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 for this chapter.
In Chapter 1 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 |
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.
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, 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.
- 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).
- Pointers may be known to be not-null, and a null check can be skipped (Chapter 10).
- 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 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 identified by the Java class sub-typing relationship; 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". Our lattice has the following structure:
Our lattice elements can be one of three types:
- The highest element is "top", denoted by T; assigning T 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 down the lattice.
In later chapters we will explore extending this lattice, as it frequently forms the heart of core optimizations we want our compiler to do.
There are other important properties of the Lattice that we discuss in Chapter 4 and Chapter 10, such as the "meet" and "join" operators and their rules.
The following visual shows how the graph looks like pre-peephole optimization:
- 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.
