Date and Blog Introduction: The blog post on 2024-01-31 is the first in the "Flambda2 Snippets" series. It focuses on the CPS-based internal representation used in the Flambda2 optimizer, inspired by Andrew Kennedy's paper.

• Table of Contents: CPS (Continuation Passing Style), Double Barrelled CPS, The Flambda2 Term Language, and Following up.

CPS (Continuation Passing Style):

• In Flambda2 IR, terms are represented in CPS style. There is a distinction between First-Class CPS (using functions to represent continuations) and Second-Class CPS (using control-flow constructs in the intermediate language, equivalent to an explicit control-flow graph).
• An example shows how a recursive function is transformed into a tail-recursive CPS form, demonstrating optimizations like avoiding allocations and making functions tail-recursive.
• Continuations can be called (taking a fixed number of parameters), OCaml functions can be applied (with a special parameter for the continuation), and branching constructions lead to calls to different continuations.
• An example shows how the original program with an if statement and a raise can be transformed using CPS to a simpler form.

Double Barrelled CPS:

• In OCaml, functions can return normally (Barrel 1) and throw exceptions (Barrel 2), requiring the ability to represent both paths in the control-flow graph.
• This leads to function definitions having two special parameters (exception continuation and normal return continuation), function applications having two special arguments, and try...with terms and raise terms being translated using continuations.

The Flambda2 Term Language:

• The CPS form has directed the implementation of the FL2 language, regrouping features into 6 categories sorted by their effect on the control-flow.
• Benefits include better code organization (dealing with control-flow at the expression level and language features at a lower level) and reduced code duplication (sharing common code for similar features).

Following up:

• The goal is to show the fundamental design choice of using a CPS-based representation in Flambda2.
• Flambda2 takes the Lambda IR as input, performs CPS conversion and Closure conversion (each worthy of a blog post), and then has the "Simplify" optimization pass.
• During "Simplify", static analysis is performed during a downward traversal, and an optimized term is rebuilt during an upward traversal.
• Examples of optimizations during "Simplify" include inlining function calls, constant propagation, dead code elimination, loopification, unboxing, and specialization of polymorphic primitives.

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