization is the process of automatically finding the optimal code sequence for one loop-free sequence of instructions. It is performed in and by a type of computer software termed a compiler. Real-world compilers generally cannot produce genuinely optimal code. While most standard compiler optimizations only improve code partly, a superoptimizer's goal is to find the optimal sequence, the canonical form. Superoptimizers can be used to improve conventional optimizers by highlighting missed opportunities so a human can write additional rules.
The term superoptimization was first coined by Alexia Massalin in the 1987 paper Superoptimizer: A Look at the Smallest Program. In 1992, the GNU Superoptimizer (GSO) was developed to integrate into the GNU Compiler Collection (GCC). Later work further developed and extended these ideas.
Typically, superoptimizing is performed via exhaustive brute-force search in the space of valid instruction sequences. This is a costly method, and thus impractical for general-purpose compilers. Yet, it has been shown to be useful in optimizing performance-critical inner loops. It is also possible to use a SMT solver to approach the problem.
In 2001, goal-directed superoptimizing was demonstrated in the Denali project by Compaq research. In 2006, answer set declarative programming was used for superoptimising in the Total Optimisation using Answer Set Technology (TOAST) project at the University of Bath,.
Publicly available superoptimizers
Several superoptimizers are available for free download.
- For the x86 family of instruction sets:
- For embedded systems:
- For the JVM:
- For LLVM IR:
- For WebAssembly
- Massalin, Henry (1987). "Superoptimizer: A look at the smallest program" (PDF). ACM SIGARCH Computer Architecture News. 15 (5): 122–126. doi:10.1145/36177.36194. Archived (PDF) from the original on 2017-07-04. Retrieved 2012-04-25.
Given an instruction set, the superoptimizer finds the shortest program to compute a function. Startling programs have been generated, many of them engaging in convoluted bit-fiddling bearing little resemblance to the source programs which defined the functions. The key idea in the superoptimizer is a probabilistic test that makes exhaustive searches practical for programs of useful size.
- Granlund, Torbjörn; Kenner, Richard (July 1992). "Eliminating branches using a superoptimizer and the GNU C compiler". ACM SIGPLAN Notices. 27 (7): 341–352. CiteSeerX 10.1.1.58.3509. doi:10.1145/143095.143146. ISBN 978-0-89791475-8. S2CID 8825539.
- "Index of /gnu/superopt". GNU Operating System. Free Software Foundation, Inc. 1995-06-14. Archived from the original on 2016-09-11. Retrieved 2016-09-03.
- Joshi, Rajeev; Nelson, Greg; Randall, Keith (2001-07-30). "Denali: a goal-directed superoptimizer". Compaq Systems Research Center. HP Labs. Hewlett-Packard Co. Archived from the original on 2016-05-27. Retrieved 2016-09-02.
- Brain, Martin; Crick, Tom; De Vos, Marina; Fitch, John (2006-08-17). "TOAST: Applying Answer Set Programming to Superoptimisation". In Etalle, Sandro; Truszczyński, Mirosław (eds.). Logic Programming. Springer-Verlag, Berlin / Heidelberg. pp. 270–284. ISBN 978-3-540-36636-2.
- "TOAST – KRRwiki". Department of Computer Science, Mathematical Foundations Group. Knowledge Representation and Reasoning (KRR) group. University of Bath. 2007-08-07. Archived from the original on 2012-11-28. Retrieved 2016-09-03.
- Bansal, Sorav; Aiken, Alex (2006-10-21). "Automatic Generation of Peephole Superoptimizers" (PDF). Stanford University. Computer Systems Lab, Stanford University. Archived (PDF) from the original on 2016-06-11. Retrieved 2016-09-02.
- Bansal, Sorav; Aiken, Alex (2006-10-25). "Binary Translation Using Peephole Superoptimizers" (PDF). Department of Computer Science. Indian Institute of Technology, Delhi. Archived (PDF) from the original on 2016-09-08. Retrieved 2016-10-17.
- Serpell, Daniel (2003). "SuperOptimizer for Microchip's PIC microcontrollers". Google Sites. Archived from the original on 2016-10-11. Retrieved 2016-09-06.
- Serpell, Daniel (2003-06-21). "PIC Microcontroller SuperOptimizer". Freecode. Slashdot Media. Archived from the original on 2016-09-17. Retrieved 2016-09-06.
- Hume, Tom (2012-08-21). "Clojure program to exhaustively search for optimal Java programs". GitHub. Archived from the original on 2018-06-10. Retrieved 2016-09-06.
- Cabrera Arteaga, Javier; Donde, Shrinish; Gu, Jian; Floros, Orestis; Satabin, Lucas; Baudry, Benoit; Monperrus, Martin (2020). "Superoptimization of WebAssembly bytecode". Conference Companion of the 4th International Conference on Art, Science, and Engineering of Programming. Porto Portugal: ACM: 36–40. arXiv:2002.10213. doi:10.1145/3397537.3397567. ISBN 978-1-4503-7507-8. S2CID 211259480.