package ocaml-base-compiler

Tail-modulo-cons optimization.

Warning: this module is unstable and part of compiler-libs.

TMC (Tail Modulo Cons) is a code transformation that rewrites transformed functions in destination-passing-style, in such a way that certain calls that were not in tail position in the original program become tail-calls in the transformed program.

As a classic example, the following program | let[@tail_mod_cons] rec map f = function | [] -> [] | x :: xs -> let y = f x in y :: map f xs | becomes (expressed in almost-source-form; the translation is in fact at the Lambda-level) | let rec map f = function | [] -> [] | x :: xs -> let y = f x in let dst = y :: Placeholder in map_dps dst 1 f xs; dst and map_dps dst offset f = function | [] -> dst.offset <- [] | x :: xs -> let y = f x in let dst' = y :: Placeholder in dst.offset <- dst'; map_dps dst 1 f fx |

In this example, the expression (y :: map f xs) had a call in non-tail-position, and it gets rewritten into tail-calls. TMC handles all such cases where the continuation of the call (what needs to be done after the return) is a "construction", the creation of a (possibly nested) data block.

The code transformation generates two versions of the input function, the "direct" version with the same type and behavior as the original one (here just map), and the "destination-passing-style" version (here map_dps).

Any call to the original function from outside the let..rec declaration gets transformed into a call into the direct version, which will itself call the destination-passing-style versions on recursive calls that may benefit from it (they are in tail-position modulo constructors).

Because of this inherent code duplication, the transformation may not always improve performance. In this implementation, TMC is opt-in, we only transform functions that the user has annotated with an attribute to request the transformation.