Memoryless computation: new results, constructions, and extensions

TL;DR

This paper explores memoryless computation, extending it to include memory use, and demonstrates how memory can lead to shorter programs and simpler instructions, generalizing the XOR swap concept.

Contribution

It provides new bounds and constructions for memoryless programs, including cases with and without memory, and quantifies the benefits of using memory in such computations.

Findings

Any function of n variables can be computed with 4n-3 updates without memory.

Using memory reduces program length and instruction complexity.

Memory allows for binary instructions, which are not sufficient without memory.

Abstract

In this paper, we are interested in memoryless computation, a modern paradigm to compute functions which generalises the famous XOR swap algorithm to exchange the contents of two variables without using a buffer. This uses a combinatorial framework for procedural programming languages, where programs are only allowed to update one variable at a time. We first consider programs which do not have any memory. We prove that any function of $n$ variables can be computed this way in only $4n-3$ variable updates. We then derive the exact number of instructions required to compute any manipulation of variables. This shows that combining variables, instead of simply moving them around, not only allows for memoryless programs, but also yields shorter programs. Second, we show that allowing programs to use memory is also incorporated in the memoryless computation framework. We then quantify the gains obtained by using memory: this leads to shorter programs and allows us to use only binary instructions, which is not sufficient in general when no memory is used.