Oscar Waddell, Dipanwita Sarkar, and R. Kent Dybvig. Fixing letrec: A faithful yet efficient implementation of Scheme's recursive binding construct. Journal of Higher-Order and Symbolic Computation 18, 3/4, 299-326, December 2005 (bibtex).
A Scheme letrec expression is easily converted into more primitive constructs via a straightforward transformation given in the Revised5 Report. This transformation, unfortunately, introduces assignments that can impede the generation of efficient code. This article presents a more judicious transformation that preserves the semantics of the revised report transformation and also detects invalid references and assignments to left-hand-side variables, yet enables the compiler to generate efficient code. A variant of letrec that enforces left-to-right evaluation of bindings is also presented and shown to add virtually no overhead.