Use isDead() instead of checking null inputs

21 files changed, 57 insertions, 34 deletions

diff --git a/chapter07/src/main/java/com/seaofnodes/simple/node/Node.java b/chapter07/src/main/java/com/seaofnodes/simple/node/Node.java
index 5759a065..941edaca 100644
--- a/chapter07/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter07/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -119,7 +119,7 @@ public abstract class Node {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public void _printLine(StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(_nid,label()));
-        if( _inputs==null ) {
+        if( isDead() ) {
             sb.append("DEAD\n");
             return;
         }
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 234930b4..cffb41e4 100644
--- a/chapter08/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter08/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -121,7 +121,7 @@ public abstract class Node {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public void _printLine(StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(_nid,label()));
-        if( _inputs==null ) {
+        if( isDead() ) {
             sb.append("DEAD\n");
             return;
         }
diff --git a/chapter09/src/main/java/com/seaofnodes/simple/node/Node.java b/chapter09/src/main/java/com/seaofnodes/simple/node/Node.java
index a5f26545..151cb6a7 100644
--- a/chapter09/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter09/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -124,7 +124,7 @@ public abstract class Node {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public void _printLine(StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(_nid,label()));
-        if( _inputs==null ) {
+        if( isDead() ) {
             sb.append("DEAD\n");
             return;
         }
diff --git a/chapter10/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter10/src/main/java/com/seaofnodes/simple/IRPrinter.java
index f08f3208..1ac32b1a 100644
--- a/chapter10/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter10/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -14,7 +14,7 @@ public class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static void _printLine( Node n, StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
@@ -46,7 +46,7 @@ public class IRPrinter {
     // Nodes as referred to as %id
     public static void _printLineLlvmFormat( Node n, StringBuilder sb ) {
         nodeId(sb, n).append(": ");
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
diff --git a/chapter11/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter11/src/main/java/com/seaofnodes/simple/IRPrinter.java
index 909dd575..3410ab64 100644
--- a/chapter11/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter11/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -14,7 +14,7 @@ public class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static void _printLine( Node n, StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
@@ -46,7 +46,7 @@ public class IRPrinter {
     // Nodes as referred to as %id
     public static void _printLineLlvmFormat( Node n, StringBuilder sb ) {
         nodeId(sb, n).append(": ");
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
diff --git a/chapter12/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter12/src/main/java/com/seaofnodes/simple/IRPrinter.java
index 909dd575..3410ab64 100644
--- a/chapter12/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter12/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -14,7 +14,7 @@ public class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static void _printLine( Node n, StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
@@ -46,7 +46,7 @@ public class IRPrinter {
     // Nodes as referred to as %id
     public static void _printLineLlvmFormat( Node n, StringBuilder sb ) {
         nodeId(sb, n).append(": ");
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
diff --git a/chapter13/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter13/src/main/java/com/seaofnodes/simple/IRPrinter.java
index 909dd575..3410ab64 100644
--- a/chapter13/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter13/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -14,7 +14,7 @@ public class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static void _printLine( Node n, StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
@@ -46,7 +46,7 @@ public class IRPrinter {
     // Nodes as referred to as %id
     public static void _printLineLlvmFormat( Node n, StringBuilder sb ) {
         nodeId(sb, n).append(": ");
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
diff --git a/chapter14/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter14/src/main/java/com/seaofnodes/simple/IRPrinter.java
index 909dd575..3410ab64 100644
--- a/chapter14/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter14/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -14,7 +14,7 @@ public class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static void _printLine( Node n, StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
@@ -46,7 +46,7 @@ public class IRPrinter {
     // Nodes as referred to as %id
     public static void _printLineLlvmFormat( Node n, StringBuilder sb ) {
         nodeId(sb, n).append(": ");
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
diff --git a/chapter15/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter15/src/main/java/com/seaofnodes/simple/IRPrinter.java
index 2b4e7ec6..bc1e2741 100644
--- a/chapter15/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter15/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -13,7 +13,7 @@ public class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static void _printLine( Node n, StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
@@ -45,7 +45,7 @@ public class IRPrinter {
     // Nodes as referred to as %id
     public static void _printLineLlvmFormat( Node n, StringBuilder sb ) {
         nodeId(sb, n).append(": ");
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
diff --git a/chapter16/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter16/src/main/java/com/seaofnodes/simple/IRPrinter.java
index 2b4e7ec6..bc1e2741 100644
--- a/chapter16/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter16/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -13,7 +13,7 @@ public class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static void _printLine( Node n, StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
@@ -45,7 +45,7 @@ public class IRPrinter {
     // Nodes as referred to as %id
     public static void _printLineLlvmFormat( Node n, StringBuilder sb ) {
         nodeId(sb, n).append(": ");
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter17/src/main/java/com/seaofnodes/simple/IRPrinter.java
index f8691931..e1e19aae 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -13,7 +13,7 @@ public class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static void _printLine( Node n, StringBuilder sb ) {
         sb.append("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
@@ -45,7 +45,7 @@ public class IRPrinter {
     // Nodes as referred to as %id
     public static void _printLineLlvmFormat( Node n, StringBuilder sb ) {
         nodeId(sb, n).append(": ");
-        if( n._inputs==null ) {
+        if( n.isDead() ) {
             sb.append("DEAD\n");
             return;
         }
diff --git a/chapter18/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter18/src/main/java/com/seaofnodes/simple/IRPrinter.java
index d563e031..bedc7de9 100644
--- a/chapter18/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter18/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -13,7 +13,7 @@ public class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static SB printLine( Node n, SB sb ) {
         sb.p("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null )
+        if( n.isDead() )
             return sb.p("DEAD\n");
         for( Node def : n._inputs )
             sb.p(def==null ? "____" : "%4d".formatted(def._nid))
diff --git a/chapter19/src/main/java/com/seaofnodes/simple/IRPrinter.java b/chapter19/src/main/java/com/seaofnodes/simple/IRPrinter.java
index 957a9184..1b5ade4a 100644
--- a/chapter19/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/chapter19/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -13,7 +13,7 @@ public abstract class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static SB printLine( Node n, SB sb ) {
         sb.p("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null )
+        if( n.isDead() )
             return sb.p("DEAD\n");
         for( Node def : n._inputs )
             sb.p(def==null ? "____" : "%4d".formatted(def._nid))
diff --git a/chapter20/src/main/java/com/seaofnodes/simple/print/IRPrinter.java b/chapter20/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
index a815e536..0e79d54d 100644
--- a/chapter20/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
+++ b/chapter20/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
@@ -17,7 +17,7 @@ public abstract class IRPrinter {
     // 1234 sssss 1234 1234 1234 1234 1234 1234 tttttt
     public static SB printLine( Node n, SB sb ) {
         sb.p("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null )
+        if( n.isDead() )
             return sb.p("DEAD\n");
         for( Node def : n._inputs )
             sb.p(def==null ? "____" : "%4d".formatted(def._nid))
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/print/IRPrinter.java b/chapter21/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
index 82090c78..ffa1d742 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
@@ -18,7 +18,7 @@ public abstract class IRPrinter {
     public static SB printLine( Node n, SB sb ) {
         if( n==null ) return sb;
         sb.p("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null )
+        if( n.isDead() )
             return sb.p("DEAD\n");
         for( Node def : n._inputs )
             sb.p(def==null ? "____" : "%4d".formatted(def._nid))
diff --git a/chapter22/src/main/java/com/seaofnodes/simple/print/IRPrinter.java b/chapter22/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
index 82090c78..ffa1d742 100644
--- a/chapter22/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
+++ b/chapter22/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
@@ -18,7 +18,7 @@ public abstract class IRPrinter {
     public static SB printLine( Node n, SB sb ) {
         if( n==null ) return sb;
         sb.p("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null )
+        if( n.isDead() )
             return sb.p("DEAD\n");
         for( Node def : n._inputs )
             sb.p(def==null ? "____" : "%4d".formatted(def._nid))
diff --git a/chapter23/src/main/java/com/seaofnodes/simple/print/IRPrinter.java b/chapter23/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
index 336cf500..046ffb4a 100644
--- a/chapter23/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
+++ b/chapter23/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
@@ -18,7 +18,7 @@ public abstract class IRPrinter {
     public static SB printLine( Node n, SB sb ) {
         if( n==null ) return sb;
         sb.p("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null )
+        if( n.isDead() )
             return sb.p("DEAD\n");
         for( Node def : n._inputs )
             sb.p(def==null ? "____" : "%4d".formatted(def._nid))
diff --git a/chapter24/src/main/java/com/seaofnodes/simple/print/IRPrinter.java b/chapter24/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
index 336cf500..046ffb4a 100644
--- a/chapter24/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
+++ b/chapter24/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
@@ -18,7 +18,7 @@ public abstract class IRPrinter {
     public static SB printLine( Node n, SB sb ) {
         if( n==null ) return sb;
         sb.p("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null )
+        if( n.isDead() )
             return sb.p("DEAD\n");
         for( Node def : n._inputs )
             sb.p(def==null ? "____" : "%4d".formatted(def._nid))
diff --git a/chapter25/src/main/java/com/seaofnodes/simple/print/IRPrinter.java b/chapter25/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
index 8b1c1e2f..f278be70 100644
--- a/chapter25/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
+++ b/chapter25/src/main/java/com/seaofnodes/simple/print/IRPrinter.java
@@ -16,7 +16,7 @@ public abstract class IRPrinter {
     public static SB printLine( Node n, SB sb ) {
         if( n==null ) return sb;
         sb.p("%4d %-7.7s ".formatted(n._nid,n.label()));
-        if( n._inputs==null )
+        if( n.isDead() )
             return sb.p("DEAD\n");
         for( Node def : n._inputs )
             sb.p(def==null ? "____" : "%4d".formatted(def._nid))
@@ -51,7 +51,7 @@ public abstract class IRPrinter {
         ArrayList<CompUnit> cus = new ArrayList<>(code._compunits.values());
         cus.sort(Comparator.comparing(cu -> cu._cname==null ? "" : cu._cname));
         for( CompUnit cu : cus ) {
-            if( cu._start==null || cu._start._inputs==null ) continue;
+            if( cu._start==null || cu._start.isDead() ) continue;
             sb.nl().p("=== ").p(cu._cname==null ? "" : cu._cname).p(" ===\n");
             printLine(cu._start,sb);
             ArrayList<Node> projs = projections(cu._start);
@@ -59,7 +59,7 @@ public abstract class IRPrinter {
 
             ArrayList<FunNode> funs = new ArrayList<>();
             for( FunNode fun : code._linker )
-                if( fun!=null && fun._inputs!=null && fun._compunit==cu )
+                if( fun!=null && !fun.isDead() && fun._compunit==cu )
                     funs.add(fun);
             funs.sort(Comparator.comparingInt(n -> n._nid));
             for( FunNode fun : funs ) printFunction(fun,sb);
@@ -104,7 +104,7 @@ public abstract class IRPrinter {
                                       IdentityHashMap<Node,Boolean> owned,
                                       IdentityHashMap<Node,Boolean> emitted,
                                       SB sb) {
-        if( n._inputs==null ) return;
+        if( n.isDead() ) return;
         for( Node def : n._inputs ) {
             if( def==null || def instanceof CFGNode || def instanceof ConstantNode ||
                 def instanceof PhiNode || isMultiChild(def) ||
@@ -121,7 +121,7 @@ public abstract class IRPrinter {
     private static void functionPost(Node n, FunNode owner,
                                      IdentityHashMap<Node,Boolean> visit,
                                      ArrayList<Node> post) {
-        if( n==null || n._inputs==null || visit.put(n,Boolean.TRUE)!=null ) return;
+        if( n==null || n.isDead() || visit.put(n,Boolean.TRUE)!=null ) return;
         if( n instanceof FunNode fun && fun!=owner ) return; // Linked callee
         if( n instanceof ParmNode &&
             input0(n)!=owner ) return;                       // Callee parameter
@@ -134,7 +134,7 @@ public abstract class IRPrinter {
         // are ownership/linkage hooks (StopCU, CallEnds, FunPtrs), never CFG
         // continuation in the function being printed.
         if( !(n instanceof ReturnNode) ) for( Node use : n._outputs ) {
-            if( use==null || use._inputs==null ) continue;
+            if( use==null || use.isDead() ) continue;
             if( n instanceof CallNode && use instanceof FunNode ) continue;
             if( use instanceof FunNode fun && fun!=owner ) continue;
             if( use instanceof ParmNode &&
@@ -161,7 +161,7 @@ public abstract class IRPrinter {
     private static ArrayList<Node> projections(Node multi) {
         ArrayList<Node> ps = new ArrayList<>();
         for( Node use : multi._outputs )
-            if( use!=null && use._inputs!=null && isMultiChild(use) ) ps.add(use);
+            if( use!=null && !use.isDead() && isMultiChild(use) ) ps.add(use);
         ps.sort(Comparator.comparingInt(IRPrinter::sortOrder));
         return ps;
     }
@@ -182,7 +182,7 @@ public abstract class IRPrinter {
     // Raw, bounds-safe graph inspection for the debugger.  In particular do
     // not call an accessor which might assert, sharpen, cache, or lazily kill.
     private static Node input0(Node n) {
-        return n==null || n._inputs==null || n._inputs.isEmpty()
+        return n==null || n._inputs.isEmpty()
             ? null : n._inputs.at(0);
     }
 
diff --git a/docs/chapter-backports.md b/docs/chapter-backports.md
index 7151dc4d..40717e5c 100644
--- a/docs/chapter-backports.md
+++ b/docs/chapter-backports.md
@@ -131,6 +131,25 @@ Historical results below are dated evidence, not a substitute for a fresh
 baseline. Logs live in ignored build directories and may no longer exist.
 Reusable implementation lessons are in `skills/chapter25-codex-notes.md`.
 
+### Issue #251: dead-node printing corrected locally, 2026-09-19
+
+[Issue #251](https://github.com/SeaOfNodes/Simple/issues/251) identifies dead-node
+checks against a null `_inputs` array. Constructors and clones allocate the
+array; `kill()` empties it without setting it to null. Line printers now use
+`isDead()` throughout 7-25, including both columnar and LLVM formats in 10-17.
+Chapter 25's whole-program printer uses the same predicate to omit dead
+functions and nodes. Its bounds-safe input accessor simply checks the input
+count. Printing does not prune the linker.
+
+`Chapter07Test.testDeadNodePrinting` in every snapshot 7-25 checks retained input
+storage, the `DEAD` marker, and live inputless nodes. Chapter 25 also tests
+omitting a dead function while preserving its linker entry. The regressions
+fail against isolated original printers in 7, 10, 18, and 25. Full `make tests`
+passed with assertions enabled in every affected snapshot, including 447 tests
+in 25. The initial Chapter 17 test assumed an inputless Start; its setup was
+corrected for that chapter's representation and its full target rerun.
+Logs: `chapter25/build/issue251-review/{original,fixed,chapter17-fixed}.log`.
+
 ### Issue #247: emulator 64-bit stores corrected locally, 2026-09-19
 
 [Issue #247](https://github.com/SeaOfNodes/Simple/issues/247) identifies `st8`
diff --git a/skills/chapter25-codex-notes.md b/skills/chapter25-codex-notes.md
index ab16e8ee..0bb324ac 100644
--- a/skills/chapter25-codex-notes.md
+++ b/skills/chapter25-codex-notes.md
@@ -216,6 +216,10 @@ Consequences:
 
 ## Side-effect-free diagnostics: B09 lessons
 
+- `Node._inputs` remains allocated after `kill()`. Use `isDead()` when printing
+  or filtering dead nodes; an empty input array alone does not imply death.
+  Bounds-safe edge inspection only needs null-node and input-count checks.
+
 - Type construction and interning are allowed during printing. Do not add
   alternate linker-key comparisons or other machinery merely to avoid interning;
   canonical types and fast identity comparisons are normal type-system operations.