MLton 20210117

# DeepFlatten

DeepFlatten is an optimization pass for the SSA2 IntermediateLanguage, invoked from SSA2Simplify.

## Description

This pass flattens into mutable fields of objects and into vectors.

For example, an (int * int) ref is represented by a 2 word object, and an (int * int) array contains pairs of int-s, rather than pointers to pairs of int-s.

## Implementation

- deep-flatten.fun

## Details and Notes

There are some performance issues with the deep flatten pass, where it consumes an excessive amount of memory.

- http://www.mlton.org/pipermail/mlton/2005-April/026990.html

- http://www.mlton.org/pipermail/mlton-user/2010-June/001626.html

- http://www.mlton.org/pipermail/mlton/2010-December/030876.html

A number of applications require compilation with -disable-pass deepFlatten to avoid exceeding available memory. It is often asked whether the deep flatten pass usually has a significant impact on performance. The standard benchmark suite was run with and without the deep flatten pass enabled when the pass was first introduced:

- http://www.mlton.org/pipermail/mlton/2004-August/025760.html

The conclusion is that it does not have a significant impact. However, these are micro benchmarks; other applications may derive greater benefit from the pass.