Add shared graph viewer to all chapters

13 files changed, 122 insertions, 295 deletions

diff --git a/chapter08/src/main/java/com/seaofnodes/simple/GraphVisualizer.java b/chapter08/src/main/java/com/seaofnodes/simple/GraphVisualizer.java
deleted file mode 100644
index d75e5bea..00000000
--- a/chapter08/src/main/java/com/seaofnodes/simple/GraphVisualizer.java
+++ /dev/null
@@ -1,256 +0,0 @@
-package com.seaofnodes.simple;
-
-import com.seaofnodes.simple.node.*;
-
-import java.util.*;
-
-/**
- * Simple visualizer that outputs GraphViz dot format.
- * The dot output must be saved to a file and run manually via dot to generate the SVG output.
- * Currently, this is done manually.
- */
-public class GraphVisualizer {
-
-    /**
-     * If set to true we put the control nodes in a separate cluster from
-     * data nodes.
-     */
-    boolean _separateControlCluster = false;
-
-    public GraphVisualizer(boolean separateControlCluster) { this._separateControlCluster = separateControlCluster; }
-    public GraphVisualizer() { this(false); }
-
-    public String generateDotOutput(Parser parser) {
-
-        // Since the graph has cycles, we need to create a flat list of all the
-        // nodes in the graph.
-        Collection<Node> all = findAll(parser);
-        StringBuilder sb = new StringBuilder();
-        sb.append("digraph chapter08 {\n");
-        sb.append("/*\n");
-        sb.append(parser.src());
-        sb.append("\n*/\n");
-
-        // To keep the Scopes below the graph and pointing up into the graph we
-        // need to group the Nodes in a subgraph cluster, and the scopes into a
-        // different subgraph cluster.  THEN we can draw edges between the
-        // scopes and nodes.  If we try to cross subgraph cluster borders while
-        // still making the subgraphs DOT gets confused.
-        sb.append("\trankdir=BT;\n"); // Force Nodes before Scopes
-
-        // Preserve node input order
-        sb.append("\tordering=\"in\";\n");
-
-        // Merge multiple edges hitting the same node.  Makes common shared
-        // nodes much prettier to look at.
-        sb.append("\tconcentrate=\"true\";\n");
-
-        // Force nested scopes to order
-        sb.append("\tcompound=\"true\";\n");
-
-        // Just the Nodes first, in a cluster no edges
-        nodes(sb, all);
-
-        // Now the scopes, in a cluster no edges
-        for( ScopeNode scope : parser._xScopes )
-            scope( sb, scope );
-
-        // Walk the Node edges
-        nodeEdges(sb, all);
-
-        // Walk the active Scope edges
-        for( ScopeNode scope : parser._xScopes )
-            scopeEdges( sb, scope );
-
-        sb.append("}\n");
-        return sb.toString();
-    }
-
-    private void nodesByCluster(StringBuilder sb, boolean doCtrl, Collection<Node> all) {
-        if (!_separateControlCluster && doCtrl) // all nodes in 1 cluster
-            return;
-        // Just the Nodes first, in a cluster no edges
-        sb.append(doCtrl ? "\tsubgraph cluster_Controls {\n" : "\tsubgraph cluster_Nodes {\n"); // Magic "cluster_" in the subgraph name
-        for (Node n : all) {
-            if (n instanceof ProjNode || n instanceof ScopeNode)
-                continue; // Do not emit, rolled into MultiNode or Scope cluster already
-            if (_separateControlCluster && doCtrl && !n.isCFG()) continue;
-            if (_separateControlCluster && !doCtrl && n.isCFG()) continue;
-            sb.append("\t\t").append(n.uniqueName()).append(" [ ");
-            String lab = n.glabel();
-            if (n instanceof MultiNode) {
-                // Make a box with the MultiNode on top, and all the projections on the bottom
-                sb.append("shape=plaintext label=<\n");
-                sb.append("\t\t\t<TABLE BORDER=\"0\" CELLBORDER=\"1\" CELLSPACING=\"0\" CELLPADDING=\"4\">\n");
-                sb.append("\t\t\t<TR><TD BGCOLOR=\"yellow\">").append(lab).append("</TD></TR>\n");
-                sb.append("\t\t\t<TR>");
-                boolean doProjTable = false;
-                for (Node use : n._outputs) {
-                    if (use instanceof ProjNode proj) {
-                        if (!doProjTable) {
-                            doProjTable = true;
-                            sb.append("<TD>").append("\n");
-                            sb.append("\t\t\t\t<TABLE BORDER=\"0\" CELLBORDER=\"1\" CELLSPACING=\"0\">").append("\n");
-                            sb.append("\t\t\t\t<TR>");
-                        }
-                        sb.append("<TD PORT=\"p").append(proj._idx).append("\"");
-                        if (proj.isCFG()) sb.append(" BGCOLOR=\"yellow\"");
-                        sb.append(">").append(proj.glabel()).append("</TD>");
-                    }
-                }
-                if (doProjTable) {
-                    sb.append("</TR>").append("\n");
-                    sb.append("\t\t\t\t</TABLE>").append("\n");
-                    sb.append("\t\t\t</TD>");
-                }
-                sb.append("</TR>\n");
-                sb.append("\t\t\t</TABLE>>\n\t\t");
-
-            } else {
-                // control nodes have box shape
-                // other nodes are ellipses, i.e. default shape
-                if (n.isCFG()) sb.append("shape=box style=filled fillcolor=yellow ");
-                else if (n instanceof PhiNode) sb.append("style=filled fillcolor=lightyellow ");
-                sb.append("label=\"").append(lab).append("\" ");
-            }
-            sb.append("];\n");
-        }
-        if (!_separateControlCluster) {
-            // Force Region & Phis to line up
-            for (Node n : all) {
-                if (n instanceof RegionNode region) {
-                    sb.append("\t\t{ rank=same; ");
-                    sb.append(region).append(";");
-                    for (Node phi : region._outputs)
-                        if (phi instanceof PhiNode) sb.append(phi.uniqueName()).append(";");
-                    sb.append("}\n");
-                }
-            }
-        }
-        sb.append("\t}\n");     // End Node cluster
-    }
-
-    private void nodes(StringBuilder sb, Collection<Node> all) {
-        nodesByCluster(sb, true, all);
-        nodesByCluster(sb, false, all);
-    }
-
-    // Build a nested scope display, walking the _prev edge
-    private void scope( StringBuilder sb, ScopeNode scope ) {
-        sb.append("\tnode [shape=plaintext];\n");
-        int level=1;
-        for( int idx = scope._scopes.size()-1; idx>=0; idx-- ) {
-            var syms = scope._scopes.get(idx);
-            String scopeName = makeScopeName(scope, level);
-            sb.append("\tsubgraph cluster_").append(scopeName).append(" {\n"); // Magic "cluster_" in the subgraph name
-            sb.append("\t\t").append(scopeName).append(" [label=<\n");
-            sb.append("\t\t\t<TABLE BORDER=\"0\" CELLBORDER=\"1\" CELLSPACING=\"0\">\n");
-            // Add the scope level
-            int scopeLevel = scope._scopes.size()-level;
-            sb.append("\t\t\t<TR><TD BGCOLOR=\"cyan\">").append(scopeLevel).append("</TD>");
-            for(String name: syms.keySet())
-                sb.append("<TD PORT=\"").append(makePortName(scopeName, name)).append("\">").append(name).append("</TD>");
-            sb.append("</TR>\n");
-            sb.append("\t\t\t</TABLE>>];\n");
-            level++;
-        }
-        // Scope clusters nest, so the graphics shows the nested scopes, so
-        // they are not closed as they are printed; so they just keep nesting.
-        // We close them all at once here.
-        sb.append( "\t}\n".repeat( level-1 ) ); // End all Scope clusters
-    }
-
-    private String makeScopeName(ScopeNode sn, int level) { return sn.uniqueName() + "_" + level; }
-    private String makePortName(String scopeName, String varName) { return scopeName + "_" + varName; }
-
-    // Walk the node edges
-    private void nodeEdges(StringBuilder sb, Collection<Node> all) {
-        // All them edge labels
-        sb.append("\tedge [ fontname=Helvetica, fontsize=8 ];\n");
-        for( Node n : all ) {
-            // Do not display the Constant->Start edge;
-            // ProjNodes handled by Multi;
-            // ScopeNodes are done separately
-            if( n instanceof ConstantNode || n instanceof ProjNode || n instanceof ScopeNode )
-                continue;
-            for( int i=0; i<n.nIns(); i++ ) {
-                Node def = n.in(i);
-                if( n instanceof PhiNode && def instanceof RegionNode ) {
-                    // Draw a dotted use->def edge from Phi to Region.
-                    sb.append('\t').append(n.uniqueName());
-                    sb.append(" -> ");
-                    sb.append(def.uniqueName());
-                    sb.append(" [style=dotted taillabel=").append(i).append("];\n");
-                } else if( def != null ) {
-                    // Most edges land here use->def
-                    sb.append('\t').append(n.uniqueName()).append(" -> ");
-                    if( def instanceof ProjNode proj ) {
-                        String mname = proj.ctrl().uniqueName();
-                        sb.append(mname).append(":p").append(proj._idx);
-                    } else sb.append(def.uniqueName());
-                    // Number edges, so we can see how they track
-                    sb.append("[taillabel=").append(i);
-                    // control edges are colored red
-                    if( def.isCFG() )
-                        sb.append(" color=red");
-                    // Backedges do not add a ranking constraint
-                    if( i==2 && (n instanceof PhiNode || n instanceof LoopNode) )
-                        sb.append(" constraint=false");
-                    sb.append("];\n");
-                }
-            }
-        }
-    }
-
-    // Walk the scope edges
-    private void scopeEdges( StringBuilder sb, ScopeNode scope ) {
-        sb.append("\tedge [style=dashed color=cornflowerblue];\n");
-        int level=1;
-        for( int i = scope._scopes.size()-1; i>=0; i-- ) {
-            var syms = scope._scopes.get(i);
-            String scopeName = makeScopeName(scope, level);
-            for( String name : syms.keySet() ) {
-                int idx = syms.get(name);
-                Node def = scope.in(idx);
-                while( def instanceof ScopeNode lazy )
-                    def = lazy.in(idx);
-                if( def==null ) continue;
-                sb.append("\t")
-                  .append(scopeName).append(":")
-                  .append('"').append(makePortName(scopeName, name)).append('"') // wrap port name with quotes because $ctrl is not valid unquoted
-                  .append(" -> ");
-                if( def instanceof ProjNode proj ) {
-                    String mname = proj.ctrl().uniqueName();
-                    sb.append(mname).append(":p").append(proj._idx);
-                } else sb.append(def.uniqueName());
-                sb.append(";\n");
-            }
-            level++;
-        }
-    }
-
-    /**
-     * Finds all nodes in the graph.
-     */
-    private Collection<Node> findAll(Parser parser) {
-        final HashMap<Integer, Node> all = new HashMap<>();
-        for( Node n : Parser.START._outputs )
-            walk(all, n);
-        for( Node n : parser._scope._inputs )
-            walk(all, n);
-        return all.values();
-    }
-
-    /**
-     * Walk a subgraph and populate distinct nodes in the all list.
-     */
-    private void walk(HashMap<Integer,Node> all, Node n) {
-        if(n == null ) return;
-        if (all.get(n._nid) != null) return; // Been there, done that
-        all.put(n._nid, n);
-        for (Node c : n._inputs)
-            walk(all, c);
-        for( Node c : n._outputs )
-            walk(all, c);
-    }
-}
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/Parser.java b/chapter08/src/main/java/com/seaofnodes/simple/Parser.java
index 69796828..5c699a54 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/Parser.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/Parser.java
@@ -1,5 +1,7 @@
 package com.seaofnodes.simple;
 
+import com.seaofnodes.graph.GraphObserver;
+
 import com.seaofnodes.simple.node.*;
 import com.seaofnodes.simple.type.*;
 
@@ -14,6 +16,12 @@ import java.util.*;
  */
 public class Parser {
 
+    // Current compilation context; the observer belongs to the parser.
+    public static Parser PARSER;
+    public GraphObserver<Node> _obs;
+    public int pos() { return _lexer._position; }
+
+
     /**
      * A Global Static, unique to each compilation.  This is a public, so we
      * can make constants everywhere without having to thread the StartNode
@@ -68,6 +76,7 @@ public class Parser {
     ScopeNode _breakScope;
 
     public Parser(String source, TypeInteger arg) {
+        PARSER = this;
         Node.reset();
         _lexer = new Lexer(source);
         _scope = new ScopeNode();
@@ -92,8 +101,7 @@ public class Parser {
 
     private Node ctrl(Node n) { return _scope.ctrl(n); }
 
-    public StopNode parse() { return parse(false); }
-    public StopNode parse(boolean show) {
+    public StopNode parse() {
         _xScopes.push(_scope);
         // Enter a new scope for the initial control and arguments
         _scope.push();
@@ -104,7 +112,6 @@ public class Parser {
         _xScopes.pop();
         if (!_lexer.isEOF()) throw error("Syntax error, unexpected " + _lexer.getAnyNextToken());
         STOP.peephole();
-        if( show ) showGraph();
         return STOP;
     }
 
@@ -144,7 +151,6 @@ public class Parser {
         else if (matchx("while")) return parseWhile();
         else if (matchx("break")) return parseBreak();
         else if (matchx("continue")) return parseContinue();
-        else if (matchx("#showGraph")) return require(showGraph(),";");
         else if (matchx(";")) return null; // Empty statement
         else return parseExpressionStatement();
     }
@@ -320,14 +326,7 @@ public class Parser {
         return ret;
     }
 
-    /**
-     * Dumps out the node graph
-     * @return {@code null}
-     */
-    private Node showGraph() {
-        System.out.println(new GraphVisualizer().generateDotOutput(this));
-        return null;
-    }
+
 
     /**
      * Parses an expression statement
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/node/AddNode.java b/chapter08/src/main/java/com/seaofnodes/simple/node/AddNode.java
index 0c66422d..7fa63426 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/node/AddNode.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/node/AddNode.java
@@ -9,8 +9,6 @@ public class AddNode extends Node {
 
     @Override public String label() { return "Add"; }
 
-    @Override public String glabel() { return "+"; }
-
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         in(1)._print0(sb.append("("), visited);
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/node/BoolNode.java b/chapter08/src/main/java/com/seaofnodes/simple/node/BoolNode.java
index 5948a1d3..3938b903 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/node/BoolNode.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/node/BoolNode.java
@@ -17,9 +17,6 @@ abstract public class BoolNode extends Node {
     public String label() { return getClass().getSimpleName(); }
 
     @Override
-    public String glabel() { return op(); }
-
-    @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         in(1)._print0(sb.append("("), visited);
         in(2)._print0(sb.append(op()), visited);
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/node/DivNode.java b/chapter08/src/main/java/com/seaofnodes/simple/node/DivNode.java
index 7207fd48..e8b7067e 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/node/DivNode.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/node/DivNode.java
@@ -10,8 +10,6 @@ public class DivNode extends Node {
 
     @Override public String label() { return "Div"; }
 
-    @Override public String glabel() { return "//"; }
-
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         in(1)._print0(sb.append("("), visited);
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/node/MinusNode.java b/chapter08/src/main/java/com/seaofnodes/simple/node/MinusNode.java
index 3d944fd4..e4d718d5 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/node/MinusNode.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/node/MinusNode.java
@@ -10,8 +10,6 @@ public class MinusNode extends Node {
 
     @Override public String label() { return "Minus"; }
 
-    @Override public String glabel() { return "-"; }
-
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         in(1)._print0(sb.append("(-"), visited);
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/node/MulNode.java b/chapter08/src/main/java/com/seaofnodes/simple/node/MulNode.java
index 2e701a4e..18e23d8b 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/node/MulNode.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/node/MulNode.java
@@ -10,8 +10,6 @@ public class MulNode extends Node {
 
     @Override public String label() { return "Mul"; }
 
-    @Override public String glabel() { return "*"; }
-
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         in(1)._print0(sb.append("("), visited);
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/node/Node.java b/chapter08/src/main/java/com/seaofnodes/simple/node/Node.java
index ba64b059..f20e9c2f 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -1,5 +1,7 @@
 package com.seaofnodes.simple.node;
 
+import com.seaofnodes.simple.Parser;
+
 import com.seaofnodes.simple.IRPrinter;
 import com.seaofnodes.simple.Utils;
 import com.seaofnodes.simple.type.Type;
@@ -77,13 +79,9 @@ public abstract class Node {
     // Easy reading label for debugger, e.g. "Add" or "Region" or "EQ"
     public abstract String label();
 
-    // Unique label for graph visualization, e.g. "Add12" or "Region30" or "EQ99"
+    // Unique label for debugging, e.g. "Add12" or "Region30" or "EQ99"
     public String uniqueName() { return label() + _nid; }
 
-    // Graphical label, e.g. "+" or "Region" or "=="
-    public String glabel() { return label(); }
-
-
     // ------------------------------------------------------------------------
 
     // Debugger Printing.
@@ -258,7 +256,7 @@ public abstract class Node {
     }
 
     // Mostly used for asserts and printing.
-    boolean isDead() { return isUnused() && nIns()==0 && _type==null; }
+    public boolean isDead() { return isUnused() && nIns()==0 && _type==null; }
 
     // Shortcuts to stop DCE mid-parse
     // Add bogus null use to keep node alive
@@ -303,12 +301,22 @@ public abstract class Node {
      * code. </li>
      * </ul>
      */
-    public final Node peephole( ) {
+    public final Node peephole() {
+        var obs = Parser.PARSER == null ? null : Parser.PARSER._obs;
+        if( obs != null ) obs.before(this);
+        Type old = _type;
+        Node n = peepholeOpt();
+        Node rez = n == null ? this : n;
+        if( obs != null ) obs.after(this, n == null && old == _type ? null : rez, false);
+        return rez;
+    }
+
+    private Node peepholeOpt() {
         // Compute initial or improved Type
         Type type = _type = compute();
 
         if (_disablePeephole)
-            return this;        // Peephole optimizations turned off
+            return null;        // Peephole optimizations turned off
 
         // Replace constant computations from non-constants with a constant node
         if (!(this instanceof ConstantNode) && type.isConstant())
@@ -322,8 +330,8 @@ public abstract class Node {
             // Recursively optimize
             return deadCodeElim(n.peephole());
 
-        return this;            // No progress
-    }
+        return null;            // No progress
+        }
 
     // m is the new Node, self is the old.
     // Return 'm', which may have zero uses but is alive nonetheless.
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/node/NotNode.java b/chapter08/src/main/java/com/seaofnodes/simple/node/NotNode.java
index 39df410d..e8d2ecf2 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/node/NotNode.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/node/NotNode.java
@@ -10,8 +10,6 @@ public class NotNode extends Node {
 
     @Override public String label() { return "Not"; }
 
-    @Override public String glabel() { return "!"; }
-
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         in(1)._print0(sb.append("(!"), visited);
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/node/PhiNode.java b/chapter08/src/main/java/com/seaofnodes/simple/node/PhiNode.java
index c7b5b357..7eeed394 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/node/PhiNode.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/node/PhiNode.java
@@ -12,8 +12,6 @@ public class PhiNode extends Node {
 
     @Override public String label() { return "Phi_"+_label; }
 
-    @Override public String glabel() { return "&phi;_"+_label; }
-
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         if( region().inProgress() )
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/node/SubNode.java b/chapter08/src/main/java/com/seaofnodes/simple/node/SubNode.java
index af66b950..5e61df95 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/node/SubNode.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/node/SubNode.java
@@ -10,8 +10,6 @@ public class SubNode extends Node {
 
     @Override public String label() { return "Sub"; }
 
-    @Override public String glabel() { return "-"; }
-
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         in(1)._print0(sb.append("("), visited);
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java b/chapter08/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java
new file mode 100644
index 00000000..585592d4
--- /dev/null
+++ b/chapter08/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java
@@ -0,0 +1,53 @@
+package com.seaofnodes.simple.print;
+
+import com.seaofnodes.graph.GraphAdapter;
+import com.seaofnodes.graph.GraphSnapshot;
+import com.seaofnodes.graph.GraphSnapshot.*;
+import com.seaofnodes.simple.node.*;
+import com.seaofnodes.simple.node.Node;
+import java.util.ArrayList;
+
+/** Chapter 8's view of the IR; browser, transport and layout are shared. */
+public class SimpleGraphAdapter extends GraphAdapter<Node> {
+    @Override protected boolean dead(Node n) { return n.isDead(); }
+    @Override protected int id(Node n) { return n._nid; }
+    @Override protected int nIns(Node n) { return n.nIns(); }
+    @Override protected Node in(Node n, int idx) { return n.in(idx); }
+    @Override protected int nOuts(Node n) { return n.nOuts(); }
+    @Override protected Node out(Node n, int idx) { return n._outputs.get(idx); }
+
+    private ArrayList<Edge> edges(Node n) {
+        var edges = new ArrayList<Edge>();
+        String[] names = n instanceof ScopeNode scope && n.nIns() != 0 ? scope.reverseNames() : null;
+        for( int i = 0; i < n.nIns(); i++ ) {
+            String name = names == null || i >= names.length ? null : names[i];
+            edges.add(new Edge(i, ref(n.in(i)), role(n, i), name));
+        }
+        return edges;
+    }
+
+    @Override protected GraphSnapshot.Node desc(Node n) {
+        Projection proj = n instanceof ProjNode p ? new Projection(ref(n.in(0)), p._idx) : null;
+        String label = n.label();
+        return new GraphSnapshot.Node(n._nid, label == null ? n.getClass().getSimpleName() : label,
+                                      n._type == null ? null : n._type.toString(), kind(n), edges(n), proj);
+    }
+
+    private Kind kind(Node n) {
+        if( n instanceof ScopeNode ) return Kind.SCOPE;
+        if( n instanceof StartNode ) return Kind.CTRL;
+        if( n instanceof LoopNode ) return Kind.LOOP;
+        if( n instanceof RegionNode ) return Kind.REGION;
+        if( n instanceof PhiNode ) return Kind.PHI;
+        return n.isCFG() ? Kind.CTRL : Kind.DATA;
+    }
+
+    private Role role(Node n, int i) {
+        if( n instanceof ScopeNode || n instanceof ConstantNode ) return Role.ASSOC;
+        if( n instanceof PhiNode ) return i == 0 ? Role.ASSOC : Role.DATA;
+        if( n instanceof ProjNode ) return n.isCFG() ? Role.CTRL : Role.DATA;
+        if( n instanceof RegionNode && i == 0 ) return Role.ASSOC;
+        if( i == 0 || n instanceof RegionNode || n instanceof StopNode ) return Role.CTRL;
+        return Role.DATA;
+    }
+}
diff --git a/chapter08/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java b/chapter08/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java
new file mode 100644
index 00000000..7861b040
--- /dev/null
+++ b/chapter08/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java
@@ -0,0 +1,40 @@
+package com.seaofnodes.simple.print;
+
+import com.seaofnodes.graph.GraphCapture;
+import com.seaofnodes.graph.GraphViewer;
+import com.seaofnodes.simple.Parser;
+import com.seaofnodes.simple.node.Node;
+import java.util.ArrayList;
+import java.io.IOException;
+
+/** Chapter 8's compilation context; capture and event bookkeeping are shared. */
+public class SimpleGraphObserver extends GraphCapture<Node> {
+    private Parser _parser;
+    private Node _ret;
+    private String _phase;
+    public SimpleGraphObserver() { super(new SimpleGraphAdapter()); }
+    public static void main(String[] args) throws IOException {
+        GraphViewer.run(new SimpleGraphObserver());
+    }
+    @Override protected void compile(String src) {
+        _parser = new Parser(src);
+        _parser._obs = this;
+        _phase = "Parse";
+        _ret = _parser.parse();
+        phase(_phase);
+    }
+    @Override protected String phase() { return _phase; }
+    @Override protected int pos() { return "Parse".equals(_phase) ? _parser.pos() : -1; }
+    @Override protected void detach() {
+        if( _parser != null ) _parser._obs = null;
+        _parser = null;
+        _ret = null;
+    }
+    @Override protected void roots(ArrayList<Node> roots) {
+        roots.add(Parser.START);
+        roots.add(_parser.STOP);
+        roots.add(_parser._scope);
+        roots.addAll(_parser._xScopes);
+        roots.add(_ret);
+    }
+}