Chapter 5: If Statement, Phi, and Region

Co-authored-by: Cliff Click <cliffc@acm.org>
Co-authored-by: XmiliaH <45106915+XmiliaH@users.noreply.github.com>
Co-authored-by: Yichen Yan <oraluben@outlook.com>
Co-authored-by: Thalia Archibald <thalia@archibald.dev>
Co-authored-by: Tamas Hadhazy <helsgithub15@gmail.com>
Co-authored-by: David Rubin <87927264+Rexicon226@users.noreply.github.com>

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-# Appendix
-## Dead Code Elimination(DCE)
-Dead Code Elimination is the process or removing nodes that are dead(unused).
-
-There are two kinds of dead code eliminations present in this chapter.
-- Delete unused nodes when popping the scope.
-- Delete old nodes after finding a better replacement(after idealize call).
-
-Both of the methods mentioned above delete the nodes recursively. However, the first case happens
-regardless of whether `peephole` is called or not.
-
-### Delete unused nodes when popping the scope
-A node is unused if it has no outputs(users).
-
-### Example 1:
-  ``` 
-  int a = 12; // the only output it has is the scope itself
-  return 1;
-  ```
-We clearly see `a` is unused.
-This gives us the following graph(without DCE):
-
-![Graph2](../docs/04-dce1.svg)
-
-To remove `a` we introduce the following technique when popping the scope.
-
-  ```java
-  var ret = (ReturnNode) parseBlock();
-  _scope.pop();
-  ```
-As of now the end of the program is hard-wired to return, after finishing parsing we pop the scope.
-The state of the scope before popping it:
-  ```
-  _scope = {ScopeNode@1422} "Scope[$ctrl:null, arg:arg][a:12]"
-  1 = {ProjNode@1528} "arg"
-  2 = {ConstantNode@1529} "12"
-  _inputs = {ArrayList@1523}  size = 3
-  ```
-The output(s) of `a`
-  ```
-  _outputs = {ArrayList@1544}  size = 1
-  0 = {ScopeNode@1422} "Scope[$ctrl:null, arg:arg][a:12]"
-  ```
-Notice how the only output of `a`is the scope itself. When `a` is introduced, it is added to the
-inputs of the scope, and the scope is added to the outputs of `a`(recall use-def chains).
-
-Since `a` is not being returned - it is unused and can be killed.
-
-`_scope.pop`:
-```java
-public void pop() { popN(_scopes.pop().size());  }
-```
-`_scope.popN`(responsible for killing the unused nodes *recursively*):
-```java 
-void popN(int n) {
-    for (int i = 0; i < n; i++) {
-        Node old_def = _inputs.removeLast(); // start from the back
-        if (old_def != null &&        // If it exists and
-            old_def.delUse(this)) {   // If we removed the last use, the old def is now dead
-            old_def.kill();           // Kill the old def
-        }
-    }
-}
-```
-First, we start off by looping through the inputs of the scope(*remember*: scope is just a node like any other, the symbols of the scope are the inputs).
-If the node returned by `removeLast` is not null, and if the only user of `old_def` is the current Node then we can delete it.
-
-`old_def.delUse(this)` - delete *this* from `old_def`'s outputs.
-```java
-protected boolean delUse( Node use ) {
-  Utils.del(_outputs, Utils.find(_outputs, use));
-  return _outputs.isEmpty();
-                                     }
-```
-Returns true if `_outputs` becomes empty after deleting `use`.
-`old_def.kill()` starts the killing process recursively.
-
-`old_def.kill()`
-```java 
-public void kill( ) {
-      assert isUnused();      // Has no uses, so it is dead
-      popN(nIns());           // Set all inputs to null, recursively killing unused Nodes
-      _type=null;             // Flag as dead
-      assert isDead();        // Really dead now
-    }
-```
-`popN` is called again, our goal is to look for nodes that might also get deleted once `a` is freed.
-The only input `a` has is the constant node with value 12. As every other node a `ConstantNode` takes in the control node first(`Start`), once we
-drop the `Start` node from the inputs of `ConstantNode`:
-``` 
-Node old_def = _inputs.removeLast();
-```
-we can also get rid of `ConstantNode`(`this`) from the outputs of `Start`:
-``` 
-old_def.delUse(this) 
-```
-### Example 2:
-```
-int a = 12; 
-int b = a + 1; 
-return 1; 
-```
-- Call `popN` on current scope.
-- Realize that `b = a + 1;` has no uses - so we can kill it.
-- In the kill function we can call `popN` but this time with the inputs of `b = a + 1;`(start killing them recursively)
-- We start the killing process from the back and examine "1" first. This has no output other than the `AddNode`. Since `this = AddNode` calling `delUse` will mark "1" as dead.
-- We now can kill "1" and call the `popN` function with one input(the control node).
-- Now we examine "a" which has 2 uses at this point, the scope and `AddNode`. Since `this = AddNode` we will remove `AddNode` from its outputs, it will still have the scope as a user - so we can't delete it just yet.
-- We move onto`a = 12`, since "a" has only one user left(the scope) and `this = scope` it can get killed easily.
-- Same as before, we go through the inputs of "a" and will delete them from the outputs of the control node.
-
-*A constant node pointing to "Start"(ignored by the graph visualizer) but still maintains a connection:*
-
-![Graph3](../docs/04-dce2.svg)
-
-## Delete old nodes after finding a better replacement
-Consider the following code:
-```
-return (2+1)+3; 
-```
-Without DCE after the `idealize` call we would end up with:
-![Graph3](../docs/04-dce3.svg)
-
-After identifying a more optimal replacement value (6) for the expression (2 + 1) + 3,
-we created a constant node to store this value.
-However, we did not remove the original expression,
-resulting in dangling nodes in our structure.
-
-To fix this, we introduce an extra `deadCodeElim` call that is responsible for deleting unused nodes.
-
-
-```java
-public final Node peephole( ) {
-    ...
-    if (!(this instanceof ConstantNode) && type.isConstant())
-        return new deadCodeElim(ConstantNode(type).peephole());
-
-    ...
-  
-    Node n = idealize();
-    if( n != null )         // Something changed
-        // Recursively optimize
-        return deadCodeElim(n.peephole());
-   
-   ...
-}
-``` 
-
-```java
-// m is the new Node, self is the old.
-// Return 'm', which may have zero uses but is alive nonetheless.
-// If self has zero uses (and is not 'm'), {@link #kill} self.
-private Node deadCodeElim(Node m) {
-    // If self is going dead and not being returned here (Nodes returned
-    // from peephole commonly have no uses (yet)), then kill self.
-    if( m != this && isUnused() ) {
-        // Killing self - and since self recursively kills self's inputs we
-        // might end up killing 'm', which we are returning as a live Node.
-        // So we add a bogus extra null output edge to stop kill().
-        m.addUse(null); // Add bogus null use to keep m alive
-        kill();            // Kill self because replacing with 'm'
-        m.delUse(null);    // Remove bogus null.
-    }
-    return m;
-}
-```
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