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diff --git a/chapter04/README.md b/chapter04/README.md index fd77392c..e7ae71d2 100644 --- a/chapter04/README.md +++ b/chapter04/README.md @@ -1,26 +1,29 @@ # Chapter 4: External Argument and Comparisons +[Previous: Chapter 3](../chapter03/README.md) | +[Next: Chapter 5](../chapter05/README.md) + # Table of Contents 1. [Extensions to Intermediate Representation](#extensions-to-intermediate-representation) 2. [Projection Nodes](#projection-nodes) 3. [Visualisation](#visualisation) -5. [Initial values](#initial-values) -6. [When Is Control Not-Null?](#when-is-control-not-null) -5. [Note Regarding Visualizations](#note-regarding-visualizations) -6. [Changes to Type System](#changes-to-type-system) -7. [Tuple Types](#tuple-types) -8. [$ctrl name binding](#ctrl-name-binding) -9. [More Peephole Optimizations](#more-peephole-optimizations) -10. [Peephole Walkthrough](#peephole-walkthrough) -11. [Dead Code Elimination(DCE)](#dead-code-eliminationdce) +4. [Initial values](#initial-values) +5. [When Is Control Not-Null?](#when-is-control-not-null) +6. [Note Regarding Visualizations](#note-regarding-visualizations) +7. [Changes to Type System](#changes-to-type-system) +8. [Tuple Types](#tuple-types) +9. [$ctrl name binding](#ctrl-name-binding) +10. [More Peephole Optimizations](#more-peephole-optimizations) +11. [Peephole Walkthrough](#peephole-walkthrough) +12. [Dead Code Elimination(DCE)](#dead-code-eliminationdce) You can also read [this chapter](https://github.com/SeaOfNodes/Simple/tree/linear-chapter04) in a linear Git revision history on the [linear](https://github.com/SeaOfNodes/Simple/tree/linear) branch and [compare](https://github.com/SeaOfNodes/Simple/compare/linear-chapter03...linear-chapter04) it to the previous chapter. -In this chapter we extend the language grammar with following features: +In this chapter we extend the language grammar with the following features: -* The program receives a single argument named `arg` of integer type from external environment. +* The program receives a single argument named `arg` of integer type from the external environment. * Expressions support comparison operators. * We introduce a scope sensitive name binding `$ctrl` for the current incoming control node. Thus, our Return is no longer hard-wired to Start, instead it @@ -69,15 +72,15 @@ In the visuals, projection nodes are shown as rectangular boxes inside the node ### Visualisation - + In the visual above, the projection nodes have been tagged by the names -associated with the outputs of the Start node. Both MultiNode that contain -Control and Control nodes are yellow. `$ctrl` and `arg` are outputs(users) of +associated with the outputs of the Start node. Both MultiNodes that contain +Control and Control nodes are yellow. `$ctrl` and `arg` are outputs (users) of StartNode(`START`) and have their MultiNode as their only input in slot 0. The `label` has no semantics and is used during debug printing. -```java +```java public ProjNode(MultiNode ctrl, int idx, String label) { super(ctrl); ``` @@ -130,37 +133,37 @@ The graph: constant. The `arg` was originally defined by a ProjNode; but when peephole is called the computed type is a constant, and the ProjNode gets replaced with a ConstantNode: - + ```java public final Node peephole( ) { - Type type = _type = compute(); + Type type = _type = compute(); /* type = {TypeInteger@1527} "2" */ // ProjNode is not a constant unlike its type. if (!(this instanceof ConstantNode) && type.isConstant()) /* Create Constant(2) */ return deadCodeElim(new ConstantNode(type).peephole()); - + /* won't get called */ ... ``` If you don't pass an argument you get `TypeInteger.BOT` -```java +```java Parser parser = new Parser("return arg; "); ... - + public Parser(String source) { this(source, TypeInteger.BOT); } - + ``` We clearly see this is not a constant: ```java -public final static TypeInteger BOT = new TypeInteger(false, 1); +public final static TypeInteger BOT = new TypeInteger(false, 1); /* _is_con = false */ ``` @@ -177,7 +180,7 @@ In later chapters, `$ctrl` will point to other control nodes, such as #### Note Regarding Visualizations From this chapter onwards we omit the edges from Constants to Start node, mainly to reduce -clutter and help draw reader's attention to the more important aspects of the graph. +clutter and help draw the reader's attention to the more important aspects of the graph. @@ -195,7 +198,7 @@ associated with a Type at a specific Node can be conveniently represented as a [lattice](https://en.wikipedia.org/wiki/Lattice_(order)). The type itself is identified by the Java class `Type` and subtypes. The type -implementation uses Java classes as convenient to deal with a Types' internal +implementation uses Java classes as a convenient way to deal with a Type's internal structure - but the Java classes have no relation to a Type's place in the lattice. @@ -239,20 +242,20 @@ In the table above `Con1` and `Con2` represent two distinct integer values, and * The `meet` of any integer with itself is that integer. * The `meet` of two unrelated integer constants is `IntBot`. -Currently, all our integer valued nodes are either a constant or a `IntBot` integer type. +Currently, all our integer valued nodes are either a constant or an `IntBot` integer type. When we start optimizing loops, we will start seeing `IntTop` values. ### Tuple Types -Tuple types need a little more explaination: they are a fixed-size grouping of +Tuple types need a little more explanation: they are a fixed-size grouping of types; literally a `Type[]`. Tuples represent a collection of otherwise unrelated types, and come from `MultiNode`s. `ProjNode`s will take the appropriate `Type` array element out of a Tuple. The `StartNode` now produces a 2-element `TypeTuple` with control and the type of `arg`: `[ctrl, TypeInteger.INTBOT]` The lattice `meet` operator on Tuples is done element by element; each array -element recursively calls `meet`. Tuples of mixed sizes are a internal +element recursively calls `meet`. Tuples of mixed sizes are an internal compiler error, and the `meet` uses Bottom for them. @@ -284,7 +287,7 @@ We would expect the compiler to output `arg+3` here, but as it stands what we ge  -We need to perform some algebraic simplifications to enable better outcome. For +We need to perform some algebraic simplifications to enable a better outcome. For example, we need to rearrange the expression as follows: ``` @@ -303,7 +306,7 @@ Here is a list of peepholes introduced in this Chapter, more will be introduced |-----------------------|-----------------------|------------------------------------------------| | (arg + 0 ) | arg | Add of zero identity | | (arg - 0 ) | arg | Sub of zero identity | -| (arg * 1 ) | arg | Multiple of one identity | +| (arg * 1 ) | arg | Multiply by one identity | | (con + arg) | (arg + con) | Move constants to right, to encourage folding | | (con * arg) | (arg * con) | Move constants to right, to encourage folding | | (con1 + (arg + con2)) | (arg + (con1 + con2)) | Move constants to right, to encourage folding | @@ -374,18 +377,18 @@ public final Node peephole( ) { } ``` -The peephole method does following: +The peephole method does the following: - Compute a Type for the node. - If the Type is a Constant and the node is not a ConstantNode, replace with a ConstantNode, recursively invoking peephole on the new constant. - Otherwise, ask the Node for a better replacement via a call to `idealize()`. - The "better replacement" is things like `(1+2)` becomes `3` and `1+(x+2))` + The "better replacement" is things like `(1+2)` becomes `3` and `1+(x+2)` becomes `(x+(1+2))`. -- - Each Node subtype is responsible for deciding what the `idealize()` step +- Each Node subtype is responsible for deciding what the `idealize()` step should do. If there is a better replacement then the node returns a non `null` value. -- - If we see a non `null` value, something changed, so we invoke `peephole()` +- If we see a non `null` value, something changed, so we invoke `peephole()` again and then also run dead code elimination on the (now dead and replaced) `this` node. |