# Chapter 13: References

# Table of Contents

1. [References](#references)
    - [Forward References](#forward-references)
    - [Self-referential references](#self-referential-references)
    - [Auto deepen forward ref types](#auto-deepen-forward-ref-types)
2. [Global Code Motion Example](#global-code-motion-walkthrough)

In this chapter, we add references and structs.

You can also read [this chapter](https://github.com/SeaOfNodes/Simple/tree/linear-chapter13) 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-chapter12...linear-chapter13) it to the previous chapter.

## References

Structs can reference other structs; references can be mutually recursive.
Any reference field can allow null or not, same as a normal variable.


### Forward References
A forward reference occurs when a type or identifier is used before its full definition is
encountered by the compiler. (incomplete types, mutual references).
When forward-reference types are encountered, the compiler may initially treat them as placeholders or "shallow" representations.


Consider the following code:
```java
struct Person {
String name;       // No string type, yet
int age;
FamilyTree? tree;  // A family tree, or not
}
...
```
At the time of the `Person` struct definition, the `FamilyTree` struct is not yet defined. This is a forward reference. To handle this, we assume the struct will be defined later.
```java 
Type t = TYPES.get(tname);
   // Assume a forward-reference type
   if( t == null ) {
       int old2 = _lexer._position;
       String id = _lexer.matchId();
       if( id==null ) {
           _lexer._position = old;
           return null;
       }
       TYPES.put(tname,t = TypeStruct.make(tname)); // Assume forward ref
       _lexer._position = old2;
   }
```
Reference fields start out as null, doing anything with them will result in a compile time error.

```java 
struct Person {
  String name;
  int age;
  FamilyTree? tree;
}
Person p = new Person;
p.tree.la = null; // Accessing unknown field 'la' from 'null'
                         
```

It turns out `p.tree` is `null`, so we can't access the field `la` from it.
```java 
// name  = "tree"
// memAlias(alias) = StoreNode
// in(3) = {ConstantNode@1651} "null"
return parsePostfix(new LoadNode(name, alias, declaredType.glb(), memAlias(alias), expr).peephole());
```
`LoadNode::idealize`
In LoadNode idealize we return st.val().
```java 
        // Simple Load-after-Store on same address.
        if( mem() instanceof StoreNode st &&
            ptr() == st.ptr() ) { // Must check same object
            assert _name.equals(st._name); // Equiv class aliasing is perfect
            return st.val(); // // <<-- {ConstantNode@1651} "null"
        }
```
`mem(): ` refers to `memAlias(alias)`. Therefore, in the next iteration, we will end up with:

```java
...
// expr {ConstantNode@1651} "null"
// ptr = expr._type
// ptr = TypeMemPtr@1510 "null"
// ptr._obj._name = $TOP
TypeStruct base = (TypeStruct)TYPES.get(ptr._obj._name);
int idx = base==null ? -1 : base.find(name); // <<-- null
if( idx == -1 ) {
  throw error("Accessing unknown field '" + name + "' from '" + ptr.str() + "'"); // throws here!!!
}
```
When we are creating the struct, we set up the initial value to Null:

`newStruct`:
```java
private Node newStruct(TypeStruct obj) {
   Node n = new NewNode(TypeMemPtr.make(obj), ctrl()).peephole().keep();
   int alias = START._aliasStarts.get(obj._name);
   for( Field field : obj._fields ) {
       memAlias(alias, new StoreNode(field._fname, alias, field._type, ctrl(), memAlias(alias), n, new ConstantNode(field._type.makeInit()).peephole(),true).peephole());
       alias++;
   }
   return n.unkeep();
}
```
Zooming in....

```java 
// parsing =  FamilyTree? tree;  
for( Field field : obj._fields ) {
    // field._type = *FamilyTree?
    // REMEMBER:
    // @Override public TypeMemPtr makeInit() { return NULLPTR; }
  memAlias(..., new ConstantNode(field._type.makeInit()).peephole(),true).peephole());
  alias++;
}
```
In a case of a reference `field._type.makeInit()` will return  a `NULLPTR`, later this is what the peephole turns into when loading
the field. `p.tree`

--- 

```java
struct N { N next; int i; }
N n = new N;
return n.next;  // Value reports as null, despite field typed as not-null
```

As shown above, we explicitly allow not-null fields to be null initialized (as
of chapter13).  We can't allow null to be stored into a not-null field, but we
can allow a not-null field to be null (initially).  This will be corrected in a
later chapter.


### Self-referential references
Self-cyclic or self-referential types are types that include a reference to themsleves as part of their defintion.

Consider the following code:
```java 
struct N { N next; int i; }
N n = new N;
n.next = null; // <<-- Compile error, cannot store null into not-null field
return n.next;
```
The field `next` is of type `N`, which is the same type as the structure `N` itself.
This creates a circular reference. To resolve this, we auto deepen the forward ref type.


#### Auto deepen forward ref types
Auto deepening refers to the process where the compiler expands the forward reference into its complete definition.

When `a.next` is encountered, we auto deepen the type so its fields are going to be set.

This is needed to resolve the stale type that the loop brings around.

```java 
if( e instanceof TypeMemPtr tmp && tmp._obj._fields==null ) {
  e = tmp.make_from((TypeStruct) TYPES.get(tmp._obj._name));
}
``` 

```java 
struct LLI { LLI? next; int i; }
LLI? head = null;
while( arg ) {
    LLI x = new LLI;
    x.next = head;
    x.i = arg;
    head = x;
    arg = arg-1;
}
if( !head ) return 0;
LLI? next = head.next;
if( next==null ) return 1;
return next.i;
```

---

Normal field syntax works:  
e.g. `person.tree.father = new Person;` or
`person.tree.father.name = "Dad";`.

Reference fields all start out null, and can be tested as such.  However, a
non-null reference field can only be stored with a non-null ref.

```java
struct N { N next; int i; }
N n = new N;
return n.next.i;  // Value reports as null, despite field typed as not-null
```

And:

```java
struct N { N next; int i; }
N n = new N;
n.next = null; // <<-- Compile error, cannot store null into not-null field
return n.next;
```

Forward references must be specified later and field access is later checked.

## Global Code Motion walkthrough
Now that we have all these new features covered consider the following example:

```java
struct Person {
  String name;
  int age;
  FamilyTree? tree;  
}
Person p  = new Person;
p.age =1;
return p.age;
```
Reverse post order: Traverse nodes before their children. (Opposite of post order).
Loop through the post order list from the end to the beginning.

RPO = `[StartNode, CProjNode("ctrl"), ReturnNode("return 1"), StopNode("return 1"), XctrlNode("Xctrl")]` -> all the control nodes.

Remember, with constants we want to schedule them to the deepest basic block.
For example a constant node will have StartNode as its in(0) but we can find a better replacement.
Early schedule:

Does not change anything here:

#### Early schedule:
```
From: Start to: Start|#1(18)
From: Start to: Start|#null(12)
From: $ctrl to: $ctrl|.name=(13)
From: $ctrl to: $ctrl|.age=(19)
From: $ctrl to: $ctrl|.tree=(17)
```
The control nodes are already set so the early schedule is useless.
All the nodes involed: Return, Constant, NewNode specific their control input explicitly so there is nothing to do.

#### Late schedule:
```
From: Start to: $ctrl(12)
From: Start to: $ctrl(18)
```
We see that the late schedule actually changes the control input of the StartNode to the $ctrl node in 2 cases.
The algo that does this is:
```java 
 // Walk up from the LCA to the early, looking for best place.  This is
// the lowest execution frequency, approximated by least loop depth and
// deepest control flow.
CFGNode best = lca;
lca = lca.idom();       // Already found best for starting LCA
for( ; lca != early.idom(); lca = lca.idom() )
    if( better(lca,best) )
        best = lca;
assert !(best instanceof IfNode);
```