The increasing popularity of
reasoning models -- language models that output a series of reasoning or thought tokens before producing an answer -- is justified, in part, by theoretical results showing that chain-of-thought (CoT) transformers can simulate Turing machines, and thus perform arbitrary computation. However, the Turing machine, while suitable for complexity-theoretic analysis, is not convenient, intuitive, or efficient for discussing algorithms. Algorithms are typically designed and analyzed at a higher level of abstraction, captured by the
Word RAM model with random-access memory and unit-cost operations on
\bigO(logn)-bit words. As a result, Word RAM algorithms can be substantially more efficient than their Turing machine counterparts, raising the question:
Can CoT transformers efficiently simulate Word RAM algorithms? For instance, can they sort
n items in
\bigO(nlogn) steps or run Dijkstra's algorithm in
\bigO(E+VlogV) steps? We answer affirmatively, up to poly-logarithmic overhead. We first establish this for finite-precision transformers with poly-logarithmic width and rightmost unique hard attention, then strengthen the result to two more practical settings with finite width and log-precision:
continuous CoT, where reasoning takes the form of vectors rather than tokens, and a
hybrid architecture in which transformer layers sit atop a recurrent (linear RNN) layer. In all three cases, we find that CoT
can efficiently simulate any Word RAM algorithm with only a poly-logarithmic overhead in
n. This overhead reduces to log-square when the Word RAM has a ``flat'' instruction set, and only logarithmic for multiplication-free flat instructions -- in stark contrast to known CoT simulations of Turing machines, which require quadratic overhead over Word RAM.