From dca50fde64f64cd105c35651e12ea09f81db2cc3 Mon Sep 17 00:00:00 2001 From: Dibyendu Majumdar Date: Tue, 26 Sep 2023 17:43:36 +0100 Subject: Chapter 2: Arithmetic MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Co-authored-by: Cliff Click Co-authored-by: Thalia Archibald Co-authored-by: Yichen Yan Co-authored-by: Tamás Hadházy <85063808+Hels15@users.noreply.github.com> Co-authored-by: XmiliaH <45106915+XmiliaH@users.noreply.github.com> Co-authored-by: Hsiang-Ying Fu <66062782+fuhsnn@users.noreply.github.com> Co-authored-by: hadha Co-authored-by: RobertObkircher --- docs/chapter02/02-demonstration-peephole.svg | 54 ++++ .../02-demonstration-pre-peephole-nokill.svg | 100 ++++++++ docs/chapter02/02-demonstration-pre-peephole.svg | 87 +++++++ docs/chapter02/02-grammar.md | 40 +++ docs/chapter02/02-post-peephole-ex1.gv | 17 ++ docs/chapter02/02-post-peephole-ex1.svg | 56 ++++ docs/chapter02/02-pre-peephole-ex1.gv | 34 +++ docs/chapter02/02-pre-peephole-ex1.svg | 168 ++++++++++++ docs/chapter02/Makefile | 12 + docs/chapter02/README.md | 282 +++++++++++++++++++++ docs/chapter02/lattice.gv | 40 +++ docs/chapter02/lattice.svg | 159 ++++++++++++ 12 files changed, 1049 insertions(+) create mode 100644 docs/chapter02/02-demonstration-peephole.svg create mode 100644 docs/chapter02/02-demonstration-pre-peephole-nokill.svg create mode 100644 docs/chapter02/02-demonstration-pre-peephole.svg create mode 100644 docs/chapter02/02-grammar.md create mode 100644 docs/chapter02/02-post-peephole-ex1.gv create mode 100644 docs/chapter02/02-post-peephole-ex1.svg create mode 100644 docs/chapter02/02-pre-peephole-ex1.gv create mode 100644 docs/chapter02/02-pre-peephole-ex1.svg create mode 100644 docs/chapter02/Makefile create mode 100644 docs/chapter02/README.md create mode 100644 docs/chapter02/lattice.gv create mode 100644 docs/chapter02/lattice.svg (limited to 'docs/chapter02') diff --git a/docs/chapter02/02-demonstration-peephole.svg b/docs/chapter02/02-demonstration-peephole.svg new file mode 100644 index 00000000..90ab6ca4 --- /dev/null +++ b/docs/chapter02/02-demonstration-peephole.svg @@ -0,0 +1,54 @@ + + + + + + +chapter02 + + +cluster_Nodes + + + + +Start1 + +Start + + + +Con_5 + +#3 + + + +Con_5->Start1 + + +0 + + + +Return6 + +Return + + + +Return6->Start1 + + +0 + + + +Return6->Con_5 + + +1 + + + \ No newline at end of file diff --git a/docs/chapter02/02-demonstration-pre-peephole-nokill.svg b/docs/chapter02/02-demonstration-pre-peephole-nokill.svg new file mode 100644 index 00000000..3d4f53df --- /dev/null +++ b/docs/chapter02/02-demonstration-pre-peephole-nokill.svg @@ -0,0 +1,100 @@ + + + + + + +chapter02 + + +cluster_Nodes + + + + +Start1 + +Start + + + +Con_2 + +#1 + + + +Con_2->Start1 + + +0 + + + +Con_3 + +#2 + + + +Con_3->Start1 + + +0 + + + +Add4 + ++ + + + +Add4->Con_2 + + +1 + + + +Add4->Con_3 + + +2 + + + +Con_5 + +#3 + + + +Con_5->Start1 + + +0 + + + +Return6 + +Return + + + +Return6->Start1 + + +0 + + + +Return6->Con_5 + + +1 + + + \ No newline at end of file diff --git a/docs/chapter02/02-demonstration-pre-peephole.svg b/docs/chapter02/02-demonstration-pre-peephole.svg new file mode 100644 index 00000000..49624652 --- /dev/null +++ b/docs/chapter02/02-demonstration-pre-peephole.svg @@ -0,0 +1,87 @@ + + + + + + +chapter02 + + +cluster_Nodes + + + + +Start1 + +Start + + + +Con_2 + +#1 + + + +Con_2->Start1 + + +0 + + + +Con_3 + +#2 + + + +Con_3->Start1 + + +0 + + + +Add4 + ++ + + + +Add4->Con_2 + + +1 + + + +Add4->Con_3 + + +2 + + + +Return5 + +Return + + + +Return5->Start1 + + +0 + + + +Return5->Add4 + + +1 + + + \ No newline at end of file diff --git a/docs/chapter02/02-grammar.md b/docs/chapter02/02-grammar.md new file mode 100644 index 00000000..ce875eda --- /dev/null +++ b/docs/chapter02/02-grammar.md @@ -0,0 +1,40 @@ +# Grammar for Chapter 1 + +```antlrv4 +grammar SimpleLanguage; + +program + : statement EOF + ; + +statement + : 'return' expression ';' + ; + +expression + : additiveExpression + ; + +additiveExpression + : multiplicativeExpression (('+' | '-') multiplicativeExpression)* + ; + +multiplicativeExpression + : unaryExpression (('*' | '/') unaryExpression)* + ; + +unaryExpression + : ('-') unaryExpression + | primaryExpression + ; + +primaryExpression + : INTEGER_LITERAL + | '(' expression ')' + ; + +INTEGER_LITERAL + : [1-9][0-9]* + | [0] + ; +``` diff --git a/docs/chapter02/02-post-peephole-ex1.gv b/docs/chapter02/02-post-peephole-ex1.gv new file mode 100644 index 00000000..6c2b23ae --- /dev/null +++ b/docs/chapter02/02-post-peephole-ex1.gv @@ -0,0 +1,17 @@ +digraph chapter02 { +/* +return 1+2*3+-5; +*/ + rankdir=BT; + ordering="in"; + concentrate="true"; + subgraph cluster_Nodes { + Start1 [ shape=box style=filled fillcolor=yellow label="Start" ]; + Con_13 [ label="#2" ]; + Return14 [ shape=box style=filled fillcolor=yellow label="Return" ]; + } + edge [ fontname=Helvetica, fontsize=8 ]; + Con_13 -> Start1[taillabel=0 style=dotted]; + Return14 -> Start1[taillabel=0 color=red]; + Return14 -> Con_13[taillabel=1]; +} diff --git a/docs/chapter02/02-post-peephole-ex1.svg b/docs/chapter02/02-post-peephole-ex1.svg new file mode 100644 index 00000000..db27e30d --- /dev/null +++ b/docs/chapter02/02-post-peephole-ex1.svg @@ -0,0 +1,56 @@ + + + + + + +chapter02 + + +cluster_Nodes + + + + +Start1 + +Start + + + +Con_13 + +#2 + + + +Con_13->Start1 + + +0 + + + +Return14 + +Return + + + +Return14->Start1 + + +0 + + + +Return14->Con_13 + + +1 + + + diff --git a/docs/chapter02/02-pre-peephole-ex1.gv b/docs/chapter02/02-pre-peephole-ex1.gv new file mode 100644 index 00000000..411de5ee --- /dev/null +++ b/docs/chapter02/02-pre-peephole-ex1.gv @@ -0,0 +1,34 @@ +digraph chapter02 { +/* +return 1+2*3+-5; +*/ + rankdir=BT; + ordering="in"; + concentrate="true"; + subgraph cluster_Nodes { + Start1 [ shape=box style=filled fillcolor=yellow label="Start" ]; + Con_2 [ label="#1" ]; + Con_3 [ label="#2" ]; + Con_4 [ label="#3" ]; + Mul5 [ label="*" ]; + Con_6 [ label="#5" ]; + Minus7 [ label="-" ]; + Add8 [ label="+" ]; + Add9 [ label="+" ]; + Return10 [ shape=box style=filled fillcolor=yellow label="Return" ]; + } + edge [ fontname=Helvetica, fontsize=8 ]; + Con_2 -> Start1[taillabel=0 style=dotted]; + Con_3 -> Start1[taillabel=0 style=dotted]; + Con_4 -> Start1[taillabel=0 style=dotted]; + Mul5 -> Con_3[taillabel=1]; + Mul5 -> Con_4[taillabel=2]; + Con_6 -> Start1[taillabel=0 style=dotted]; + Minus7 -> Con_6[taillabel=1]; + Add8 -> Mul5[taillabel=1]; + Add8 -> Minus7[taillabel=2]; + Add9 -> Con_2[taillabel=1]; + Add9 -> Add8[taillabel=2]; + Return10 -> Start1[taillabel=0 color=red]; + Return10 -> Add9[taillabel=1]; +} diff --git a/docs/chapter02/02-pre-peephole-ex1.svg b/docs/chapter02/02-pre-peephole-ex1.svg new file mode 100644 index 00000000..29c76bd8 --- /dev/null +++ b/docs/chapter02/02-pre-peephole-ex1.svg @@ -0,0 +1,168 @@ + + + + + + +chapter02 + + +cluster_Nodes + + + + +Start1 + +Start + + + +Con_2 + +#1 + + + +Con_2->Start1 + + +0 + + + +Con_3 + +#2 + + + +Con_3->Start1 + + +0 + + + +Con_4 + +#3 + + + +Con_4->Start1 + + +0 + + + +Mul5 + +* + + + +Mul5->Con_3 + + +1 + + + +Mul5->Con_4 + + +2 + + + +Con_6 + +#5 + + + +Con_6->Start1 + + +0 + + + +Minus7 + +- + + + +Minus7->Con_6 + + +1 + + + +Add8 + ++ + + + +Add8->Mul5 + + +1 + + + +Add8->Minus7 + + +2 + + + +Add9 + ++ + + + +Add9->Con_2 + + +1 + + + +Add9->Add8 + + +2 + + + +Return10 + +Return + + + +Return10->Start1 + + +0 + + + +Return10->Add9 + + +1 + + + diff --git a/docs/chapter02/Makefile b/docs/chapter02/Makefile new file mode 100644 index 00000000..12659b8f --- /dev/null +++ b/docs/chapter02/Makefile @@ -0,0 +1,12 @@ +GV := $(wildcard *.gv) +SVG := $(patsubst %.gv,%.svg,$(GV)) + +.PHONY: all +all: $(SVG) + +%.svg: %.gv + dot -Tsvg $< > $@ + +.PHONY: clean +clean: + @rm -f $(SVG) diff --git a/docs/chapter02/README.md b/docs/chapter02/README.md new file mode 100644 index 00000000..0e3f2a8d --- /dev/null +++ b/docs/chapter02/README.md @@ -0,0 +1,282 @@ +# Chapter 2: Arithmetic + + +# Table of Contents + +1. [Extensions to Intermediate Representation](#extensions-to-intermediate-representation) +2. [Peephole Optimizations](#peephole-optimizations) +3. [Constant Folding and Constant Propagation](#constant-folding-and-constant-propagation) +4. [Nodes Pre Peephole Optimization](#nodes-pre-peephole-optimization) +5. [Post-peephole](#post-peephole) +6. [Demonstration](#demonstration) + +You can also read [this chapter](https://github.com/SeaOfNodes/Simple/tree/linear-chapter02) 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-chapter01...linear-chapter02) it to the previous chapter. + + +In this chapter we extend the language grammar to include arithmetic operations such as addition, subtraction, +multiplication, division, and unary minus. This allows us to write statements such as: + +``` +return 1 + 2 * 3 + -5; +``` + +Here is the [complete language grammar](02-grammar.md) for this chapter. + +## Extensions to Intermediate Representation + +In [Chapter 1](../chapter01/README.md) we introduced following nodes. + +| 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 | End of function | Predecessor control node, Data node value | Return value of the function | +| Constant | Data | Constants such as integer literals | None, however Start node is set as input to enable graph walking | Value of the constant | + +We extend the set of nodes by adding following additional node types. + +| Node Name | Type | Description | Inputs | Value | +|-----------|------|-------------------------------|------------------------------------------------------------|-----------------------------| +| Add | Data | Add two values | Two data nodes, values are added, order not important | Result of the add operation | +| Sub | Data | Subtract a value from another | Two data nodes, values are subtracted, order matters | Result of the subtract | +| Mul | Data | Multiply two values | Two data nodes, values are multiplied, order not important | Result of the multiply | +| Div | Data | Divide a value by another | Two data nodes, values are divided, order matters | Result of the division | +| Minus | Data | Negate a value | One data node, value is negated | Result of the unary minus | + + +## *Value* equality vs *Reference* equality + +In much of Simple, Nodes are looked up (`find()` calls) via *reference* +equality. This is by far the most common case. In [Chapter +9](../chapter09/README.md) we introduce *value* equality for the first time. +In both cases the choice of value-vs-reference equality is intentional: it is +*never* correct to "just pick one or the other kind of equality". When in +doubt check the context: only *Global Value Numbering* uses value equality; +everywhere we mean reference equality. + + +## Peephole Optimizations + +Nodes in the graph can be peephole-optimized. The graph is viewed through a +"peephole", a small chunk of graph, and if a certain pattern is detected we +locally rewrite the graph. + +During parsing, these peephole optimizations are particularly easy to check and +apply: there are no uses (yet) of a just-created Node from a just-parsed piece +of syntax, so there's no effort to the "rewrite" part of the problem. We just +replace in-place before installing Nodes into the graph. + +This replacement might allow us to *kill* the unused inputs from the replaced +Node, basically doing a modest Dead Code Elimination during parsing. + +E.g. Suppose we already parsed out a constant `1`, and a constant `2`; then when we +parse an `Add(1,2)`, the peephole rule for constant math replaces the Add with a +constant `3`. At this point, we also *kill* the unused `Add`, which recursively +may *kill* the unused constants `1` and `2`. + + + +## Constant Folding and Constant Propagation + +In this chapter and next we focus on a particular peephole optimization: +constant folding and constant propagation. Since we do not have non-constant values +until [Chapter 4](../chapter04/README.md), the main feature we demonstrate now is constant folding. +However, we introduce some additional ideas into the compiler at this stage, to +set the scene for Chapter 4. + +It is useful for the compiler to know at various points of the program whether +a node's value is a constant. The compiler can use this knowledge to perform various +optimizations such as: + +* Evaluate expressions at compile time and replace an expression with a + constant. This idea can be extended in a number of ways and is called + [Partial Evaluation](https://en.wikipedia.org/wiki/Partial_evaluation). +* The compiler may be able to identify regions of code that are dead and no + longer needed, such as when a conditional branch always take one of the + branches ([Chapter 6](../chapter06/README.md)). +* Pointers may be known to be not-null, and a null check can be skipped ([Chapter 10](../chapter10/README.md)). +* Array indices may be known to be in-range, and a range check can be skipped. +* Many additional optimizations are possible when the compiler learns more + about the possible set of Node values. + +In order to achieve above, we need a way to talk about what kinds of values a Node can take +on at runtime - we call these Types, and we annotate Nodes with Types. + +The Type annotation serves two purposes: + +* it defines the set of operations allowed on the Node +* it defines the set of values the Node takes on + +The Type itself is represented as a Java class; the Java classes are used as +convenient to represent the Type's structure; all types are subtypes of the +class `Type`. For now, we only have the following hierarchy of types: + +``` +Type ++-- TypeInteger +``` + +It turns out that the set of values associated with a Type at a specific Node +can be conveniently represented as a *lattice* +[wiki/Lattice_(order)](https://en.wikipedia.org/wiki/Lattice_(order)). Our lattice has the following structure: + +![Lattice](./lattice.svg) + +Our lattice elements can be one of three types: + +* The highest element is "top", denoted by ⊤; assigning ⊤ means that the Node's + value may or may not be a compile time constant. +* All elements in the middle are constants. +* The lowest is "bottom", denoted by ⊥; assigning ⊥ means that we know that the + Node's value is **not** a compile time constant. + +An invariant of peephole optimizations is that the type of a Node always moves +*up* the lattice (towards "top"); peepholes are *pessmistic* assuming the worst +until they can prove better. A later *optimistic* optimization will start all +Nodes at *top* and move Types *down* the lattice as eager assumptions are +proven wrong. + +In later chapters we will explore extending this lattice, as it frequently +forms the heart of core optimizations we want our compiler to do. + +We add a `_type` field to every Node, to store its current computed best +`Type`. We need a field to keep the optimizer runtime linear, and later when +doing an optimistic version of constant propagation (called [Sparse Conditional +Constant Propagation](https://en.wikipedia.org/wiki/Sparse_conditional_constant_propagation)). + +We add a `_type` field even to `Start` and `Return`, and later to all control +Nodes - because the Sea of Nodes does not distinguish between control and data. +Both nodes are equally peepholed and optimized, and this will be covered +starting in [Chapter 4](../chapter04/README.md) and [Chapter +5](../chapter05/README.md). + + +There are other important properties of the Lattice that we discuss in [Chapter +4](../chapter04/README.md) and [Chapter 10](../chapter10/README.md), such as the "meet" and "join" operators and their rules. + + +## Nodes Pre Peephole Optimization + +The following visual shows how the graph looks like pre-peephole optimization: + +![Example Visual](./02-pre-peephole-ex1.svg) + +* Control nodes appear as square boxes with yellow background +* Control edges are in bold red +* The edges from Constants to Start are shown in dotted lines as these are not true control edges +* We label each edge with its position in the node's list of inputs. + + +## Post-peephole + +![Example Visual](./02-post-peephole-ex1.svg) + + +## Demonstration +To demonstrate how the optimisation works, let's consider the following code: +``` +return 1+2; +``` +This is parsed as: +```java + var lhs = parseMultiplication(); + if (match("+")) return new AddNode(lhs, parseAddition()).peephole(); +``` +When the peephole is called on the AddNode node it first calls the compute function: +```java + public final Node peephole( ) { + // Compute initial or improved Type + Type type = _type = compute(); +``` +```java + public Type compute() { + if( in(1)._type instanceof TypeInteger i0 && + in(2)._type instanceof TypeInteger i1 ) { + if (i0.isConstant() && i1.isConstant()) + return TypeInteger.constant(i0.value()+i1.value()); + } + return Type.BOTTOM; + } +``` +In our case both of the operands(`1`, `2`) are constants, and instance of +`TypeInteger`. +Since `compute` returns a type, and we know that both of our operands are constants, we can do +constant propagation/folding where we compute the result of the expression so that instead of having `1+2` we can +have `3`. + +The 'TypeInteger' interface inherits from type but also has a field that stores the value observed for that type. +```java + private final long _con +``` + +AddNode has the layout: + +![Example Visual](./02-demonstration-pre-peephole.svg) + +Since, we already know the value of both of the operands, we can just add them together and replace the +old node with a constant node that just holds the result of the expression. + +To do this - we have to recognize this happens when the type computed for the node is already a constant, but the node +is still not an instance of constant node. + +This can be captured by: +```java +// Replace constant computations from non-constants with a constant node +if (!(this instanceof ConstantNode) && type.isConstant()) { +} +``` +Note how we return a constant type when doing constant folding +```java +return TypeInteger.constant(i0.value()+i1.value()); +``` +We can create a constant node in the following way: +```java +return new ConstantNode(type).peephole(); +``` +This recursively calls the peephole algorithm. +Now, we end up with this: + +![Example Visual](./02-demonstration-pre-peephole-nokill.svg) + +Notice, how we added the constant node holding a constant value of `3`. +The remaining add node is unused and can be killed. + +We can call the kill function before this way: +```java +// Replace constant computations from non-constants with a constant node +if (!(this instanceof ConstantNode) && type.isConstant()) { + kill(); // Kill `this` because replacing with a Constant + return new ConstantNode(type).peephole(); +``` +This is responsible for deleting the dangling add node. + +```java +public void kill( ) { + assert isUnused(); // Has no uses, so it is dead + for( int i=0; i⊤:int>, fillcolor=lightblue, shape=oval]; + INT_BOT [label=<⊥:int>, fillcolor=lightblue, shape=oval]; + + INT_MIN [label="-n" , fillcolor=lightblue]; + INT_MINDOTS [label="...", fillcolor=lightblue shape=oval]; + INT_MINUS [label="-1" , fillcolor=lightblue]; + INT_ZERO [label="0" , fillcolor=lightblue]; + INT_PLUS [label="1" , fillcolor=lightblue]; + INT_MAXDOTS [label="...", fillcolor=lightblue, shape=oval]; + INT_MAX [label="+n" , fillcolor=lightblue]; + + INT_TOP -> INT_MIN; + INT_TOP -> INT_MINDOTS; + INT_TOP -> INT_MINUS; + INT_TOP -> INT_ZERO; + INT_TOP -> INT_PLUS; + INT_TOP -> INT_MAXDOTS; + INT_TOP -> INT_MAX; + INT_MIN -> INT_BOT; + INT_MINDOTS -> INT_BOT; + INT_MINUS -> INT_BOT; + INT_ZERO -> INT_BOT; + INT_PLUS -> INT_BOT; + INT_MAXDOTS -> INT_BOT; + INT_MAX -> INT_BOT; + + INT_MIN -> INT_MINDOTS -> INT_MINUS -> INT_ZERO -> INT_PLUS -> INT_MAXDOTS -> INT_MAX [style=invis]; + + // Additional customization + {rank=same; INT_MIN; INT_MINDOTS; INT_MINUS; INT_ZERO; INT_PLUS; INT_MAXDOTS; INT_MAX; } +} diff --git a/docs/chapter02/lattice.svg b/docs/chapter02/lattice.svg new file mode 100644 index 00000000..b11d57aa --- /dev/null +++ b/docs/chapter02/lattice.svg @@ -0,0 +1,159 @@ + + + + + + +Lattice + + + +INT_TOP + +⊤ +:int + + + +INT_MIN + +-n + + + +INT_TOP->INT_MIN + + + + + +INT_MINDOTS + +... + + + +INT_TOP->INT_MINDOTS + + + + + +INT_MINUS + +-1 + + + +INT_TOP->INT_MINUS + + + + + +INT_ZERO + +0 + + + +INT_TOP->INT_ZERO + + + + + +INT_PLUS + +1 + + + +INT_TOP->INT_PLUS + + + + + +INT_MAXDOTS + +... + + + +INT_TOP->INT_MAXDOTS + + + + + +INT_MAX + ++n + + + +INT_TOP->INT_MAX + + + + + +INT_BOT + +⊥ +:int + + + +INT_MIN->INT_BOT + + + + + + +INT_MINDOTS->INT_BOT + + + + + + +INT_MINUS->INT_BOT + + + + + + +INT_ZERO->INT_BOT + + + + + + +INT_PLUS->INT_BOT + + + + + + +INT_MAXDOTS->INT_BOT + + + + + + +INT_MAX->INT_BOT + + + + + -- cgit v1.2.3