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# Chapter 1: Introduction
[Introduction](../README.md) |
[Next: Chapter 2](../chapter02/README.md)
# Table of Contents
1. [Implementation Language](#implementation-language)
2. [Assumptions](#assumptions)
3. [Architecture](#architecture)
4. [Data Structures](#data-structures)
5. [Intermediate Representation as a Graph of Nodes](#intermediate-representation-as-a-graph-of-nodes)
6. [Nodes are in a Graph](#nodes-are-in-a-graph)
7. [Types of Nodes](#types-of-nodes)
8. [Unique Node ID](#unique-node-id)
9. [Start Node](#start-node)
10. [Constant Node](#constant-node)
11. [Return Node](#return-node)
12. [Representation](#representation)
You can also read [this chapter](https://github.com/SeaOfNodes/Simple/tree/linear-chapter01) in a linear Git revision history on the [linear](https://github.com/SeaOfNodes/Simple/tree/linear) branch.
In this chapter we aim to compile simple scripts such as:
```
return 1;
```
We implement the `return` statement.
The `return` statement accepts an `expression` as an argument.
The only `expression` type is an integer literal, as shown in the example above.
To implement this simple language, we introduce a few key components and data structures.
Here is the [complete language grammar](docs/01-grammar.md) for this chapter.
## Implementation Language
Our implementation language is Java. We chose Java as it is widely available and understood.
## Assumptions
We assume that the reader is familiar with traditional linear intermediate representations, and is familiar with terms such as Basic Block, Control Flow Graph, etc. No attempt is made to explain these topics.
If necessary the reader can consult a standard compiler textbook.
## Architecture
We construct the intermediate Sea of Nodes (SoN) representation directly as we
parse the language. There is no Abstract Syntax Tree representation. The reason
for this is to demonstrate a key benefit of the SoN IR: a number of pessimistic
peephole optimizations can be performed while parsing a language. This aspect
is more fully explored from [Chapter 2](../chapter02/README.md) onwards, and
eventually leads to a full-fledged program typing without needing any
type-analysis passes.
### Intermediate Representation as a Graph of Nodes
The intermediate representation is a graph of Node objects. The `Node` class is the base type for objects in the IR graph.
The `Node` class provides common capabilities that are inherited by all subtypes.
Subtypes implements semantics relevant to that subtype, e.g. AddNodes add and MulNodes multiply.
Each `Node` represents an "instruction" as it may appear in traditional IRs.
### Nodes are in a Graph
The key idea of the Sea of Nodes IR is that each Node is linked to other Nodes
by def-use dependencies. As this is such an important and fundamental aspect
of the IR, it is important to understand how we implement this, and depict this
in graph visuals.
The base `Node` class maintains a list of Nodes that are inputs to it. An
input is an edge from a "use" to a "def". What this means is that if `B` is a
definition, and `A` uses `B`, then `B` has a def->use edge from `B` to `A` and
correspondingly A has an input use->def edge from `A` to `B`.
Visually we show an arrow from the "use" to the "def". Here is an example:
From an implementation point of view, our `Node` type also maintains a both links.
This means that in the above scenario:
* Since `A` is a "use" of `B`, then `B` will appear in `A`'s list of inputs.
* Conversely, `B` maintains a list of outputs, and `A` will appear in this list.
* A key invariant is that these edges must be in sync at all times.
Here is how this looks in the `Node` class:
```java
public abstract class Node {
/**
* Inputs to the node. These are use-def references to Nodes.
* <p>
* Generally fixed length, ordered, nulls allowed, no unused trailing space.
* Ordering is required because e.g. "a/b" is different from "b/a".
* The first input (offset 0) is often a Control node.
*/
public final ArrayList<Node> _inputs;
/**
* Outputs reference Nodes that are not null and have this Node as an
* input. These nodes are users of this node, thus these are def-use
* references to Nodes.
* <p>
* Outputs directly match inputs, making a directed graph that can be
* walked in either direction. These outputs are typically used for
* efficient optimizations but otherwise have no semantic meaning.
*/
public final ArrayList<Node> _outputs;
}
```
### Types of Nodes
There are two categories of Nodes in the intermediate representation.
* **Control Nodes** - these represent the Control Flow Graph (CFG) of the compiled program
* **Data Nodes** - these capture the data semantics
The following control and data nodes appear in this chapter.
| Node Name | Type | Description | Inputs | Value |
|-----------|---------|-----------------------------------------------|------------------------------------------------------------------|-------------------------------------------------------|
| Start | Control | Start of function | None | None for now as we do not have function arguments yet |
| Return | Control | Represents the termination of a function | Predecessor control node, Data node value | Return value of the function |
| Constant | Data | Represents constants such as integer literals | None, however Start node is set as input to enable graph walking | Value of the constant |
Within a traditional basic block, instructions are executed in sequence. In the
Sea of Nodes model, the correct sequence of instructions is determined by a
scheduling algorithm that depends only on dependencies between nodes (including
control dependencies) that are explicit as edges in the graph. This enables a
number of optimizations at very little cost (nearly always small constant time)
because all dependencies are always available.
### Unique Node ID
Each node is assigned a unique dense integer Node ID when created. This ID is
useful for debugging, efficiently computing equality and e.g. as an index into
a bit vector, which in turn is used to efficiently visit a (possibly cyclic)
graph. We discuss Node equality in [Chapter 9](../chapter09/README.md).
### Start Node
The Start node represents the start of the function. For now, we do not have any inputs to Start because our function does not
yet accept parameters. When we add parameters the value of Start will be a tuple, and will require Projections to extract the values.
We discuss this in detail in [Chapter 4](../chapter04/README.md).
### Constant Node
A Constant node represents a constant value. At present, the only constants
that we allow are integer literals; therefore Constants contain an integer
value. As we add other types of constants, we will refactor how we represent
Constants.
Constants have no semantic inputs. However, we set Start as an input to
Constants to enable a forward graph walk. This edge carries no semantic
meaning, and it is present _solely_ to allow visitation.
The Constant's value is the value stored in it.
### Return Node
The Return node has two inputs. The first input is a control node and the
second is the data node that supplies the return value.
In this presentation, Return ends the program, since multiple `return` statements are not possible.
The Stop node will be introduced in [Chapter 5](../chapter05/README.md) when we implement `if` statements.
The Return's output is the value from the data node.
### Representation
Here is a visualization of the program:
```
return 1;
```
* Control nodes appear as rectangular boxes with pale yellow backgrounds
* Control edges are red and thicker than data edges
* The edges from Constants to Start are shown as dashed gray lines as these are not true control edges
* We label each edge with its position in the `_inputs` array, thus `0` means the edge is `_inputs[0]`.
## Reading Material
Our data structures are based upon the descriptions provided in the following papers:
* [From Quads to Graphs: An Intermediate Representation's Journey](http://softlib.rice.edu/pub/CRPC-TRs/reports/CRPC-TR93366-S.pdf)
* [Combining Analyses, Combining Optimizations](https://dl.acm.org/doi/pdf/10.1145/201059.201061)
* [A Simple Graph-Based Intermediate Representation](https://www.oracle.com/technetwork/java/javase/tech/c2-ir95-150110.pdf)
* [Global Code Motion Global Value Numbering](https://courses.cs.washington.edu/courses/cse501/06wi/reading/click-pldi95.pdf)
* [EasySSA](https://www.dropbox.com/scl/fi/0ww4sgl3ynep9hhe3i4xn/EasySSA.pdf?rlkey=2cp78hzxke62flkmyneiebzoz&dl=0)
* [SeaOfNodes](https://www.dropbox.com/scl/fi/cxykfvlzsmlcatyg6rlbt/SeaOfNodes.pdf?rlkey=z6o7y3rwr6atrejilcze6r8x0&e=1&dl=0)
Following the lead from above, we represent our intermediate representation using an object oriented data model. Details of the
representation follow.
|
