We introduce Perceus, an algorithm for precise reference counting with
reuse and specialization. Starting from a functional core language with
explicit control-flow, Perceus emits precise reference counting
instructions such that (cycle-free) programs are garbage free, where only live
references are retained. This enables further optimizations,
like reuse analysis that allows for guaranteed in-place updates at
runtime. This in turn enables a novel programming paradigm that we call
functional but in-place (FBIP). Much like tail-call optimization
enables writing loops with regular function calls, reuse analysis enables
writing in-place mutating algorithms in a purely functional way. We give
a novel formalization of reference counting in a linear resource
calculus, and prove that Perceus is sound and garbage free. We show
evidence that Perceus, as implemented in Koka, has good performance
and is competitive with other state-of-the-art memory collectors.