mirror of
https://github.com/enso-org/enso.git
synced 2024-12-23 16:32:18 +03:00
72 lines
2.7 KiB
Markdown
72 lines
2.7 KiB
Markdown
---
|
|
layout: developer-doc
|
|
title: Demand Analysis
|
|
category: runtime
|
|
tags: [runtime, demand-analysis, execution]
|
|
order: 2
|
|
---
|
|
|
|
# Demand Analysis
|
|
|
|
Demand analysis is the process of deciding when to compute the value of a
|
|
suspended computation in a language which supports suspended computation.
|
|
|
|
<!-- MarkdownTOC levels="2,3" autolink="true" -->
|
|
|
|
- [Determining When To Force](#determining-when-to-force)
|
|
- [Avoiding Pathological Force-Thunk Chains](#avoiding-pathological-force-thunk-chains)
|
|
- [The Demand Analysis Algorithm](#the-demand-analysis-algorithm)
|
|
|
|
<!-- /MarkdownTOC -->
|
|
|
|
## Determining When To Force
|
|
|
|
As Enso supports dynamic dispatch for methods, we cannot always (even in the
|
|
presence of a typechecker), statically determine whether or not the function
|
|
that will eventually be called at any given call site. This means that we have
|
|
to come up with a more efficient way to handle suspended arguments.
|
|
|
|
This is done by making the _function application_ responsible for determining
|
|
when a value passed to it should be evaluated. It works as follows:
|
|
|
|
1. _All_ function arguments are passed suspended.
|
|
2. The function application is provided with knowledge of its argument
|
|
declarations.
|
|
3. As a result, the _application_ is responsible for deciding which arguments
|
|
should be evaluated up front, and which should be deferred.
|
|
4. In conjunction with this, the suspended arguments to the application need to
|
|
be explicitly forced at their use sites in the function body.
|
|
|
|
## Avoiding Pathological Force-Thunk Chains
|
|
|
|
The above approach does, when done naively, result in a severe performance
|
|
pathology when passing suspended arguments into functions also expecting
|
|
suspended arguments. Without intervention, the suspended argument gets wrapped
|
|
in a chain of thunks that has a significant performance cost to resolve, when
|
|
necessary.
|
|
|
|
In order to avoid this issue, we do the following:
|
|
|
|
1. Where a suspended argument is passed into a function, we pass it _without_
|
|
explicitly forcing it.
|
|
2. All other uses of that argument are forced.
|
|
|
|
## The Demand Analysis Algorithm
|
|
|
|
The algorithm for performing demand analysis on Enso code is as follows.
|
|
|
|
```
|
|
for (node <- ir):
|
|
if (node is a usage):
|
|
if (node is not used in a function application):
|
|
node = force(node)
|
|
```
|
|
|
|
This, however, is not entirely sufficient to support codegen. At the time of
|
|
generating truffle code, we want to know whether a given usage in an argument to
|
|
a function application needs to be wrapped in a suspension or left alone (as is
|
|
the case for a suspended term passed to a function).
|
|
|
|
To this end, we instead explicitly mark the arguments to the application with
|
|
whether or not they should be suspended during codegen.
|