Backport fix for set_def killing new_def (#21)

1 files changed, 6 insertions, 3 deletions

diff --git a/chapter02/src/main/java/com/seaofnodes/simple/node/Node.java b/chapter02/src/main/java/com/seaofnodes/simple/node/Node.java
index 43dea8e1..7c39083e 100644
--- a/chapter02/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter02/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -154,6 +154,12 @@ public abstract class Node {
      */
     void set_def(int idx, Node new_def ) {
         Node old_def = in(idx);
+        if( old_def == new_def ) return; // No change
+        // If new def is not null, add the corresponding def->use edge
+        // This needs to happen before removing the old node's def->use edge as
+        // the new_def might get killed if the old node kills it recursively.
+        if( new_def != null )
+            new_def._outputs.add(this);
         if( old_def != null ) { // If the old def exists, remove a use->def edge
             ArrayList<Node> outs = old_def._outputs;
             int lidx = outs.size()-1; // Last index
@@ -168,9 +174,6 @@ public abstract class Node {
         }
         // Set the new_def over the old (killed) edge
         _inputs.set(idx,new_def);
-        // If new def is not null, add the corresponding use->def edge
-        if( new_def != null )
-            new_def._outputs.add(this);
     }