= 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

* https://github.com/MLton/mlton/blob/master/mlton/ssa/deep-flatten.fun[`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.
