Chapter 17: Syntax Sugar

Co-authored-by: Tamás Hadházy <85063808+Hels15@users.noreply.github.com>
Co-authored-by: tannal <tannal2409@gmail.com>
Co-authored-by: hadha <helsgithub15@gmail.com>
Co-authored-by: RobertObkircher <robert.obkircher@gmail.com>

32 files changed, 859 insertions, 489 deletions

diff --git a/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java b/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
index 29841266..9698abd9 100644
--- a/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
+++ b/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java
@@ -16,43 +16,6 @@ public abstract class GlobalCodeMotion {
         schedLate( stop);
     }
 
-    // ------------------------------------------------------------------------
-    // Backwards walk on the CFG only, looking for unreachable code - which has
-    // to be an infinite loop.  Insert a bogus never-taken exit to Stop, so the
-    // loop becomes reachable.  Also, set loop nesting depth
-    public static void fixLoops(StopNode stop) {
-        // Backwards walk from Stop, looking for unreachable code
-        BitSet visit = new BitSet();
-        HashSet<CFGNode> unreach = new HashSet<>();
-        unreach.add(Parser.START);
-        for( Node ret : stop._inputs )
-            ((ReturnNode)ret).walkUnreach(visit,unreach);
-        if( unreach.isEmpty() ) return;
-
-        // Forwards walk from unreachable, looking for loops with no exit test.
-        visit.clear();
-        for( CFGNode cfg : unreach )
-            walkInfinite(cfg,visit,stop);
-        // Set loop depth on remaining graph
-        unreach.clear();
-        visit.clear();
-        for( Node ret : stop._inputs )
-            ((ReturnNode)ret).walkUnreach(visit,unreach);
-        assert unreach.isEmpty();
-    }
-
-
-    // Forwards walk over previously unreachable, looking for loops with no
-    // exit test.
-    static private void walkInfinite( CFGNode n, BitSet visit, StopNode stop ) {
-        if( visit.get(n._nid) ) return; // Been there, done that
-        visit.set(n._nid);
-        if( n instanceof LoopNode loop )
-            loop.forceExit(stop);
-        for( Node use : n._outputs )
-            if( use instanceof CFGNode cfg )
-                walkInfinite(cfg,visit,stop);
-    }
 
     // ------------------------------------------------------------------------
     // Visit all nodes in CFG Reverse Post-Order, essentially defs before uses
@@ -66,7 +29,7 @@ public abstract class GlobalCodeMotion {
         // Reverse Post-Order on CFG
         for( int j=rpo.size()-1; j>=0; j-- ) {
             CFGNode cfg = rpo.get(j);
-            cfg.loopDepth();    // Pre-compute loop depth
+            cfg.loopDepth();
             for( Node n : cfg._inputs )
                 _schedEarly(n,visit);
             if( cfg instanceof RegionNode )
@@ -138,14 +101,13 @@ public abstract class GlobalCodeMotion {
 
                 // All uses done?
                 for( Node use : n._outputs )
-                    if( use!=null && late[use._nid]==null && !(use instanceof PhiNode) )
+                    if( use!=null && late[use._nid]==null )
                         continue outer; // Nope, await all uses done
 
                 // Loads need their memory inputs' uses also done
                 if( n instanceof LoadNode ld )
                     for( Node memuse : ld.mem()._outputs )
-                        if( !(memuse instanceof PhiNode) &&
-                            late[memuse._nid]==null &&
+                        if( late[memuse._nid]==null &&
                             // Load-use directly defines memory
                             (memuse._type instanceof TypeMem ||
                              // Load-use indirectly defines memory
@@ -211,7 +173,7 @@ public abstract class GlobalCodeMotion {
 
     // Least loop depth first, then largest idepth
     private static boolean better( CFGNode lca, CFGNode best ) {
-        return lca._loopDepth < best._loopDepth ||
+        return lca.loopDepth() < best.loopDepth() ||
                 (lca.idepth() > best.idepth() || best instanceof IfNode);
     }
 
@@ -251,9 +213,9 @@ public abstract class GlobalCodeMotion {
     //
     private static CFGNode anti_dep( LoadNode load, CFGNode stblk, CFGNode defblk, CFGNode lca, Node st ) {
         // Walk store blocks "reach" from its scheduled location to its earliest
-        for( ; stblk != defblk; stblk = stblk.idom() ) {
+        for( ; stblk != defblk.idom(); stblk = stblk.idom() ) {
             // Store and Load overlap, need anti-dependence
-            if( !(stblk instanceof IfNode) && stblk._anti==load._nid ) {
+            if( stblk._anti==load._nid ) {
                 lca = stblk._idom(lca,null); // Raise Loads LCA
                 if( lca == stblk && st != null && st._inputs.find(load) == -1 ) // And if something moved,
                     st.addDef(load);   // Add anti-dep as well
diff --git a/src/main/java/com/seaofnodes/simple/IRPrinter.java b/src/main/java/com/seaofnodes/simple/IRPrinter.java
index 418cda29..7ef55a3f 100644
--- a/src/main/java/com/seaofnodes/simple/IRPrinter.java
+++ b/src/main/java/com/seaofnodes/simple/IRPrinter.java
@@ -216,10 +216,11 @@ public class IRPrinter {
             // Print block header
             sb.append("%-13.13s".formatted(label(blk)+":"));
             sb.append( "     ".repeat(4) ).append(" [[  ");
-            if( blk instanceof RegionNode || blk instanceof StopNode )
+            if( blk instanceof StartNode ) ;
+            else if( blk instanceof RegionNode || blk instanceof StopNode )
                 for( int i=(blk instanceof StopNode ? 0 : 1); i<blk.nIns(); i++ )
                     label(sb,blk.cfg(i));
-            else if( !(blk instanceof StartNode) )
+            else
                 label(sb,blk.cfg(0));
             sb.append(" ]]  \n");
 
diff --git a/src/main/java/com/seaofnodes/simple/IterPeeps.java b/src/main/java/com/seaofnodes/simple/IterPeeps.java
index 96ae350e..36f053f9 100644
--- a/src/main/java/com/seaofnodes/simple/IterPeeps.java
+++ b/src/main/java/com/seaofnodes/simple/IterPeeps.java
@@ -2,23 +2,23 @@ package com.seaofnodes.simple;
 
 import com.seaofnodes.simple.node.*;
 
-import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.BitSet;
 import java.util.Random;
 
 /**
- * The IterOptim runs after parsing. It iterates the peepholes to a fixed point
+ * The IterPeeps runs after parsing. It iterates the peepholes to a fixed point
  * so that no more peepholes apply.  This should be linear because peepholes rarely
  * (never?)  increase code size.  The graph should monotonically reduce in some
  * dimension, which is usually size.  It might also reduce in e.g. number of
  * MulNodes or Load/Store nodes, swapping out more "expensive" Nodes for cheaper
  * ones.
- *
+ * <br>
  * The theoretical overall worklist is mindless just grabbing the next thing and
- * doing it.  If the graph changes, put the neighbors on the worklist.  Lather,
- * Rinse, Repeat until the worklist runs dry.
- *
+ * doing it.  If the graph changes, put the neighbors on the worklist.
+ * <br>
+ * Lather, Rinse, Repeat until the worklist runs dry.
+ * <p>
  * The main issues we have to deal with:
  *
  * <ul>
@@ -28,7 +28,7 @@ import java.util.Random;
  *   is stable and correct between peepholes.  In our case `Node.subsume` does
  *   most of the munging, building on our prior stable Node utilities.</li>
  *
- * <li>Changing a Node also changes the graph "neighborhood".  The neigbors need to
+ * <li>Changing a Node also changes the graph "neighborhood".  The neighbors need to
  *   be checked to see if THEY can also peephole, and so on.  After any peephole
  *   or graph update we put a Nodes uses and defs on the worklist.</li>
  *
@@ -38,7 +38,7 @@ import java.util.Random;
  *   check is linear at each peephole step... so quadratic overall.  Its a useful
  *   assert, but one we can disable once the overall algorithm is stable - and
  *   then turn it back on again when some new set of peepholes is misbehaving.
- *   The code for this is turned on in `IterOptim.iterate` as `assert
+ *   The code for this is turned on in `IterPeeps.iterate` as `assert
  *   progressOnList(stop);`</li>
  * </ul>
  */
@@ -113,9 +113,12 @@ public abstract class IterPeeps {
         MID_ASSERT = true;
         int old_cnt = Node.ITER_CNT, old_nop = Node.ITER_NOP_CNT;
         Node changed = stop.walk( n -> {
-                if( WORK.on(n) ) return null;
-                Node m = n.peepholeOpt();
-                if( m==null ) return null;
+                Node m = n;
+                if( n.compute().isa(n._type) && (!n.iskeep() || n._nid<=5) ) { // Types must be forwards, even if on worklist
+                    if( WORK.on(n) ) return null;
+                    m = n.peepholeOpt();
+                    if( m==null ) return null;
+                }
                 System.err.println("BREAK HERE FOR BUG");
                 return m;
             });
diff --git a/src/main/java/com/seaofnodes/simple/Parser.java b/src/main/java/com/seaofnodes/simple/Parser.java
index a8064e17..adc47edc 100644
--- a/src/main/java/com/seaofnodes/simple/Parser.java
+++ b/src/main/java/com/seaofnodes/simple/Parser.java
@@ -2,7 +2,6 @@ package com.seaofnodes.simple;
 
 import com.seaofnodes.simple.node.*;
 import com.seaofnodes.simple.type.*;
-import static com.seaofnodes.simple.Utils.TODO;
 
 import java.util.*;
 
@@ -91,12 +90,12 @@ public class Parser {
     public final Stack<ScopeNode> _xScopes = new Stack<>();
 
     ScopeNode _continueScope;
-    ScopeNode _breakScope;
+    ScopeNode _breakScope;      // Merge all the while-breaks here
 
     // Mapping from a type name to a Type.  The string name matches
     // `type.str()` call.  No TypeMemPtrs are in here, because Simple does not
     // have C-style '*ptr' references.
-    public final HashMap<String, Type> TYPES;
+    public static HashMap<String, Type> TYPES;
 
     // Mapping from a type name to the constructor for a Type.
     public final HashMap<String, StructNode> INITS;
@@ -141,6 +140,8 @@ public class Parser {
             put("u16" ,TypeInteger.U16);
             put("u32" ,TypeInteger.U32);
             put("u8"  ,TypeInteger.U8 );
+            put("val" ,Type.TOP);    // Marker type, indicates type inference
+            put("var" ,Type.BOTTOM); // Marker type, indicates type inference
         }};
     }
 
@@ -148,7 +149,7 @@ public class Parser {
     public static Node find(int nid) { return START.find(nid); }
 
     private Node ctrl() { return _scope.ctrl(); }
-    private Node ctrl(Node n) { return _scope.ctrl(n); }
+    private <N extends Node> N ctrl(N n) { return _scope.ctrl(n); }
 
     public StopNode parse() { return parse(false); }
     public StopNode parse(boolean show) {
@@ -163,11 +164,11 @@ public class Parser {
         _scope.define(ScopeNode.ARG0, TypeInteger.BOT, false, new  ProjNode(START, 2, ScopeNode.ARG0).peephole());
 
         // Parse whole program
-        parseBlock();
+        parseBlock(false);
 
         if( ctrl()._type==Type.CONTROL )
             STOP.addReturn(new ReturnNode(ctrl(), ZERO, _scope).peephole());
-        _scope.pop();
+        _scope.kill();
         _xScopes.pop();
         for( StructNode init : INITS.values() )
             init.unkeep().kill();
@@ -187,9 +188,9 @@ public class Parser {
      * Does not parse the opening or closing '{}'
      * @return a {@link Node} or {@code null}
      */
-    private Node parseBlock() {
+    private Node parseBlock(boolean inCon) {
         // Enter a new scope
-        _scope.push();
+        _scope.push(inCon);
         while (!peek('}') && !_lexer.isEOF())
             parseStatement();
         // Exit scope
@@ -208,18 +209,19 @@ public class Parser {
     private Node parseStatement() {
         if( false ) return null;
         else if (matchx("return")  ) return parseReturn();
-        else if (match ("{")       ) return require(parseBlock(),"}");
+        else if (match ("{")       ) return require(parseBlock(false),"}");
         else if (matchx("if")      ) return parseIf();
         else if (matchx("while")   ) return parseWhile();
+        else if (matchx("for")     ) return parseFor();
         else if (matchx("break")   ) return parseBreak();
         else if (matchx("continue")) return parseContinue();
         else if (matchx("struct")  ) return parseStruct();
         else if (matchx("#showGraph")) return require(showGraph(),";");
         else if (matchx(";")       ) return null; // Empty statement
-        // declarations of vars with struct type are handled in parseExpressionStatement due
-        // to ambiguity
-        else return parseExpressionStatement();
+        // Declaration or normal assignment/expression
+        else return parseDeclarationStatement();
     }
+
     /**
      * Parses a while statement
      *
@@ -229,11 +231,40 @@ public class Parser {
      * @return a {@link Node}, never {@code null}
      */
     private Node parseWhile() {
+        require("(");
+        return parseLooping(false);
+    }
 
-        var savedContinueScope = _continueScope;
-        var savedBreakScope    = _breakScope;
 
+    /**
+     * Parses a for statement
+     *
+     * <pre>
+     *     for( var x=init; test; incr ) body
+     * </pre>
+     * @return a {@link Node}, never {@code null}
+     */
+    private Node parseFor() {
+        // {   var x=init,y=init,...;
+        //     while( pred ) {
+        //         body;
+        //         next;
+        //     }
+        // }
         require("(");
+        _scope.push();          // Scope for the index variables
+        if( !match(";") )       // Can be empty init "for(;test;next) body"
+            parseDeclarationStatement(); // Non-empty init
+        Node rez = parseLooping(true);
+        _scope.pop();           // Exit index variable scope
+        return rez;
+    }
+
+    // Shared by `for` and `while`
+    private Node parseLooping( boolean doFor ) {
+
+        var savedContinueScope = _continueScope;
+        var savedBreakScope    = _breakScope;
 
         // Loop region has two control inputs, the first is the entry
         // point, and second is back edge that is set after loop is parsed
@@ -254,19 +285,32 @@ public class Parser {
         _xScopes.push(_scope = _scope.dup(true)); // The true argument triggers creating phis
 
         // Parse predicate
-        var pred = require(parseExpression(), ")");
+        var pred = peek(';') ? con(1) : parseAsgn();
+        require( doFor ? ";" : ")" );
+
         // IfNode takes current control and predicate
-        Node ifNode = new IfNode(ctrl(), pred).peephole();
+        Node ifNode = new IfNode(ctrl(), pred.keep()).peephole();
         // Setup projection nodes
         Node ifT = new CProjNode(ifNode.  keep(), 0, "True" ).peephole().keep();
         Node ifF = new CProjNode(ifNode.unkeep(), 1, "False").peephole();
 
+        // for( ;;next ) body
+        int nextPos = -1, nextEnd = -1;
+        if( doFor ) {
+            // Skip the next expression and parse it later
+            nextPos = pos();
+            skipAsgn();
+            nextEnd = pos();
+            require(")");
+        }
+
         // Clone the body Scope to create the break/exit Scope which accounts for any
         // side effects in the predicate.  The break/exit Scope will be the final
         // scope after the loop, and its control input is the False branch of
         // the loop predicate.  Note that body Scope is still our current scope.
         ctrl(ifF);
         _xScopes.push(_breakScope = _scope.dup());
+        _breakScope.addGuards(ifF,pred,true); // Up-cast predicate
 
         // No continues yet
         _continueScope = null;
@@ -274,7 +318,9 @@ public class Parser {
         // Parse the true side, which corresponds to loop body
         // Our current scope is the body Scope
         ctrl(ifT.unkeep());     // set ctrl token to ifTrue projection
+        _scope.addGuards(ifT,pred.unkeep(),false); // Up-cast predicate
         parseStatement();       // Parse loop body
+        _scope.removeGuards(ifT);
 
         // Merge the loop bottom into other continue statements
         if (_continueScope != null) {
@@ -283,6 +329,16 @@ public class Parser {
             _scope = _continueScope;
         }
 
+        // Now append the next code onto the body code
+        if( doFor ) {
+            int old = pos(nextPos);
+            if( !peek(')') )
+              parseAsgn();
+            if( pos() != nextEnd )
+                throw errorSyntax( "Unexpected code after expression" );
+            pos(old);
+        }
+
         // The true branch loops back, so whatever is current _scope.ctrl gets
         // added to head loop as input.  endLoop() updates the head scope, and
         // goes through all the phis that were created earlier.  For each phi,
@@ -302,7 +358,8 @@ public class Parser {
         // the scope is the exit scope after the exit test.
         _xScopes.pop();
         _xScopes.push(exit);
-        return _scope = exit;
+        _scope = exit;
+        return _scope;
     }
 
     private ScopeNode jumpTo(ScopeNode toScope) {
@@ -319,14 +376,41 @@ public class Parser {
             return cur;
         // toScope is either the break scope, or a scope that was created here
         assert toScope._lexSize.size() <= _breakScope._lexSize.size();
-        toScope.ctrl(toScope.mergeScopes(cur));
+        toScope.ctrl(toScope.mergeScopes(cur).peephole());
         return toScope;
     }
 
     private void checkLoopActive() { if (_breakScope == null) throw Parser.error("No active loop for a break or continue"); }
 
-    private Node parseBreak   () { checkLoopActive(); return (   _breakScope = require(jumpTo(    _breakScope ),";"));  }
     private Node parseContinue() { checkLoopActive(); return (_continueScope = require(jumpTo( _continueScope ),";"));  }
+    private Node parseBreak   () {
+        checkLoopActive();
+        // At the time of the break, and loop-exit conditions are only valid if
+        // they are ALSO valid at the break.  It is the intersection of
+        // conditions here, not the union.
+        _breakScope.removeGuards(_breakScope.ctrl());
+        _breakScope = require(jumpTo(_breakScope ),";");
+        _breakScope.addGuards(_breakScope.ctrl(), null, false);
+        return _breakScope;
+    }
+
+    // Look for an unbalanced `)`, skipping balanced
+    private Type skipAsgn() {
+        int paren=0;
+        while( true )
+            // Next X char handles skipping complex comments
+            switch( _lexer.nextXChar() ) {
+            case Character.MAX_VALUE:
+                throw Utils.TODO();
+            case ')':
+                if( --paren<0 )
+                    return posT(pos()-1); // Leave the `)` behind
+                break;
+            case '(': paren ++; break;
+            default: break;
+            }
+    }
+
 
     /**
      * Parses a statement
@@ -337,9 +421,16 @@ public class Parser {
      * @return a {@link Node}, never {@code null}
      */
     private Node parseIf() {
-        require("(");
         // Parse predicate
-        var pred = require(parseExpression(), ")").keep();
+        require("(");
+        var pred = require(parseAsgn(), ")");
+        return parseTrinary(pred,true,"else");
+    }
+
+    // Parse a conditional expression, merging results.
+    private Node parseTrinary( Node pred, boolean stmt, String fside ) {
+        pred.keep();
+
         // IfNode takes current control and predicate
         Node ifNode = new IfNode(ctrl(), pred).peephole();
         // Setup projection nodes
@@ -353,31 +444,48 @@ public class Parser {
 
         // Parse the true side
         ctrl(ifT.unkeep());     // set ctrl token to ifTrue projection
-        _scope.upcast(ifT,pred,false); // Up-cast predicate
-        parseStatement();       // Parse true-side
+        _scope.addGuards(ifT,pred,false); // Up-cast predicate
+        Node lhs = stmt ? parseStatement() : parseAsgn().keep(); // Parse true-side
+        _scope.removeGuards(ifT);
+
         ScopeNode tScope = _scope;
 
         // Parse the false side
         _scope = fScope;        // Restore scope, then parse else block if any
         ctrl(ifF.unkeep());     // Ctrl token is now set to ifFalse projection
-        _scope.upcast(ifF,pred,true); // Up-cast predicate
-        if (matchx("else")) {
-            parseStatement();
+        // Up-cast predicate, even if not else clause, because predicate can
+        // remain true if the true clause exits: `if( !ptr ) return 0; return ptr.fld;`
+        _scope.addGuards(ifF,pred,true);
+        boolean doRHS = match(fside);
+        Node rhs = doRHS
+            ? (stmt ? parseStatement() : parseAsgn())
+            : (stmt ? null             : con(lhs._type.makeZero()));
+        _scope.removeGuards(ifF);
+        if( doRHS )
             fScope = _scope;
-        }
         pred.unkeep();
 
+        // Check for `if(pred) int x=17;`
         if( tScope.nIns() != ndefs || fScope.nIns() != ndefs )
             throw error("Cannot define a new name on one arm of an if");
 
+        // Check the trinary widening int/flt
+        if( !stmt ) {
+            rhs = widenInt( rhs, lhs._type ).keep();
+            lhs = widenInt( lhs.unkeep(), rhs._type ).keep();
+            rhs.unkeep();
+        }
+
         // Merge results
         _scope = tScope;
         _xScopes.pop();       // Discard pushed from graph display
 
-        return ctrl(tScope.mergeScopes(fScope));
+        RegionNode r = ctrl(tScope.mergeScopes(fScope));
+        Node ret = stmt ? r : peep(new PhiNode("",lhs._type.meet(rhs._type),r,lhs.unkeep(),rhs));
+        r.peephole();
+        return ret;
     }
 
-
     /**
      * Parses a return statement; "return" already parsed.
      * The $ctrl edge is killed.
@@ -388,7 +496,7 @@ public class Parser {
      * @return an expression {@link Node}, never {@code null}
      */
     private Node parseReturn() {
-        var expr = require(parseExpression(), ";");
+        var expr = require(parseAsgn(), ";");
         Node ret = STOP.addReturn(new ReturnNode(ctrl(), expr, _scope).peephole());
         ctrl(XCTRL);            // Kill control
         return ret;
@@ -403,101 +511,120 @@ public class Parser {
         return null;
     }
 
-    /** Parse: name [=expr]
+    /** Parse: [name '='] expr
      */
-    private Node parseAsgn(Type t, boolean xfinal) {
-        boolean isDecl = t!=null; // Having a type is a declaration, missing one is updating a prior name
-        int old = _lexer._position;
-        String name = requireId();
-        Node expr;
-        if( peek(';') || peek(',') ) {
-            // Bare "name" is not allowed
-            if( t==null ) throw errorSyntax("expression");
-            // Does this declaration need an initializer and not getting one?
-            Type init = xfinal || t instanceof TypeMemPtr tmp && !tmp._nil
-                // Must be initialized in the "new" constructor
-                ? Type.TOP
-                // Else takes the default
-                : t.makeInit();
-            expr = con(init);
-
-        } else if( !match("=") ) {     // Something else
-            _lexer._position = old;
-            return null;
-        } else
-            expr = parseExpression();
-
-        // Defining a new variable vs updating an old one
-        if( !isDecl ) { // Assigning over an existing name
-            // Lookup
-            ScopeMinNode.Var def = _scope.lookup(name);
-            if( def==null )
-                throw error("Undefined name '" + name + "'");
-            // TOP fields are for late-initialized fields; these have never
-            // been written to, and this must be the final write.  Other writes
-            // need to check the final bit.
-            if( _scope.in(def._idx)._type!=Type.TOP && def._final )
-                throw error("Cannot reassign final '"+name+"'");
-            t = def._type; // Declared field type
-        }
+    private Node parseAsgn() {
+        int old = pos();
+        String name = _lexer.matchId();
+        // Just a plain expression, no assignment.
+        // Distinguish `var==expr` from `var=expr`
+        if( name==null || KEYWORDS.contains(name) || !matchOpx('=','=') )
+            {  pos(old);  return parseExpression();  }
+
+        // Parse assignment expression
+        Node expr = parseAsgn();
+
+        // Final variable to update
+        ScopeMinNode.Var def = _scope.lookup(name);
+        if( def==null )
+            throw error("Undefined name '" + name + "'");
+
+        // TOP fields are for late-initialized fields; these have never
+        // been written to, and this must be the final write.  Other writes
+        // outside the constructor need to check the final bit.
+        if( _scope.in(def._idx)._type!=Type.TOP && def._final && !_scope.inCon() )
+            throw error("Cannot reassign final '"+name+"'");
+
+        // Lift expression, based on type
+        Node lift = liftExpr(expr, def.type(), def._final);
+        // Update
+        _scope.update(name,lift);
+        // Return un-lifted expr
+        return expr;
+    }
 
+    // Make finals deep; widen ints to floats; narrow wide int types.
+    // Early error if types do not match variable.
+    private Node liftExpr( Node expr, Type t, boolean xfinal ) {
+        assert !(expr._type instanceof TypeMemPtr tmp) || !tmp.isFRef();
+        // Final is deep on ptrs
+        if( xfinal && t instanceof TypeMemPtr tmp ) {
+            t = tmp.makeRO();
+            expr = peep(new ReadOnlyNode(expr));
+        }
         // Auto-widen int to float
-        if( expr._type instanceof TypeInteger && t instanceof TypeFloat )
-            expr = new ToFloatNode(expr).peephole();
+        expr = widenInt( expr, t );
         // Auto-narrow wide ints to narrow ints
         expr = zsMask(expr,t);
-        // Auto-deepen forward ref types
-        Type e = expr._type;
-        if( e instanceof TypeMemPtr tmp && tmp._obj._fields==null )
-            e = TYPES.get(tmp._obj._name);
         // Type is sane
-        if( !e.isa(t) )
-            throw error("Type " + e.str() + " is not of declared type " + t.str());
-
-        if( isDecl ) {
-            if( !_scope.define(name,t,xfinal,expr) )
-                throw error("Redefining name '" + name + "'");
-        } else
-            _scope.update(name,expr);
+        if( !expr._type.isa(t) )
+            throw error("Type " + expr._type.str() + " is not of declared type " + t.str());
         return expr;
     }
 
-    /** Parse final: [!]asgn
-     */
-    private Node parseFinal(Type t) {
-        boolean xfinal = t!=null && match("!");
-        return parseAsgn(t,xfinal);
+    private Node widenInt( Node expr, Type t ) {
+        return expr._type instanceof TypeInteger && t instanceof TypeFloat
+            ? peep(new ToFloatNode(expr)) : expr;
     }
 
     /**
-     * Parses an expression statement or a declaration statement where type is a struct
-     *
-     * <pre>
-     * type decl [, decl]*;  // Define many names with the same type
-     * asgn;                 // Assign or load a variable
-     * expr;                 // Something else
-     * name;                 // Bare name is an error
-
-     * </pre>
-     * @return an expression {@link Node}, never {@code null}
+     * Parse declaration or expression statement
+     * declStmt = type var['=' exprAsgn][, var['=' exprAsgn]]* ';' | exprAsgn ';'
+     * <p>
+     * exprAsgn = var '=' exprAsgn | expr
      */
-    private Node parseExpressionStatement() {
-        Node n;
+    private Node parseDeclarationStatement() {
         Type t = type();
-        if( t != null ) {
-            // now parse final [, final]*
-            n = parseFinal(t);
-            while( match(",") )
-                n = parseFinal(t);
-
-            // Parse "asgn;" which is just "name = expr;"
-        } else if( ( n = parseAsgn(null,false)) == null ) {
-            // Something else
-            n = parseExpression();
-        }
+        if( t == null )
+            return require(parseAsgn(),";");
+
+        // now parse var['=' asgnexpr] in a loop
+        Node n = parseDeclaration(t);
+        while( match(",") )
+            n = parseDeclaration(t);
         return require(n,";");
     }
 
+    /** Parse final: [!]var['=' asgn]
+     */
+    private Node parseDeclaration(Type t) {
+        assert t!=null;
+        // Has var/val instead of a user-declared type
+        boolean inferType = t==Type.TOP || t==Type.BOTTOM;
+        boolean hasBang = match("!");
+        String name = requireId();
+
+        // Optional initializing expression follows
+        boolean xfinal = false;
+        Node expr;
+        if( match("=") ) {
+            expr = parseAsgn();
+            // `val` is always final
+            xfinal = (t==Type.TOP) ||
+                // var is always not-final, final if no Bang AND TMP since primitives are not-final by default
+                (t!=Type.BOTTOM && !hasBang && (t instanceof TypeMemPtr));
+            // var/val, then type comes from expression
+            if( inferType )
+                t = expr._type.glb();
+
+        } else {
+            if( inferType && !_scope.inCon() )
+                throw errorSyntax("=expression");
+            expr = con(t.makeInit());
+        }
+
+        // Lift expression, based on type
+        Node lift = liftExpr(expr, t, xfinal);
+        if( xfinal && t instanceof TypeMemPtr tmp )
+            t = tmp.makeRO();
+
+        // Define a new name,
+        if( !_scope.define(name,t,xfinal,lift) )
+            throw error("Redefining name '" + name + "'");
+        return null;
+    }
+
+
 
     /**
      * Parse a struct declaration, and return the following statement.
@@ -510,11 +637,12 @@ public class Parser {
         if (_xScopes.size() > 1) throw errorSyntax("struct declarations can only appear in top level scope");
         String typeName = requireId();
         Type t = TYPES.get(typeName);
-        if( t!=null && !(t instanceof TypeMemPtr tmp && tmp._obj._fields==null) )
+        if( t!=null && !(t instanceof TypeMemPtr tmp && tmp.isFRef() ) )
             throw errorSyntax("struct '" + typeName + "' cannot be redefined");
 
         // A Block scope parse, and inspect the scope afterward for fields.
-        _scope.push();
+        _scope.push(true);
+        _xScopes.push(_scope);
         require("{");
         while (!peek('}') && !_lexer.isEOF())
             parseStatement();
@@ -527,7 +655,7 @@ public class Parser {
         for( int i=lexlen; i<varlen; i++ ) {
             s.addDef(_scope.in(i));
             ScopeMinNode.Var v = _scope._vars.at(i);
-            fs[i-lexlen] = Field.make(v._name,v._type,ALIAS++,v._final);
+            fs[i-lexlen] = Field.make(v._name,v.type(),ALIAS++,v._final);
         }
         TypeStruct ts = s._ts = TypeStruct.make(typeName, fs);
         TYPES.put(typeName, TypeMemPtr.make(ts));
@@ -535,6 +663,7 @@ public class Parser {
         // Done with struct/block scope
         require("}");
         require(";");
+        _xScopes.pop();
         _scope.pop();
         return null;
     }
@@ -544,44 +673,45 @@ public class Parser {
     // 'id' which the caller must parse.  This lets us distinguish forward ref
     // types (which ARE valid here) from local vars in an (optional) forward
     // ref type position.
+
+    // t = int|i8|i16|i32|i64|u8|u16|u32|u64|byte|bool | flt|f32|f64 | val | var | struct[?]
     private Type type() {
-        int old1 = _lexer._position;
+        int old1 = pos();
         String tname = _lexer.matchId();
         if( tname==null ) return null;
         // Convert the type name to a type.
         Type t0 = TYPES.get(tname);
-        Type t1 = t0 == null ? TypeMemPtr.make(TypeStruct.make(tname)) : t0; // Null: assume a forward ref type
+        // No new types as keywords
+        if( t0 == null && KEYWORDS.contains(tname) )
+            return posT(old1);
+        Type t1 = t0 == null ? TypeMemPtr.make(TypeStruct.makeFRef(tname)) : t0; // Null: assume a forward ref type
         // Nest arrays and '?' as needed
+        Type t2 = t1;
         while( true ) {
-            assert !(t1 instanceof TypeStruct);
+            assert !(t2 instanceof TypeStruct);
             if( match("?") ) {
-                if( !(t1 instanceof TypeMemPtr tmp) )
+                if( !(t2 instanceof TypeMemPtr tmp) )
                     throw error("Type "+t0+" cannot be null");
-                if( tmp._nil ) throw error("Type "+t1+" already allows null");
-                t1 = TypeMemPtr.make(tmp._obj,true);
-                continue;
-            }
-            if( match("[]") ) {
-                t1 = typeAry(t1);
-                continue;
-            }
-            break;
+                if( tmp._nil ) throw error("Type "+t2+" already allows null");
+                t2 = TypeMemPtr.make(tmp._obj,true);
+            } else if( match("[]") ) {
+                t2 = typeAry(t2);
+            } else
+                break;
         }
 
         // Check no forward ref
-        if( t0 != null ) return t1;
+        if( t0 != null ) return t2;
         // Check valid forward ref, after parsing all the type extra bits.
         // Cannot check earlier, because cannot find required 'id' until after "[]?" syntax
-        int old2 = _lexer._position;
+        int old2 = pos();
         String id = _lexer.matchId();
-        _lexer._position = old2; // Reset lexer to reparse
-        if( id==null ) {
-            _lexer._position = old1; // Reset lexer to reparse
-            return null;        // Not a type
-        }
+        pos(old2);              // Reset lexer to reparse
+        if( id==null || _scope.lookup(id)!=null )
+            return posT(old1);  // Reset lexer to reparse
         // Yes a forward ref, so declare it
         TYPES.put(tname,t1);
-        return t1;
+        return t2;
     }
 
     // Make an array type of t
@@ -599,16 +729,18 @@ public class Parser {
         return tary;
     }
 
-
     /**
      * Parse an expression of the form:
      *
      * <pre>
-     *     expr : compareExpr
+     *     expr : bitwise [? expr [: expr]]
      * </pre>
      * @return an expression {@link Node}, never {@code null}
      */
-    private Node parseExpression() { return parseBitwise(); }
+    private Node parseExpression() {
+        Node expr = parseBitwise();
+        return match("?") ? parseTrinary(expr,false,":") : expr;
+    }
 
     /**
      * Parse an bitwise expression
@@ -627,7 +759,7 @@ public class Parser {
             else if( match("^") ) lhs = new XorNode(lhs,null);
             else break;
             lhs.setDef(2,parseComparison());
-            lhs = lhs.peephole();
+            lhs = peep(lhs);
         }
         return lhs;
     }
@@ -655,9 +787,9 @@ public class Parser {
             else break;
             // Peepholes can fire, but lhs is already "hooked", kept alive
             lhs.setDef(idx,parseShift());
-            lhs = lhs.widen().peephole();
+            lhs = peep(lhs.widen());
             if( negate )        // Extra negate for !=
-                lhs = new NotNode(lhs).peephole();
+                lhs = peep(new NotNode(lhs));
         }
         return lhs;
     }
@@ -679,7 +811,7 @@ public class Parser {
             else if( match(">>") ) lhs = new SarNode(lhs,null);
             else break;
             lhs.setDef(2,parseAddition());
-            lhs = lhs.widen().peephole();
+            lhs = peep(lhs.widen());
         }
         return lhs;
     }
@@ -700,7 +832,7 @@ public class Parser {
             else if( match("-") ) lhs = new SubNode(lhs,null);
             else break;
             lhs.setDef(2,parseMultiplication());
-            lhs = lhs.widen().peephole();
+            lhs = peep(lhs.widen());
         }
         return lhs;
     }
@@ -721,7 +853,7 @@ public class Parser {
             else if( match("/") ) lhs = new DivNode(lhs,null);
             else break;
             lhs.setDef(2,parseUnary());
-            lhs = lhs.widen().peephole();
+            lhs = peep(lhs.widen());
         }
         return lhs;
     }
@@ -730,13 +862,31 @@ public class Parser {
      * Parse a unary minus expression.
      *
      * <pre>
-     *     unaryExpr : ('-') | '!') unaryExpr | postfixExpr | primaryExpr
+     *     unaryExpr : ('-') unaryExpr | '!') unaryExpr | postfixExpr | primaryExpr | '--' Id | '++' Id
      * </pre>
      * @return a unary expression {@link Node}, never {@code null}
      */
     private Node parseUnary() {
-        if (match("-")) return new MinusNode(parseUnary()).widen().peephole();
-        if (match("!")) return new   NotNode(parseUnary()).peephole();
+        // Pre-dec/pre-inc
+        int old = pos();
+        if( match("--") || match("++") ) {
+            int delta = _lexer.peek(-1)=='+' ? 1 : -1; // Pre vs post
+            String name = _lexer.matchId();
+            if( name!=null ) {
+                ScopeMinNode.Var n = _scope.lookup(name);
+                if( n != null ) {
+                    if( n._final )
+                        throw error("Cannot reassign final '"+n._name+"'");
+                    Node expr = zsMask(peep(new AddNode(_scope.in(n),con(delta))),n.type());
+                    _scope.update(n,expr);
+                    return expr;
+                }
+            }
+            // Reset, try again
+            pos(old);
+        }
+        if (match("-")) return peep(new MinusNode(parseUnary()).widen());
+        if (match("!")) return peep(new   NotNode(parseUnary()));
         return parsePostfix(parsePrimary());
     }
 
@@ -744,23 +894,55 @@ public class Parser {
      * Parse a primary expression:
      *
      * <pre>
-     *     primaryExpr : integerLiteral | Identifier | true | false | null | new Type | '(' expression ')'
+     *     primaryExpr : integerLiteral | true | false | null | new Type | '(' expression ')' | Id['++','--']
      * </pre>
      * @return a primary {@link Node}, never {@code null}
      */
     private Node parsePrimary() {
         if( _lexer.isNumber(_lexer.peek()) ) return parseLiteral();
-        if( match("(") ) return require(parseExpression(), ")");
         if( matchx("true" ) ) return con(1);
         if( matchx("false") ) return ZERO;
         if( matchx("null" ) ) return con(TypeMemPtr.NULLPTR);
+        if( match("(") ) return require(parseAsgn(), ")");
         if( matchx("new"  ) ) return alloc();
         // Expect an identifier now
-        String name = _lexer.matchId();
-        if( name == null) throw errorSyntax("an identifier or expression");
-        ScopeMinNode.Var n = _scope.lookup(name);
-        if( n!=null ) return _scope.in(n);
-        throw error("Undefined name '" + name + "'");
+        ScopeMinNode.Var n = requireLookupId("an identifier or expression");
+        Node rvalue = _scope.in(n);
+        if( rvalue._type == Type.BOTTOM )
+            throw error("Cannot read uninitialized field '"+n._name+"'");
+        // Check for assign-update, x += e0;
+        char ch = _lexer.matchOperAssign();
+        if( ch==0  ) return rvalue;
+        if( n._final )
+            throw error("Cannot reassign final '"+n._name+"'");
+        Node op = switch(ch) {
+        case '+'      -> new AddNode(rvalue,null);
+        case '-'      -> new SubNode(rvalue,null);
+        case '*'      -> new MulNode(rvalue,null);
+        case '/'      -> new DivNode(rvalue,null);
+        case        1 -> new AddNode(rvalue,con( 1));
+        case (char)-1 -> new AddNode(rvalue,con(-1));
+        default -> throw Utils.TODO();
+        };
+        // Return pre-value (x+=1) or post-value (x++)
+        boolean pre = op.in(2)==null;
+        // Parse RHS argument as needed
+        if( pre )
+            { op.keep().setDef(2,parseAsgn());  op.unkeep(); }
+        else rvalue.keep();     // Keep post-value across peeps
+        op = zsMask(peep(op),n.type());
+        _scope.update(n,op);
+        return pre ? op : rvalue.unkeep();
+    }
+
+    ScopeMinNode.Var requireLookupId(String msg) {
+        String id = _lexer.matchId();
+        if( id == null || KEYWORDS.contains(id) )
+            throw errorSyntax(msg);
+        ScopeMinNode.Var n = _scope.lookup(id);
+        if( n==null )
+            throw error("Undefined name '" + id + "'");
+        return n;
     }
 
     /**
@@ -771,7 +953,7 @@ public class Parser {
         if( t==null ) throw error("Expected a type");
         // Parse ary[ length_expr ]
         if( match("[") ) {
-            Node len = parseExpression().keep();
+            Node len = parseAsgn();
             if( !(len._type instanceof TypeInteger) )
                 throw error("Cannot allocate an array with length "+len._type);
             require("]");
@@ -796,15 +978,15 @@ public class Parser {
             // Push a scope, and pre-assign all struct fields.
             _scope.push();
             for( int i=0; i<fs.length; i++ )
-                _scope.define(fs[i]._fname, fs[i]._type, fs[i]._final, s.in(i));
+                _scope.define(fs[i]._fname, fs[i]._type, fs[i]._final, s.in(i)._type==Type.TOP ? con(Type.BOTTOM) : s.in(i));
             // Parse the constructor body
-            parseBlock();
+            parseBlock(true);
             require("}");
             init = _scope._inputs;
         }
         // Check that all fields are initialized
         for( int i=idx; i<init.size(); i++ )
-            if( init.get(i)._type == Type.TOP )
+            if( /*init.at(i)._type == Type.TOP ||*/ init.at(i)._type == Type.BOTTOM )
                 throw error("'"+tmp._obj._name+"' is not fully initialized, field '" + fs[i-idx]._fname + "' needs to be set in a constructor");
         Node ptr = newStruct(tmp._obj, con(tmp._obj.offset(fs.length)), idx, init );
         if( hasConstructor )
@@ -843,8 +1025,8 @@ public class Parser {
     private Node newArray(TypeStruct ary, Node len) {
         int base = ary.aryBase ();
         int scale= ary.aryScale();
-        Node size = new AddNode(con(base),new ShlNode(len,con(scale)).peephole()).peephole();
-        ALTMP.clear();  ALTMP.add(len); ALTMP.add(con(ary._fields[1]._type.makeInit()));
+        Node size = peep(new AddNode(con(base),peep(new ShlNode(len.keep(),con(scale)))));
+        ALTMP.clear();  ALTMP.add(len.unkeep()); ALTMP.add(con(ary._fields[1]._type.makeInit()));
         return newStruct(ary,size,0,ALTMP);
     }
 
@@ -868,7 +1050,7 @@ public class Parser {
      */
     private Node parsePostfix(Node expr) {
         String name = null;
-        if( match(".") )      name = requireId().intern();
+        if( match(".") )      name = requireId();
         else if( match("#") ) name = "#";
         else if( match("[") ) name = "[]";
         else return expr;       // No postfix
@@ -882,44 +1064,66 @@ public class Parser {
         // spoil the user experience with error messages.
         if( ctrl()._type==Type.XCONTROL )
             // Exit out via parsing the trailing expression
-            return matchOpx('=','=') ? parseExpression() : parsePostfix(con(Type.TOP));
+            return matchOpx('=','=') ? parseAsgn() : parsePostfix(con(Type.TOP));
 
         // Sanity check field name for existing
         TypeMemPtr tmp = (TypeMemPtr)TYPES.get(ptr._obj._name);
-        if( tmp == null ) throw error("Accessing unknown field '" + name + "' from '" + ptr.str() + "'");
+        if( tmp == null ) throw error("Accessing unknown field '" + name + "' from '" + ptr + "'");
         TypeStruct base = tmp._obj;
         int fidx = base.find(name);
         if( fidx == -1 ) throw error("Accessing unknown field '" + name + "' from '" + ptr.str() + "'");
 
         // Get field type and layout offset from base type and field index fidx
         Field f = base._fields[fidx];  // Field from field index
+        Type tf = f._type;
+        if( tf instanceof TypeMemPtr ftmp && ftmp.isFRef() )
+            tf = ftmp.makeFrom(((TypeMemPtr)(TYPES.get(ftmp._obj._name)))._obj);
 
-        Node off = name.equals("[]")       // If field is an array body
+        // Field offset; fixed for structs, computed for arrays
+        Node off = (name.equals("[]")       // If field is an array body
             // Array index math
-            ? new AddNode(con(base.aryBase()),new ShlNode(require(parseExpression(),"]"),con(base.aryScale())).peephole()).peephole()
+            ? peep(new AddNode(con(base.aryBase()),peep(new ShlNode(require(parseAsgn(),"]"),con(base.aryScale())))))
             // Struct field offsets are hardwired
-            : con(base.offset(fidx));
+            : con(base.offset(fidx))).keep();
 
         // Disambiguate "obj.fld==x" boolean test from "obj.fld=x" field assignment
         if( matchOpx('=','=') ) {
-            Node val = parseExpression();
-            // Auto-truncate when storing to narrow fields
-            val = zsMask(val,f._type);
-            Node st = new StoreNode(name, f._alias, f._type, memAlias(f._alias), expr, off, val, false);
+            Node val = parseAsgn().keep();
+            Node lift = liftExpr( val, tf, f._final );
+
+            Node st = new StoreNode(name, f._alias, tf, memAlias(f._alias), expr, off.unkeep(), lift, false);
             // Arrays include control, as a proxy for a safety range check.
             // Structs don't need this; they only need a NPE check which is
             // done via the type system.
             if( base.isAry() )  st.setDef(0,ctrl());
             memAlias(f._alias, st.peephole());
-            return val;        // "obj.a = expr" returns the expression while updating memory
+            return val.unkeep();        // "obj.a = expr" returns the expression while updating memory
         }
 
-        Node load = new LoadNode(name, f._alias, f._type.glb(), memAlias(f._alias), expr, off);
+        Node load = new LoadNode(name, f._alias, tf.glb(), memAlias(f._alias), expr.keep(), off);
         // Arrays include control, as a proxy for a safety range check
         // Structs don't need this; they only need a NPE check which is
         // done via the type system.
-        //if( base.isAry() ) load.setDef(0,ctrl());
-        return parsePostfix(load.peephole());
+        if( base.isAry() ) load.setDef(0,ctrl());
+        load = peep(load);
+
+        // ary[idx]++ or ptr.fld++
+        if( matchx("++") || matchx("--") ) {
+            if( f._final && !f._fname.equals("[]") )
+                throw error("Cannot reassign final '"+f._fname+"'");
+            Node inc = peep(new AddNode(load,con( _lexer.peek(-1)=='+' ? 1 : -1)));
+            Node val = zsMask(inc,tf);
+            Node st = new StoreNode(name, f._alias, tf, memAlias(f._alias), expr.unkeep(), off, val, false);
+            // Arrays include control, as a proxy for a safety range check.
+            // Structs don't need this; they only need a NPE check which is
+            // done via the type system.
+            if( base.isAry() )  st.setDef(0,ctrl());
+            memAlias(f._alias, peep(st));
+            // And use the original loaded value as the result
+        } else expr.unkill();
+        off.unkill();
+
+        return parsePostfix(load);
     }
 
 
@@ -930,16 +1134,16 @@ public class Parser {
             if( !(val._type instanceof TypeFloat tval && t instanceof TypeFloat t0 && !tval.isa(t0)) )
                 return val;
             // Float rounding
-            return new RoundF32Node(val).peephole();
+            return peep(new RoundF32Node(val));
         }
         if( t0._min==0 )        // Unsigned
-            return new AndNode(val,con(t0._max)).peephole();
+            return peep(new AndNode(val,con(t0._max)));
         // Signed extension
         int shift = Long.numberOfLeadingZeros(t0._max)-1;
         Node shf = con(shift);
         if( shf._type==TypeInteger.ZERO )
             return val;
-        return new SarNode(new ShlNode(val,shf.keep()).peephole(),shf.unkeep()).peephole();
+        return peep(new SarNode(peep(new ShlNode(val,shf.keep())),shf.unkeep()));
     }
 
 
@@ -954,6 +1158,10 @@ public class Parser {
     private ConstantNode parseLiteral() { return con(_lexer.parseNumber()); }
     public static Node con( long con ) { return con(TypeInteger.constant(con));  }
     public static ConstantNode con( Type t ) { return (ConstantNode)new ConstantNode(t).peephole();  }
+    public Node peep( Node n ) {
+        // Peephole, then improve with lexically scoped guards
+        return _scope.upcastGuard(n.peephole());
+    }
 
     //////////////////////////////////
     // Utilities for lexical analysis
@@ -967,14 +1175,29 @@ public class Parser {
     private boolean peek(char ch) { return _lexer.peek(ch); }
     private boolean peekIsId() { return _lexer.peekIsId(); }
 
+    private int pos() { return _lexer._position; }
+    private int pos(int pos) {
+        int old = _lexer._position;
+        _lexer._position = pos;
+        return old;
+    }
+    private Type posT(int pos) { _lexer._position = pos; return null; }
+
     // Require and return an identifier
     private String requireId() {
         String id = _lexer.matchId();
-        if (id != null && !KEYWORDS.contains(id) ) return id;
+        if (id != null && !KEYWORDS.contains(id) ) return id.intern();
         throw error("Expected an identifier, found '"+id+"'");
     }
 
-
+    private String matchId() {
+        int old = pos();
+        String id = _lexer.matchId();
+        if( id==null ) return null;
+        if( !KEYWORDS.contains(id) ) return id;
+        pos(old);
+        return null;
+    }
 
     // Require an exact match
     private Parser require(String syntax) { require(null, syntax); return this; }
@@ -1031,6 +1254,8 @@ public class Parser {
             return isEOF() ? Character.MAX_VALUE   // Special value that causes parsing to terminate
                     : (char) _input[_position];
         }
+        // Just crash if misused
+        public byte peek(int off) { return _input[_position+off]; }
 
         private char nextChar() {
             char ch = peek();
@@ -1059,6 +1284,8 @@ public class Parser {
             }
         }
 
+        // Next non-white-space character, or EOF
+        public char nextXChar() { skipWhiteSpace(); return nextChar(); }
 
         // Return true, if we find "syntax" after skipping white space; also
         // then advance the cursor past syntax.
@@ -1165,12 +1392,28 @@ public class Parser {
 
         //
         private boolean isPunctuation(char ch) {
-            return "=;[]<>()+-/*".indexOf(ch) != -1;
+            return "=;[]<>()+-/*&|^".indexOf(ch) != -1;
         }
 
         private String parsePunctuation() {
             int start = _position;
             return new String(_input, start, 1);
         }
+
+        // Next oper= character, or 0.
+        // As a convenience, mark "++" as a char 1 and "--" as char -1 (65535)
+        // Disallow e.g. "arg--1" which should parse as "arg - -1"
+        public char matchOperAssign() {
+            skipWhiteSpace();
+            if( _position+2 >= _input.length ) return 0;
+            char ch0 = (char)_input[_position];
+            if( "+-/*&|^".indexOf(ch0) == -1 ) return 0;
+            char ch1 = (char)_input[_position+1];
+            if(               ch1 == '=' ) { _position += 2; return ch0; }
+            if( isIdLetter((char)_input[_position+2]) ) return 0;
+            if( ch0 == '+' && ch1 == '+' ) { _position += 2; return (char) 1; }
+            if( ch0 == '-' && ch1 == '-' ) { _position += 2; return (char)-1; }
+            return 0;
+        }
     }
 }
diff --git a/src/main/java/com/seaofnodes/simple/node/AddNode.java b/src/main/java/com/seaofnodes/simple/node/AddNode.java
index 97d2bb9b..4007df91 100644
--- a/src/main/java/com/seaofnodes/simple/node/AddNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/AddNode.java
@@ -36,7 +36,7 @@ public class AddNode extends Node {
         return TypeInteger.BOT;
     }
 
-    private static boolean overflow( long x, long y ) {
+    static boolean overflow( long x, long y ) {
         if( (x ^    y ) < 0 ) return false; // unequal signs, never overflow
         return (x ^ (x + y)) < 0; // sum has unequal signs, so overflow
     }
@@ -44,6 +44,7 @@ public class AddNode extends Node {
     public Node idealize () {
         Node lhs = in(1);
         Node rhs = in(2);
+        if( rhs.err() != null ) return null;
         Type t2 = rhs._type;
 
         // Add of 0.  We do not check for (0+x) because this will already
diff --git a/src/main/java/com/seaofnodes/simple/node/CFGNode.java b/src/main/java/com/seaofnodes/simple/node/CFGNode.java
index 9e9d6970..052ad8fc 100644
--- a/src/main/java/com/seaofnodes/simple/node/CFGNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/CFGNode.java
@@ -28,15 +28,6 @@ public abstract class CFGNode extends Node {
     // Block head is Start, Region, CProj, but not e.g. If, Return, Stop
     public boolean blockHead() { return false; }
 
-    // Should be exactly 1 tail from a block head
-    public CFGNode blockTail() {
-        assert blockHead();
-        for( Node n : _outputs )
-            if( n instanceof CFGNode cfg )
-                return cfg;
-        return null;
-    }
-
     // ------------------------------------------------------------------------
     /**
      * Immediate dominator tree depth, used to approximate a real IDOM during
@@ -64,27 +55,78 @@ public abstract class CFGNode extends Node {
         return lhs;
     }
 
-    // Loop nesting depth
-    public int _loopDepth;
-    public int loopDepth() { return _loopDepth==0 ? (_loopDepth = cfg(0).loopDepth()) : _loopDepth; }
-
     // Anti-dependence field support
     public int _anti;           // Per-CFG field to help find anti-deps
 
     // ------------------------------------------------------------------------
-    // Support routines for Global Code Motion
+    // Loop nesting
+    public LoopNode loop() { return _ltree._head; }
+    public int loopDepth() { return _ltree==null ? 0 : _ltree.depth(); }
+
+    LoopTree _ltree;
+    int _pre;                   // Pre-order numbers for loop tree finding
+    private static class LoopTree {
+        LoopTree _par;
+        final LoopNode _head;
+        int _depth;
+        LoopTree(LoopNode head) { _head = head; }
+        @Override public String toString() { return "LOOP"+_head._nid; }
+        int depth() {
+            return _depth==0 ? (_par==null ? 0 : (_depth = _par.depth()+1)) : _depth;
+        }
+    }
 
-    // Tik-tok recursion pattern.  This method is final, and every caller does
-    // this work.
-    public final void walkUnreach( BitSet visit, HashSet<CFGNode> unreach ) {
-        if( visit.get(_nid) ) return;
-        visit.set(_nid);
-        _walkUnreach(visit,unreach);
-        unreach.remove(this); // Since we reached here... Node was not unreachable
+    // ------------------------------------------------------------------------
+    // Tag all CFG Nodes with their containing LoopNode; LoopNodes themselves
+    // also refer to *their* containing LoopNode, as well as have their depth.
+    // Start is a LoopNode which contains all at depth 1.
+    public void buildLoopTree(StopNode stop) {
+        _ltree = stop._ltree = Parser.XCTRL._ltree = new LoopTree((StartNode)this);
+        _bltWalk(2,stop, new BitSet());
     }
+    private int _bltWalk(int pre, StopNode stop, BitSet post) {
+        // Pre-walked?
+        if( _pre!=0 ) return pre;
+        _pre = pre++;
+        // Pre-walk
+        for( Node use : _outputs )
+            if( use instanceof CFGNode usecfg )
+                pre = usecfg._bltWalk(pre,stop,post);
 
-    // Tik-tok recursion pattern; not-final; callers override this.
-    void _walkUnreach( BitSet visit, HashSet<CFGNode> unreach ) {
-        cfg(0).walkUnreach(visit,unreach);
+        // Post-order work: find innermost loop
+        LoopTree inner = null, ltree;
+        for( Node use : _outputs ) {
+            if( !(use instanceof CFGNode usecfg) ) continue;
+            // Child visited but not post-visited?
+            if( !post.get(usecfg._nid) ) {
+                // Must be a backedge to a LoopNode then
+                ltree = usecfg._ltree = new LoopTree((LoopNode)usecfg);
+            } else {
+                // Take child's loop choice, which must exist
+                ltree = usecfg._ltree;
+                // If falling into a loop, use the target loop's parent instead
+                if( ltree._head == usecfg ) {
+                    if( ltree._par == null )
+                        // This loop never had an If test choose to take its
+                        // exit, i.e. it is a no-exit infinite loop.
+                        ltree._par = ltree._head.forceExit(stop)._ltree;
+                    ltree = ltree._par;
+                }
+            }
+            // Sort inner loops.  The decision point is some branch far removed
+            // from either loop head OR either backedge so requires pre-order
+            // numbers to figure out innermost.
+            if( inner == null ) { inner = ltree; continue; }
+            if( inner == ltree ) continue; // No change
+            LoopTree outer = ltree._head._pre > inner._head._pre ? inner : ltree;
+            inner =          ltree._head._pre > inner._head._pre ? ltree : inner;
+            inner._par = outer;
+        }
+        // Set selected loop
+        if( inner!=null )
+            _ltree = inner;
+        // Tag as post-walked
+        post.set(_nid);
+        return pre;
     }
 }
diff --git a/src/main/java/com/seaofnodes/simple/node/CProjNode.java b/src/main/java/com/seaofnodes/simple/node/CProjNode.java
index 3d6fd1fa..d2f1bbe1 100644
--- a/src/main/java/com/seaofnodes/simple/node/CProjNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/CProjNode.java
@@ -52,10 +52,6 @@ public class CProjNode extends CFGNode {
     }
 
     @Override
-    public Node getBlockStart() {
-        return ctrl() instanceof IfNode ? this : ctrl().getBlockStart();
-    }
-    @Override
     boolean eq( Node n ) { return _idx == ((CProjNode)n)._idx; }
 
     @Override
diff --git a/src/main/java/com/seaofnodes/simple/node/IfNode.java b/src/main/java/com/seaofnodes/simple/node/IfNode.java
index 3cce89ab..513b4721 100644
--- a/src/main/java/com/seaofnodes/simple/node/IfNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/IfNode.java
@@ -43,6 +43,9 @@ public class IfNode extends CFGNode implements MultiNode {
         // Else true branch is reachable
         if( t.isConstant() )
             return t.makeInit()==t ? TypeTuple.IF_FALSE : TypeTuple.IF_TRUE;
+        // Integers allow non-zero ranges: "1-65535"
+        if( t.makeZero().meet(t) != t )
+            return TypeTuple.IF_TRUE;
 
         return TypeTuple.IF_BOTH;
     }
@@ -60,12 +63,4 @@ public class IfNode extends CFGNode implements MultiNode {
         return null;
     }
 
-    @Override void _walkUnreach( BitSet visit, HashSet<CFGNode> unreach ) {
-        for( Node proj : _outputs )
-            if( ((CProjNode)proj)._loopDepth == 0 )
-                unreach.add((CProjNode)proj);
-        super._walkUnreach(visit,unreach);
-    }
-
-    @Override public Node getBlockStart() { return ctrl().getBlockStart(); }
 }
diff --git a/src/main/java/com/seaofnodes/simple/node/LoadNode.java b/src/main/java/com/seaofnodes/simple/node/LoadNode.java
index d15d4f83..1528f2e9 100644
--- a/src/main/java/com/seaofnodes/simple/node/LoadNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/LoadNode.java
@@ -33,10 +33,13 @@ public class LoadNode extends MemOpNode {
 
     @Override
     public Type compute() {
-        if( mem()._type instanceof TypeMem mem &&
-            // No constant folding if ptr might null-check
-            _declaredType != mem._t && err()==null ) {
-            return  _declaredType.join(mem._t);
+        if( mem()._type instanceof TypeMem mem ) {
+            // Update declared forward ref to the actual
+            if( _declaredType.isFRef() && mem._t instanceof TypeMemPtr tmp && !tmp.isFRef() )
+                _declaredType = tmp;
+            // No lifting if ptr might null-check
+            if( err()==null )
+                return _declaredType.join(mem._t);
         }
         return _declaredType;
     }
@@ -60,13 +63,15 @@ public class LoadNode extends MemOpNode {
         // Load-after-Store on same address, but bypassing provably unrelated
         // stores.  This is a more complex superset of the above two peeps.
         // "Provably unrelated" is really weak.
+        if( ptr instanceof ReadOnlyNode ro )
+            ptr = ro.in(1);
         Node mem = mem();
         outer:
         while( true ) {
             switch( mem ) {
             case StoreNode st:
                 if( ptr == st.ptr() && off() == st.off() )
-                    return st.val(); // Proved equal
+                    return castRO(st.val()); // Proved equal
                 // Can we prove unequal?  Offsets do not overlap?
                 if( !off()._type.join(st.off()._type).isHigh() && // Offsets overlap
                     !neverAlias(ptr,st.ptr()) )                   // And might alias
@@ -79,11 +84,13 @@ public class LoadNode extends MemOpNode {
             case ProjNode mproj:
                 if( mproj.in(0) instanceof NewNode nnn1 ) {
                     if( ptr instanceof ProjNode pproj && pproj.in(0) == mproj.in(0) )
-                        return nnn1.in(nnn1.findAlias(_alias)); // Load from New init
+                        return castRO(nnn1.in(nnn1.findAlias(_alias))); // Load from New init
                     if( !(ptr instanceof ProjNode pproj && pproj.in(0) instanceof NewNode nnn2) )
                         break outer; // Cannot tell, ptr not related to New
                     mem = nnn1.in(_alias);// Bypass unrelated New
                     break;
+                } else if( mproj.in(0) instanceof StartNode ) {
+                    break outer;
                 } else throw Utils.TODO();
             default:
                 throw Utils.TODO();
@@ -98,7 +105,11 @@ public class LoadNode extends MemOpNode {
         //   else       ptr.x = e1;                    : e1;
         //   val = ptr.x;                   ptr.x = val;
         if( mem() instanceof PhiNode memphi && memphi.region()._type == Type.CONTROL && memphi.nIns()== 3 &&
-            off() instanceof ConstantNode ) {
+            // Offset can be hoisted
+            off() instanceof ConstantNode &&
+            // Pointer can be hoisted
+            hoistPtr(ptr,memphi)  ) {
+
             // Profit on RHS/Loop backedge
             if( profit(memphi,2) ||
                 // Else must not be a loop to count profit on LHS.
@@ -111,10 +122,12 @@ public class LoadNode extends MemOpNode {
 
         return null;
     }
+
     private Node ld( int idx ) {
         Node mem = mem(), ptr = ptr();
         return new LoadNode(_name,_alias,_declaredType,mem.in(idx),ptr instanceof PhiNode && ptr.in(0)==mem.in(0) ? ptr.in(idx) : ptr, off()).peephole();
     }
+
     private static boolean neverAlias( Node ptr1, Node ptr2 ) {
         return ptr1.in(0) != ptr2.in(0) &&
             // Unrelated allocations
@@ -122,6 +135,25 @@ public class LoadNode extends MemOpNode {
             ptr2 instanceof ProjNode && ptr2.in(0) instanceof NewNode;
     }
 
+    private static boolean hoistPtr(Node ptr, PhiNode memphi ) {
+        // Can I hoist ptr above the Region?
+        if( !(memphi.region() instanceof RegionNode r) )
+            return false;       // Dead or dying Region/Phi
+        // If ptr from same Region, then yes, just use hoisted split pointers
+        if( ptr instanceof PhiNode pphi && pphi.region() == r )
+            return true;
+
+        // No, so can we lift this ptr?
+        CFGNode cptr = ptr.cfg0();
+        if( cptr != null )
+            // Pointer is controlled high
+            // TODO: Really needs to be the LCA of all inputs is high
+            return cptr.idepth() <= r.idepth();
+
+        // Dunno without a longer walk
+        return false;
+    }
+
     // Profitable if we find a matching Store on this Phi arm.
     private boolean profit(PhiNode phi, int idx) {
         Node px = phi.in(idx);
@@ -130,4 +162,11 @@ public class LoadNode extends MemOpNode {
         if( px instanceof StoreNode st1 && ptr()==st1.ptr() && off()==st1.off() ) { px.addDep(this); return true; }
         return false;
     }
+
+    // Read-Only is a deep property, and cannot be cast-away
+    private Node castRO(Node rez) {
+        if( ptr()._type.isFinal() && !rez._type.isFinal() )
+            return new ReadOnlyNode(rez).peephole();
+        return rez;
+    }
 }
diff --git a/src/main/java/com/seaofnodes/simple/node/LoopNode.java b/src/main/java/com/seaofnodes/simple/node/LoopNode.java
index 22862cb9..ecda9c63 100644
--- a/src/main/java/com/seaofnodes/simple/node/LoopNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/LoopNode.java
@@ -32,38 +32,18 @@ public class LoopNode extends RegionNode {
     // Bypass Region idom, same as the default idom() using use in(1) instead of in(0)
     @Override public CFGNode idom(Node dep) { return entry(); }
 
-    @Override public int loopDepth() {
-        if( _loopDepth!=0 ) return _loopDepth; // Was already set
-        _loopDepth = entry()._loopDepth+1;     // Entry depth plus one
-        // One-time tag loop exits
-        for( CFGNode idom = back(); idom!=this; idom = idom.idom() ) {
-            // Walk idom in loop, setting depth
-            idom._loopDepth = _loopDepth;
-            // Loop exit hits the CProj before the If, instead of jumping from
-            // Region directly to If.
-            if( idom instanceof CProjNode proj ) {
-                assert proj.in(0) instanceof IfNode; // Loop exit test
-                // Find the loop exit CProj, and set loop_depth
-                for( Node use : proj.in(0)._outputs )
-                    if( use instanceof CProjNode proj2 && proj2 != idom )
-                        proj2._loopDepth = _loopDepth-1;
-            }
-        }
-        return _loopDepth;
-    }
-
     // If this is an unreachable loop, it may not have an exit.  If it does not
     // (i.e., infinite loop), force an exit to make it reachable.
-    public void forceExit( StopNode stop ) {
+    public StopNode forceExit( StopNode stop ) {
         // Walk the backedge, then immediate dominator tree util we hit this
         // Loop again.  If we ever hit a CProj from an If (as opposed to
         // directly on the If) we found our exit.
         CFGNode x = back();
         while( x != this ) {
             if( x instanceof CProjNode exit && exit.in(0) instanceof IfNode iff ) {
-                Node other = iff.cproj(1-exit._idx);
-                if( !(other.out(0) instanceof LoopNode loop) || loop.entry()==exit )
-                    return;         // Found an exit, not an infinite loop
+                CFGNode other = iff.cproj(1-exit._idx);
+                if( other.loopDepth() < loopDepth() )
+                    return stop; // Found an exit, not an infinite loop
             }
             x = x.idom();
         }
@@ -76,5 +56,6 @@ public class LoopNode extends RegionNode {
         CProjNode f = new CProjNode(iff,1,"False");
         setDef(2,f);
         stop.addDef(new ReturnNode(t,Parser.ZERO,null));
+        return stop;
     }
 }
diff --git a/src/main/java/com/seaofnodes/simple/node/MemOpNode.java b/src/main/java/com/seaofnodes/simple/node/MemOpNode.java
index c7a1efec..82aab8ac 100644
--- a/src/main/java/com/seaofnodes/simple/node/MemOpNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/MemOpNode.java
@@ -1,6 +1,5 @@
 package com.seaofnodes.simple.node;
 
-import com.seaofnodes.simple.type.Field;
 import com.seaofnodes.simple.type.Type;
 import com.seaofnodes.simple.type.TypeMemPtr;
 
@@ -11,7 +10,9 @@ public abstract class MemOpNode extends Node {
 
     public final String _name;
     public final int _alias;
-    public final Type _declaredType;
+    // Declared type; not final because it might be a forward-reference
+    // which will be lazily improved when the reference is declared.
+    public Type _declaredType;
 
     public MemOpNode(String name, int alias, Type glb, Node mem, Node ptr, Node off) {
         super(null, mem, ptr, off);
diff --git a/src/main/java/com/seaofnodes/simple/node/MinusNode.java b/src/main/java/com/seaofnodes/simple/node/MinusNode.java
index 8a2c9406..760a4274 100644
--- a/src/main/java/com/seaofnodes/simple/node/MinusNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/MinusNode.java
@@ -2,6 +2,7 @@ package com.seaofnodes.simple.node;
 
 import com.seaofnodes.simple.type.Type;
 import com.seaofnodes.simple.type.TypeInteger;
+import com.seaofnodes.simple.Utils;
 
 import java.util.BitSet;
 
@@ -22,7 +23,9 @@ public class MinusNode extends Node {
     public Type compute() {
         if( in(1)._type instanceof TypeInteger i0 ) {
             if( i0.isHigh() ) return TypeInteger.TOP;
-            return i0==TypeInteger.BOT ? TypeInteger.BOT : TypeInteger.make(-i0._max,-i0._min);
+            if( i0==TypeInteger.BOT || i0._min == Long.MIN_VALUE || i0._max == Long.MIN_VALUE )
+                return TypeInteger.BOT;
+            return TypeInteger.make(-i0._max,-i0._min);
         }
         return TypeInteger.TOP.meet(in(1)._type);
     }
diff --git a/src/main/java/com/seaofnodes/simple/node/Node.java b/src/main/java/com/seaofnodes/simple/node/Node.java
index 462a403c..3827fdac 100644
--- a/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -153,7 +153,7 @@ public abstract class Node implements OutNode {
      * @param new_def the new definition
      * @return new_def for flow coding
      */
-    public Node setDef(int idx, Node new_def ) {
+    public <N extends Node> N setDef(int idx, N new_def ) {
         unlock();
         Node old_def = in(idx);
         if( old_def == new_def ) return new_def; // No change
@@ -267,6 +267,10 @@ public abstract class Node implements OutNode {
     }
     // Test "keep" status
     public boolean iskeep() { return _outputs.find(null) != -1; }
+    public void unkill() {
+        if( unkeep().isUnused() )
+            kill();
+    }
 
 
     // Replace self with nnn in the graph, making 'this' go dead
@@ -278,6 +282,7 @@ public abstract class Node implements OutNode {
             int idx = n._inputs.find(this);
             n._inputs.set(idx,nnn);
             nnn.addUse(n);
+            IterPeeps.addAll(n._outputs);
         }
         kill();
     }
@@ -419,7 +424,7 @@ public abstract class Node implements OutNode {
      * And more folding:
      * <pre>   ary + ((idx<<2) + 16)</pre>
      * And during code-gen:
-     * <pre>   MOV4 Rary,Ridx,16 // or some such hardware-specific notation </pre>
+     * <pre>   MOV4 Rary,[Ridx<<2 +16] // or some such hardware-specific notation </pre>
      * <p>
      * {@link #idealize} has a very specific calling convention:
      * <ul>
@@ -542,9 +547,6 @@ public abstract class Node implements OutNode {
     /** Is this Node Memory related */
     public boolean isMem() { return false; }
 
-    /** Return block start from a isCFG() */
-    public Node getBlockStart() { return null; }
-
     /** Pinned in the schedule; these are data nodes whose input#0 is not allowed to change */
     public boolean isPinned() { return false; }
 
diff --git a/src/main/java/com/seaofnodes/simple/node/NotNode.java b/src/main/java/com/seaofnodes/simple/node/NotNode.java
index ffd3e01e..664d7c7b 100644
--- a/src/main/java/com/seaofnodes/simple/node/NotNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/NotNode.java
@@ -28,15 +28,14 @@ public class NotNode extends Node {
             // top->top, bot->bot, null->1, *void->0, not-null ptr->0, ptr/nil->bot
             // If input in null then true
             // If input is not null ptr then false
-            if( p0 == TypeMemPtr.TOP  )    return TypeInteger.TOP;
-            if( p0 == TypeMemPtr.NULLPTR ) return TypeInteger.constant(1);
-            if( !p0._nil )                 return TypeInteger.constant(0);
-            return TypeInteger.BOT;
+            if( p0 == TypeMemPtr.NULLPTR ) return TypeInteger.TRUE;
+            if( !p0._nil )                 return TypeInteger.FALSE;
+            return TypeInteger.BOOL;
         case Type t:
             if( t0.getClass() != Type.class )
                 // Only doing NOT on ints and ptrs
                 throw Utils.TODO();
-            return t0==Type.TOP ? Type.TOP : Type.BOTTOM;
+            return TypeInteger.BOOL;
         }
     }
 
diff --git a/src/main/java/com/seaofnodes/simple/node/PhiNode.java b/src/main/java/com/seaofnodes/simple/node/PhiNode.java
index f0140ce6..cb266ca3 100644
--- a/src/main/java/com/seaofnodes/simple/node/PhiNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/PhiNode.java
@@ -47,7 +47,7 @@ public class PhiNode extends Node {
         // During parsing Phis have to be computed type pessimistically.
         if( r.inProgress() ) return _declaredType;
         // Set type to local top of the starting type
-        Type t = _declaredType.glb().dual();
+        Type t = _declaredType.lub();
         for (int i = 1; i < nIns(); i++)
             // If the region's control input is live, add this as a dependency
             // to the control because we can be peeped should it become dead.
@@ -101,7 +101,7 @@ public class PhiNode extends Node {
             if( in(2)._type == in(2)._type.makeInit() ) nullx = 2;
             if( nullx != -1 ) {
                 Node val = in(3-nullx);
-                if( r.idom(this) instanceof IfNode iff && iff.pred().addDep(this)==val ) {
+                if( r.idom(this).addDep(this) instanceof IfNode iff && iff.pred().addDep(this)==val ) {
                     // Must walk the idom on the null side to make sure we hit False.
                     CFGNode idom = (CFGNode)r.in(nullx);
                     while( idom.nIns() > 0 && idom.in(0) != iff ) idom = idom.idom();
diff --git a/src/main/java/com/seaofnodes/simple/node/ReadOnlyNode.java b/src/main/java/com/seaofnodes/simple/node/ReadOnlyNode.java
new file mode 100644
index 00000000..fddcaa67
--- /dev/null
+++ b/src/main/java/com/seaofnodes/simple/node/ReadOnlyNode.java
@@ -0,0 +1,29 @@
+package com.seaofnodes.simple.node;
+
+import com.seaofnodes.simple.type.Type;
+import com.seaofnodes.simple.type.TypeMemPtr;
+
+import java.util.BitSet;
+
+/**
+ * Cast a pointer to read-only
+ */
+public class ReadOnlyNode extends Node {
+    public ReadOnlyNode( Node n ) { super(null,n); }
+    @Override public String label() { return "ReadOnly"; }
+
+    @Override
+    StringBuilder _print1(StringBuilder sb, BitSet visited) {
+        return in(1)._print0(sb.append("(const)"),visited);
+    }
+    @Override public Type compute() {
+        Type t = in(1)._type;
+        return t instanceof TypeMemPtr tmp ? tmp.makeRO() : t;
+    }
+
+    @Override public Node idealize() {
+        if( in(1)._type instanceof TypeMemPtr tmp && tmp.isFinal() )
+            return in(1);
+        return null;
+    }
+}
diff --git a/src/main/java/com/seaofnodes/simple/node/RegionNode.java b/src/main/java/com/seaofnodes/simple/node/RegionNode.java
index 941de76b..fb39af17 100644
--- a/src/main/java/com/seaofnodes/simple/node/RegionNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/RegionNode.java
@@ -18,7 +18,7 @@ public class RegionNode extends CFGNode {
 
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
-        return sb.append(label()).append(_nid);
+        return sb.append(label());
     }
 
     @Override public boolean isMultiHead() { return true; }
@@ -72,65 +72,9 @@ public class RegionNode extends CFGNode {
             p1.in(0) instanceof IfNode iff )
             return iff.ctrl();
 
-
-        // Look for direct uses of a guarded expression, and replace with the guard result.
-        if( !(idom() instanceof StartNode) && // Dead or dying
-            !(this instanceof LoopNode) ) {
-            VISIT.clear();
-            boolean progress = false;
-            outer:
-            for( int i=1; i<nIns(); i++ ) {
-                CFGNode idom = cfg(i);
-                while( !(idom instanceof CProjNode cproj) ) {
-                    idom = idom.idom();
-                    if( idom==null || idom.idepth() < idom().idepth() )
-                        continue outer;
-                }
-                if( cproj.cfg0() instanceof IfNode iff && cproj.compute()==Type.CONTROL ) {
-                    VISIT.set(iff.pred()._nid);
-                    for( Node out : _outputs )
-                        if( out instanceof PhiNode phi )
-                            progress |= walk(iff,cproj,phi.in(i));
-                }
-            }
-            if( progress )
-                return this;
-        }
-
         return null;
     }
 
-    // A walk that can be cutoff early
-    private static final BitSet VISIT = new BitSet();
-    private boolean walk( IfNode iff, CProjNode cproj, Node n ) {
-        if( VISIT.get(n._nid) ) return false;
-        VISIT.set(n._nid);
-        if( n instanceof CFGNode ) return false;
-        if( n.in(0)!=null && n.cfg0().idepth() < iff.idepth() )
-            return false;
-        boolean progress = false;
-        Node pred = iff.pred();
-        if( pred._type.isHighOrConst() ) return false; // This will collapse the IF already
-        for( int i=1; i<n.nIns(); i++ ) {
-            // Using a tested value
-            if( n.in(i) == pred && !(n instanceof CastNode) ) {
-                // Checking pred for T/F
-                Type t = pred._type;
-                Type tcast = cproj._idx==0 ? t.nonZero() : t.makeInit();
-                if( t!=tcast ) { // Progress to cast
-                    Node cast = new CastNode( tcast, cproj, pred ).peephole();
-                    n.setDef( i, cast );
-                    IterPeeps.add(n);
-                    progress = true;
-                } else pred.addDep( this );
-            }
-            if( n.in(i)!=null )
-                progress |= walk(iff,cproj,n.in(i));
-        }
-        return progress;
-    }
-
-
     private int findDeadInput() {
         for( int i=1; i<nIns(); i++ )
             if( in(i)._type==Type.XCONTROL )
@@ -164,13 +108,6 @@ public class RegionNode extends CFGNode {
         return lca;
     }
 
-    @Override void _walkUnreach( BitSet visit, HashSet<CFGNode> unreach ) {
-        for( int i=1; i<nIns(); i++ )
-            cfg(i).walkUnreach(visit,unreach);
-    }
-
-    @Override public int loopDepth() { return _loopDepth==0 ? (_loopDepth = cfg(1).loopDepth()) : _loopDepth; }
-
     // True if last input is null
     public final boolean inProgress() {
         return nIns()>1 && in(nIns()-1) == null;
@@ -180,6 +117,4 @@ public class RegionNode extends CFGNode {
     @Override boolean eq( Node n ) {
         return !inProgress();
     }
-
-    @Override public Node getBlockStart() { return this; }
 }
diff --git a/src/main/java/com/seaofnodes/simple/node/ReturnNode.java b/src/main/java/com/seaofnodes/simple/node/ReturnNode.java
index de4eba52..b586a44f 100644
--- a/src/main/java/com/seaofnodes/simple/node/ReturnNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/ReturnNode.java
@@ -47,6 +47,4 @@ public class ReturnNode extends CFGNode {
             return ctrl();
         return null;
     }
-
-    @Override public Node getBlockStart() { return ctrl().getBlockStart(); }
 }
diff --git a/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java b/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java
index e4874e15..9134f9c3 100644
--- a/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java
@@ -14,16 +14,29 @@ public class ScopeMinNode extends Node {
     /** The tracked fields are now complex enough to deserve a array-of-structs layout
      */
     public static class Var {
-        public final int _idx;
-        public final String _name;
-        public final Type _type;
-        public final boolean _final;
+        public final int _idx;       // index in containing scope
+        public final String _name;   // Declared name
+        private Type _type;          // Declared type
+        public final boolean _final; // Final field
         public Var(int idx, String name, Type type, boolean xfinal) {
             _idx = idx;
             _name = name;
             _type = type;
             _final = xfinal;
         }
+        public Type type() {
+            if( !_type.isFRef() ) return _type;
+            // Update self to no longer use the forward ref type
+            Type def = Parser.TYPES.get(((TypeMemPtr)_type)._obj._name);
+            return (_type=_type.meet(def));
+        }
+        public Type lazyGLB() {
+            Type t = type();
+            return t instanceof TypeMemPtr ? t : t.glb();
+        }
+        @Override public String toString() {
+            return _type.toString()+(_final ? " ": " !")+_name;
+        }
     }
 
     public ScopeMinNode() { _type = TypeMem.BOT; }
diff --git a/src/main/java/com/seaofnodes/simple/node/ScopeNode.java b/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
index 11e54d82..86aa7665 100644
--- a/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
@@ -1,8 +1,6 @@
 package com.seaofnodes.simple.node;
 
 import com.seaofnodes.simple.IterPeeps;
-import com.seaofnodes.simple.Parser;
-import com.seaofnodes.simple.Utils;
 import com.seaofnodes.simple.Ary;
 import com.seaofnodes.simple.type.*;
 import java.util.*;
@@ -29,13 +27,22 @@ public class ScopeNode extends ScopeMinNode {
     // Since of each nested lexical scope
     public final Ary<Integer> _lexSize;
 
+    // True if parsing inside of a constructor
+    public final Ary<Boolean> _inCons;
+
+    // Extra guards; tested predicates and casted results
+    private final Ary<Node> _guards;
 
     // A new ScopeNode
-    public ScopeNode() { _vars = new Ary<>(Var.class); _lexSize = new Ary<>(Integer.class); }
+    public ScopeNode() {
+        _vars   = new Ary<>(Var    .class);
+        _lexSize= new Ary<>(Integer.class);
+        _inCons = new Ary<>(Boolean.class);
+        _guards = new Ary<>(Node   .class);
+    }
 
     @Override public String label() { return "Scope"; }
 
-
     @Override
     StringBuilder _print1(StringBuilder sb, BitSet visited) {
         sb.append("Scope[ ");
@@ -43,7 +50,7 @@ public class ScopeNode extends ScopeMinNode {
         for( int i=0; i<nIns(); i++ ) {
             if( j < _lexSize._len && i == _lexSize.at(j) ) { sb.append("| "); j++; }
             Var v = _vars.get(i);
-            v._type.print(sb);
+            v.type().print(sb);
             sb.append(" ");
             if( v._final ) sb.append("!");
             sb.append(v._name);
@@ -75,17 +82,24 @@ public class ScopeNode extends ScopeMinNode {
      *
      * @return Node that was bound
      */
-    public Node ctrl(Node n) { return setDef(0,n); }
+    public <N extends Node> N ctrl(N n) { return setDef(0,n); }
 
-    public void push() {
+    public void push() { push(false); }
+    public void push(boolean inCon) {
+        assert _lexSize._len==_inCons._len;
         _lexSize.push(_vars.size());
+        _inCons.push(inCon);
     }
     public void pop() {
+        assert _lexSize._len==_inCons._len;
         int n = _lexSize.pop();
+        _inCons.pop();
         popUntil(n);
         _vars.setLen(n);
     }
 
+    public boolean inCon() { return _inCons.last(); }
+
     // Find name in reverse, return an index into _vars or -1.  Linear scan
     // instead of hashtable, but probably doesn't matter until the scan
     // typically hits many dozens of variables.
@@ -125,7 +139,7 @@ public class ScopeNode extends ScopeMinNode {
     public Var lookup( String name ) {
         int idx = find(name);
         // -1 is missed in all scopes, not found
-        return idx == -1 ? null : _update(_vars.at(idx),null);
+        return idx == -1 ? null : update(_vars.at(idx),null);
     }
 
     /**
@@ -137,10 +151,10 @@ public class ScopeNode extends ScopeMinNode {
     public void update( String name, Node n ) {
         int idx = find(name);
         assert idx>=0;
-        _update(_vars.at(idx),n);
+        update(_vars.at(idx),n);
     }
 
-    Var _update( ScopeNode.Var v, Node st ) {
+    public Var update( ScopeNode.Var v, Node st ) {
         Node old = in(v._idx);
         if( old instanceof ScopeNode loop ) {
             // Lazy Phi!
@@ -151,7 +165,7 @@ public class ScopeNode extends ScopeMinNode {
                 // Set real Phi in the loop head
                 // The phi takes its one input (no backedge yet) from a recursive
                 // lookup, which might have insert a Phi in every loop nest.
-                : loop.setDef(v._idx,new PhiNode(v._name, v._type instanceof TypeMemPtr ? v._type : v._type.glb(), loop.ctrl(), loop.in(loop._update(v,null)._idx),null).peephole());
+                : loop.setDef(v._idx,new PhiNode(v._name, v.lazyGLB(), loop.ctrl(), loop.in(loop.update(v,null)._idx),null).peephole());
             setDef(v._idx,old);
         }
         if( st!=null ) setDef(v._idx,st); // Set new value
@@ -178,8 +192,15 @@ public class ScopeNode extends ScopeMinNode {
         // 1) duplicate the name bindings of the ScopeNode across all stack levels
         // 2) Make the new ScopeNode a user of all the nodes bound
         // 3) Ensure that the order of defs is the same to allow easy merging
-        dup._vars.addAll(_vars);
+        dup._vars   .addAll(_vars   );
         dup._lexSize.addAll(_lexSize);
+        dup._inCons .addAll(_inCons );
+        dup._guards .addAll(_guards );
+        // The dup'd guards all need dup'd keepers, to keep proper accounting
+        // when later removing all guards
+        for( Node n : _guards )
+            if( !(n instanceof CFGNode) )
+                n.keep();
         dup.addDef(ctrl());     // Control input is just copied
 
         // Memory input is a shallow copy
@@ -208,13 +229,13 @@ public class ScopeNode extends ScopeMinNode {
      * @param that The ScopeNode to be merged into this
      * @return A new node representing the merge point
      */
-    public Node mergeScopes(ScopeNode that) {
+    public RegionNode mergeScopes(ScopeNode that) {
         RegionNode r = (RegionNode) ctrl(new RegionNode(null,ctrl(), that.ctrl()).keep());
         mem()._merge(that.mem(),r);
         this ._merge(that      ,r);
         that.kill();            // Kill merged scope
         IterPeeps.add(r);
-        return r.unkeep().peephole();
+        return r.unkeep();
     }
 
     private void _merge(ScopeNode that, RegionNode r) {
@@ -223,9 +244,9 @@ public class ScopeNode extends ScopeMinNode {
                 // If we are in lazy phi mode we need to a lookup
                 // by name as it will trigger a phi creation
                 Var v = _vars.at(i);
-                Node lhs = this.in(this._update(v,null));
-                Node rhs = that.in(that._update(v,null));
-                setDef(i, new PhiNode(v._name, v._type, r, lhs, rhs).peephole());
+                Node lhs = this.in(this.update(v,null));
+                Node rhs = that.in(that.update(v,null));
+                setDef(i, new PhiNode(v._name, v.type(), r, lhs, rhs).peephole());
             }
     }
 
@@ -264,42 +285,78 @@ public class ScopeNode extends ScopeMinNode {
 
     // Up-casting: using the results of an If to improve a value.
     // E.g. "if( ptr ) ptr.field;" is legal because ptr is known not-null.
-
-    // This Scope looks for direct variable uses, or certain simple
-    // combinations, and replaces the variable with the upcast variant.
-    public Node upcast( Node ctrl, Node pred, boolean invert ) {
+    public void addGuards( Node ctrl, Node pred, boolean invert ) {
+        assert ctrl instanceof CFGNode;
+        _guards.add(ctrl);      // Marker to distinguish 0,1,2 guards
+        // add pred & its cast to the normal input list, with special Vars
+        if( ctrl._type == Type.XCONTROL || pred==null || pred.isDead() )
+            return;           // Dead, do not add any guards
         // Invert the If conditional
         if( invert )
             pred = pred instanceof NotNode not ? not.in(1) : IterPeeps.add(new NotNode(pred).peephole());
-
-        // Direct use of a value as predicate.  This is a zero/null test.
-        if( _inputs.find(pred) != -1 ) {
-            if( !(pred._type instanceof TypeMemPtr tmp) )
-                // Must be an `int`, since int and ptr are the only two value types
-                // being tested. No representation for a generic not-null int, so no upcast.
-                return null;
-            if( tmp.isa(TypeMemPtr.VOIDPTR) )
-                return null;    // Already not-null, no reason to upcast
-            // Upcast the ptr to not-null ptr, and replace in scope
-            return replace(pred,new CastNode(TypeMemPtr.VOIDPTR,ctrl,pred).peephole());
+        // This is a zero/null test.
+        // Compute the positive test type.
+        Type tnz   = pred._type.nonZero();
+        Type tcast = tnz.join(pred._type);
+        if( tcast != pred._type ) {
+            Node cast = new CastNode(tnz,ctrl,pred.keep()).peephole().keep();
+            _guards.add(pred);
+            _guards.add(cast);
+            replace(pred,cast);
         }
 
+        // Compute the negative test type.
         if( pred instanceof NotNode not ) {
-            // Direct use of a !value as predicate.  This is a zero/null test.
-            if( _inputs.find(not.in(1)) != -1 ) {
-                Type tinit = not.in(1)._type.makeInit();
-                if( not.in(1)._type.isa(tinit) ) return null; // Already zero/null, no reason to upcast
-                return replace(not.in(1), new ConstantNode(tinit).peephole());
+            Node npred = not.in(1);
+            Type tzero = npred._type.makeZero();
+            Type tzcast= tzero.join(npred._type);
+            if( tzcast != npred._type ) {
+                Node cast = new CastNode(tzero,ctrl,npred.keep()).peephole().keep();
+                _guards.add(npred);
+                _guards.add( cast);
+                replace(npred,cast);
             }
         }
-        // Apr/9/2024: Attempted to replace X with Y if guarded by a test of
-        // X==Y.  This didn't seem to help very much, or at least in the test
-        // cases seen so far was a very minor help.
+    }
+
+    // Remove matching pred/cast pairs from this guarded region.
+    public void removeGuards( Node ctrl ) {
+        assert ctrl instanceof CFGNode;
+        // 0,1 or 2 guards
+        while( true ) {
+            Node g = _guards.pop();
+            if( g == ctrl ) break;
+            if( g instanceof CFGNode ) continue;
+            g            .unkill(); // Pop/kill cast
+            _guards.pop().unkill(); // Pop/kill pred
+        }
+    }
 
-        // No upcast
-        return null;
+    // If we find a guarded instance of pred, replace with the upcasted version
+    public Node upcastGuard( Node pred ) {
+        // If finding an instanceof pred, replace with cast.
+        // Otherwise, just pred itself.
+        int i = _guards._len;
+        while( i > 0 ) {
+            Node  cast = _guards.at(--i);
+            if( cast instanceof CFGNode ) continue; // Marker between guard sets
+            Node xpred = _guards.at(--i);
+            if( xpred == pred )
+                return cast;
+        }
+        return pred;
     }
 
+    // Kill guards also
+    @Override public void kill() {
+        for( Node n : _guards )
+            if( !(n instanceof CFGNode) )
+                n.unkill();
+        _guards.clear();
+        super.kill();
+    }
+
+
     private Node replace( Node old, Node cast ) {
         assert old!=null && old!=cast;
         for( int i=0; i<nIns(); i++ )
diff --git a/src/main/java/com/seaofnodes/simple/node/ShlNode.java b/src/main/java/com/seaofnodes/simple/node/ShlNode.java
index b67d985a..6fd951e4 100644
--- a/src/main/java/com/seaofnodes/simple/node/ShlNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/ShlNode.java
@@ -44,7 +44,7 @@ public class ShlNode extends Node {
             if( (shl.value()&63)==0 )
                 return lhs;
             // (x + c) << i  =>  (x << i) + (c << i)
-            if( lhs instanceof AddNode add && add.in(2)._type instanceof TypeInteger c && c.isConstant() && c!=TypeInteger.ZERO ) {
+            if( lhs instanceof AddNode add && add.addDep(this).in(2)._type instanceof TypeInteger c && c.isConstant() && c!=TypeInteger.ZERO ) {
                 long sum = c.value() << shl.value();
                 if( Integer.MIN_VALUE <= sum  && sum <= Integer.MAX_VALUE )
                     return new AddNode( new ShlNode(add.in(1),rhs).peephole(), Parser.con(sum) );
diff --git a/src/main/java/com/seaofnodes/simple/node/StartNode.java b/src/main/java/com/seaofnodes/simple/node/StartNode.java
index c03a4b06..ab6e4931 100644
--- a/src/main/java/com/seaofnodes/simple/node/StartNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/StartNode.java
@@ -14,11 +14,11 @@ import static com.seaofnodes.simple.Utils.TODO;
  * In ch10 we also add mem aliases as structs get defined; each field in struct
  * adds a distinct alias to Start's tuple.
  */
-public class StartNode extends CFGNode implements MultiNode {
+public class StartNode extends LoopNode implements MultiNode {
 
     final Type _arg;
 
-    public StartNode(Type arg) { super(); _arg = arg; _type = compute(); }
+    public StartNode(Type arg) { super(null); _arg = arg; _type = compute(); }
 
     @Override public String label() { return "Start"; }
 
@@ -41,9 +41,4 @@ public class StartNode extends CFGNode implements MultiNode {
     @Override public int idepth() { return 0; }
     @Override public CFGNode idom(Node dep) { return null; }
 
-    @Override void _walkUnreach( BitSet visit, HashSet<CFGNode> unreach ) { }
-
-    @Override public int loopDepth() { return (_loopDepth=1); }
-
-    @Override public Node getBlockStart() { return this; }
 }
diff --git a/src/main/java/com/seaofnodes/simple/node/StopNode.java b/src/main/java/com/seaofnodes/simple/node/StopNode.java
index 0eacc1f4..867225cb 100644
--- a/src/main/java/com/seaofnodes/simple/node/StopNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/StopNode.java
@@ -1,6 +1,7 @@
 package com.seaofnodes.simple.node;
 
 import com.seaofnodes.simple.type.Type;
+import com.seaofnodes.simple.Parser;
 import com.seaofnodes.simple.IterPeeps;
 import com.seaofnodes.simple.GlobalCodeMotion;
 import com.seaofnodes.simple.GraphVisualizer;
@@ -63,8 +64,6 @@ public class StopNode extends CFGNode {
     }
     @Override public CFGNode idom(Node dep) { return null; }
 
-    @Override public int loopDepth() { return (_loopDepth=1); }
-
     public Node addReturn(Node node) {
         return addDef(node);
     }
@@ -77,13 +76,10 @@ public class StopNode extends CFGNode {
         return this;
     }
     StopNode GCM(boolean show) {
-        // Break infinite loops, forcing a Never-branch to exit
-        GlobalCodeMotion.fixLoops(this);
+        Parser.START.buildLoopTree(this);
         if( show )
             System.out.println(new GraphVisualizer().generateDotOutput(this,null,null));
         GlobalCodeMotion.buildCFG(this);
         return this;
     }
-
-    @Override public Node getBlockStart() { return this; }
 }
diff --git a/src/main/java/com/seaofnodes/simple/node/StoreNode.java b/src/main/java/com/seaofnodes/simple/node/StoreNode.java
index 7b1bddd5..62b49a1a 100644
--- a/src/main/java/com/seaofnodes/simple/node/StoreNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/StoreNode.java
@@ -42,9 +42,15 @@ public class StoreNode extends MemOpNode {
     public Type compute() {
         Type val = val()._type;
         TypeMem mem = (TypeMem)mem()._type; // Invariant
-        Type t = mem._alias==_alias
-            ? val.meet(mem._t)  // Meet into existing memory
-            : Type.BOTTOM;
+        Type t = Type.BOTTOM;               // No idea on field contents
+        // Same alias, lift val to the declared type and then meet into other fields
+        if( mem._alias == _alias ) {
+            // Update declared forward ref to the actual
+            if( _declaredType.isFRef() && val instanceof TypeMemPtr tmp && !tmp.isFRef() )
+                _declaredType = tmp;
+            val = val.join(_declaredType);
+            t = val.meet(mem._t);
+        }
         return TypeMem.make(_alias,t);
     }
 
@@ -108,6 +114,9 @@ public class StoreNode extends MemOpNode {
     String err() {
         String err = super.err();
         if( err != null ) return err;
+        TypeMemPtr tmp = (TypeMemPtr)ptr()._type;
+        if( tmp._obj.field(_name)._final )
+            return "Cannot modify final field '"+_name+"'";
         Type t = val()._type;
         return _init || t.isa(_declaredType) ? null : "Cannot store "+t+" into field "+_declaredType+" "+_name;
     }
diff --git a/src/main/java/com/seaofnodes/simple/node/SubNode.java b/src/main/java/com/seaofnodes/simple/node/SubNode.java
index 009fb30e..5d6c2643 100644
--- a/src/main/java/com/seaofnodes/simple/node/SubNode.java
+++ b/src/main/java/com/seaofnodes/simple/node/SubNode.java
@@ -29,6 +29,11 @@ public class SubNode extends Node {
             t2 instanceof TypeInteger i2 ) {
             if (i1.isConstant() && i2.isConstant())
                 return TypeInteger.constant(i1.value()-i2.value());
+            // Fold ranges like {2-3} - {0-1} into {1-3}.
+            if( !AddNode.overflow(i1._min,-i2._max) &&
+                !AddNode.overflow(i1._max,-i2._min) &&
+                i2._min != Long.MIN_VALUE  )
+                return TypeInteger.make(i1._min-i2._max,i1._max-i2._min);
         }
         // Sub of same is 0
         if( in(1)==in(2) )
diff --git a/src/main/java/com/seaofnodes/simple/type/Field.java b/src/main/java/com/seaofnodes/simple/type/Field.java
index e2c0aa59..349c1fb0 100644
--- a/src/main/java/com/seaofnodes/simple/type/Field.java
+++ b/src/main/java/com/seaofnodes/simple/type/Field.java
@@ -31,8 +31,10 @@ public class Field extends Type {
         return new Field(fname,type,alias,xfinal).intern();
     }
     public Field makeFrom( Type type ) {
-        return new Field(_fname,type,_alias,_final).intern();
+        return type == _type ? this : new Field(_fname,type,_alias,_final).intern();
     }
+    @Override public Field makeRO() { return _final ? this : make(_fname,_type.makeRO(),_alias,true);  }
+    @Override public boolean isFinal() { return _final && _type.isFinal(); }
 
     public static final Field TEST = make("test",TypeInteger.ZERO,-2,false);
     public static void gather(ArrayList<Type> ts) { ts.add(TEST); }
@@ -41,16 +43,20 @@ public class Field extends Type {
         Field fld = (Field)that; // Invariant
         assert _fname.equals(fld._fname);
         assert _alias==fld._alias;
-        assert _final==fld._final;
-        return make(_fname,_type.meet(fld._type),_alias,_final);
+        return make(_fname,_type.meet(fld._type),_alias,_final | fld._final);
     }
 
     @Override
-    public Field dual() { return make(_fname,_type.dual(),_alias,_final); }
+    public Field dual() { return make(_fname,_type.dual(),_alias,!_final); }
 
-    @Override
-    public Field glb() {
-        return make(_fname,_type.glb(),_alias,_final);
+    @Override public Field glb() {
+        Type glb = _type.glb();
+        return glb==_type ? this : make(_fname,glb,_alias,_final);
+    }
+
+    @Override public Field lub() {
+        Type lub = _type.lub();
+        return lub==_type ? this : make(_fname,lub,_alias,_final);
     }
 
     // Override in subclasses
@@ -64,7 +70,7 @@ public class Field extends Type {
 
     @Override
     public StringBuilder print( StringBuilder sb ) {
-        return _type.print(sb.append(_final?"!":"").append(_fname).append(":").append(_alias).append(" : "));
+        return _type.print(sb.append(_final?"":"!").append(_fname).append(":").append(_alias).append(" : "));
     }
 
     @Override public String str() { return _fname; }
diff --git a/src/main/java/com/seaofnodes/simple/type/Type.java b/src/main/java/com/seaofnodes/simple/type/Type.java
index 989eff9e..4ebb2b6d 100644
--- a/src/main/java/com/seaofnodes/simple/type/Type.java
+++ b/src/main/java/com/seaofnodes/simple/type/Type.java
@@ -81,8 +81,6 @@ public class Type {
      */
     public StringBuilder typeName( StringBuilder sb) { return print(sb); }
 
-    public Type makeInit() { return null; }
-    public Type nonZero() { return glb(); }
 
     // ----------------------------------------------------------
 
@@ -174,10 +172,28 @@ public class Type {
     // True if this "isa" t; e.g. 17 isa TypeInteger.BOT
     public boolean isa( Type t ) { return meet(t)==t; }
 
-    /**
-     * Compute greatest lower bound in the lattice
-     */
-    public Type glb() { return _type==TCTRL ? XCONTROL : BOTTOM; }
+    /** Compute greatest lower bound in the lattice */
+    public Type glb() { return _type==TCTRL ? CONTROL : BOTTOM; }
+    /** Compute least upper bound in the lattice */
+    public Type lub() { return _type==TCTRL ? XCONTROL : TOP; }
+
+    // Make an initial/default version of this type.  Typically, 0 for integers
+    // and null for nullable pointers.
+    public Type makeInit() { return this; }
+    // Make a zero version of this type, 0 for integers and null for pointers.
+    public Type makeZero() { return null; }
+    // Make a non-zero version of this type, if possible.  Integers attempt to
+    // exclude zero from their range and pointers become not-null.
+    public Type nonZero() { return glb(); }
+
+    // Is forward-reference
+    public boolean isFRef() { return false; }
+
+    // All reachable struct Fields are final
+    public boolean isFinal() { return true; }
+
+    // Make all reachable struct Fields final
+    public Type makeRO() { return this; }
 
     // ----------------------------------------------------------
 
diff --git a/src/main/java/com/seaofnodes/simple/type/TypeFloat.java b/src/main/java/com/seaofnodes/simple/type/TypeFloat.java
index dc36c082..f85abbed 100644
--- a/src/main/java/com/seaofnodes/simple/type/TypeFloat.java
+++ b/src/main/java/com/seaofnodes/simple/type/TypeFloat.java
@@ -104,10 +104,10 @@ public class TypeFloat extends Type {
         return make((byte)-_sz,0);
     }
 
-    @Override
-    public Type glb() { return BOT; }
-
+    @Override public Type glb() { return BOT; }
+    @Override public Type lub() { return TOP; }
     @Override public TypeFloat makeInit() { return ZERO; }
+    @Override public TypeFloat makeZero() { return ZERO; }
 
     @Override
     int hash() { return (int)(Double.hashCode(_con) ^ _sz); }
diff --git a/src/main/java/com/seaofnodes/simple/type/TypeInteger.java b/src/main/java/com/seaofnodes/simple/type/TypeInteger.java
index b475055d..b2979d7b 100644
--- a/src/main/java/com/seaofnodes/simple/type/TypeInteger.java
+++ b/src/main/java/com/seaofnodes/simple/type/TypeInteger.java
@@ -81,6 +81,7 @@ public class TypeInteger extends Type {
         if( this==I16 || this==U16              ) return 1; // 1<<1 == 2 bytes
         if( this==I32 || this==U32              ) return 2; // 1<<2 == 4 bytes
         if( this==BOT                           ) return 3; // 1<<3 == 8 bytes
+        if( isHighOrConst() ) return 0;
         throw Utils.TODO();
     }
 
@@ -114,7 +115,9 @@ public class TypeInteger extends Type {
 
     @Override public TypeInteger dual() { return make(_max,_min); }
     @Override public TypeInteger glb() { return BOT; }
+    @Override public TypeInteger lub() { return TOP; }
     @Override public TypeInteger makeInit() { return ZERO; }
+    @Override public TypeInteger makeZero() { return ZERO; }
     @Override public TypeInteger nonZero() {
         if( isHigh() ) return this;
         if( _min==0 ) return make(1,Math.max(_max,1)); // specifically good on BOOL
diff --git a/src/main/java/com/seaofnodes/simple/type/TypeMem.java b/src/main/java/com/seaofnodes/simple/type/TypeMem.java
index 32491a11..33c6a8ce 100644
--- a/src/main/java/com/seaofnodes/simple/type/TypeMem.java
+++ b/src/main/java/com/seaofnodes/simple/type/TypeMem.java
@@ -21,7 +21,7 @@ public class TypeMem extends Type {
     private TypeMem(int alias, Type t) { super(TMEM); _alias = alias; _t = t; }
 
     public static TypeMem make(int alias, Type t) { return new TypeMem(alias,t).intern(); }
-    public static final TypeMem TOP = make( 0, Type.TOP);
+    public static final TypeMem TOP = make(-1, Type.TOP   );
     public static final TypeMem BOT = make(-1, Type.BOTTOM);
 
     public static void gather(ArrayList<Type> ts) { ts.add(make(1,TypeInteger.ZERO)); ts.add(BOT); }
@@ -40,13 +40,11 @@ public class TypeMem extends Type {
 
     @Override
     public Type dual() {
-        if( _alias== 0 ) return BOT;
-        if( _alias==-1 ) return TOP;
         return make(_alias,_t.dual());
     }
 
-    @Override
-    public Type glb() { return make(_alias,_t.glb()); }
+    @Override public Type glb() { return make(_alias,_t.glb()); }
+    @Override public Type lub() { return make(_alias,_t.lub()); }
 
     @Override
     int hash() { return 9876543 + _alias + _t.hashCode(); }
diff --git a/src/main/java/com/seaofnodes/simple/type/TypeMemPtr.java b/src/main/java/com/seaofnodes/simple/type/TypeMemPtr.java
index ebcda0d8..c24fccf3 100644
--- a/src/main/java/com/seaofnodes/simple/type/TypeMemPtr.java
+++ b/src/main/java/com/seaofnodes/simple/type/TypeMemPtr.java
@@ -33,7 +33,10 @@ public class TypeMemPtr extends Type {
     }
     public static TypeMemPtr make(TypeStruct obj, boolean nil) { return new TypeMemPtr(obj, nil).intern(); }
     public static TypeMemPtr make(TypeStruct obj) { return make(obj, false); }
-    public TypeMemPtr makeFrom(TypeStruct obj) { return make(obj, _nil); }
+    public TypeMemPtr makeFrom(TypeStruct obj) { return obj==_obj ? this : make(obj, _nil); }
+    public TypeMemPtr makeFrom(boolean nil) { return nil==_nil ? this : make(_obj, nil); }
+    @Override public TypeMemPtr makeRO() { return makeFrom(_obj.makeRO()); }
+    @Override public boolean isFinal() { return _obj.isFinal(); }
 
     // An abstract pointer, pointing to either a Struct or an Array.
     // Can also be null or not.
@@ -55,9 +58,12 @@ public class TypeMemPtr extends Type {
     @Override
     public TypeMemPtr dual() { return TypeMemPtr.make(_obj.dual(), !_nil); }
 
-    @Override
-    public TypeMemPtr glb() { return make(_obj.glb(),true); }
-    @Override public TypeMemPtr makeInit() { return NULLPTR; }
+    @Override public TypeMemPtr glb() { return make(_obj.glb(),true ); }
+    @Override public TypeMemPtr lub() { return make(_obj.lub(),false); }
+    // Is forward-reference
+    @Override public boolean isFRef() { return _obj.isFRef(); }
+    @Override public Type makeInit() { return _nil ? NULLPTR : Type.TOP; }
+    @Override public Type makeZero() { return NULLPTR; }
     @Override public Type nonZero() { return VOIDPTR; }
 
     @Override public boolean isHigh() { return this==TOP; }
diff --git a/src/main/java/com/seaofnodes/simple/type/TypeStruct.java b/src/main/java/com/seaofnodes/simple/type/TypeStruct.java
index 4e4d1305..b3dbcad0 100644
--- a/src/main/java/com/seaofnodes/simple/type/TypeStruct.java
+++ b/src/main/java/com/seaofnodes/simple/type/TypeStruct.java
@@ -36,21 +36,29 @@ public class TypeStruct extends Type {
     public static final TypeStruct BOT = make("$BOT",new Field[0]);
     public static final TypeStruct TEST = make("test",new Field[]{Field.TEST});
     // Forward-ref version
-    public static TypeStruct make(String name) { return make(name, (Field[])null); }
+    public static TypeStruct makeFRef(String name) { return make(name, (Field[])null); }
+    // Make a read-only version
+    @Override public TypeStruct makeRO() {
+        if( isFinal() ) return this;
+        Field[] flds = new Field[_fields.length];
+        for( int i=0; i<flds.length; i++ )
+            flds[i] = _fields[i].makeRO();
+        return make(_name,flds);
+    }
 
     // Array
     public static TypeStruct makeAry(TypeInteger len, int lenAlias, Type body, int bodyAlias) {
         assert !(body instanceof TypeMemPtr tmp && !tmp._nil);
         return make("[" + body.str() + "]",
-                    Field.make("#" ,len , lenAlias,true),
-                    Field.make("[]",body,bodyAlias,true));
+                    Field.make("#" ,len , lenAlias,true ),
+                    Field.make("[]",body,bodyAlias,false));
     }
 
     // A pair of self-cyclic types
-    private static final TypeStruct S1F = make("S1");
-    private static final TypeStruct S2F = make("S2");
-    public  static final TypeStruct S1  = make("S1", new Field[]{ Field.make("a", TypeInteger.BOT, -1, false), Field.make("s2",TypeMemPtr.make(S2F,false),-2, false) });
-    private static final TypeStruct S2  = make("S2", new Field[]{ Field.make("b", TypeFloat  .BOT, -3, false), Field.make("s1",TypeMemPtr.make(S1F,false),-4, false) });
+    private static final TypeStruct S1F = makeFRef("S1");
+    private static final TypeStruct S2F = makeFRef("S2");
+    public  static final TypeStruct S1  = make("S1", Field.make("a", TypeInteger.BOT, -1, false), Field.make("s2",TypeMemPtr.make(S2F,false),-2, false) );
+    private static final TypeStruct S2  = make("S2", Field.make("b", TypeFloat  .BOT, -3, false), Field.make("s1",TypeMemPtr.make(S1F,false),-4, false) );
 
     private static final TypeStruct ARY = makeAry(TypeInteger.BOT,-1,TypeFloat.BOT,-2);
 
@@ -63,6 +71,10 @@ public class TypeStruct extends Type {
                 return i;
         return -1;
     }
+    public Field field(String fname) {
+        int idx = find(fname);
+        return idx== -1 ? null : _fields[idx];
+    }
 
     // Find field index by alias
     public int findAlias( int alias ) {
@@ -89,8 +101,8 @@ public class TypeStruct extends Type {
         // And the answer is: "don't ask".
         if( !_name.equals(that._name) )
             return BOT;         // It's a struct; that's about all we know
-        if( this._fields==null ) return this;
-        if( that._fields==null ) return that;
+        if( this._fields==null ) return that;
+        if( that._fields==null ) return this;
         if( _fields.length != that._fields.length ) return BOT;
         // Just do field meets
         Field[] flds = new Field[_fields.length];
@@ -115,8 +127,7 @@ public class TypeStruct extends Type {
     }
 
     // Keeps the same struct, but lower-bounds all fields.
-    @Override
-    public TypeStruct glb() {
+    @Override public TypeStruct glb() {
         if( _glb() ) return this;
         // Need to glb each field
         Field[] flds = new Field[_fields.length];
@@ -131,6 +142,33 @@ public class TypeStruct extends Type {
                  return false;
         return true;
     }
+    @Override public TypeStruct lub() {
+        if( _lub() ) return this;
+        // Need to glb each field
+        Field[] flds = new Field[_fields.length];
+        for( int i=0; i<_fields.length; i++ )
+            flds[i] = _fields[i].lub();
+        return make(_name,flds);
+    }
+    private boolean _lub() {
+        if( _fields!=null )
+          for( Field f : _fields )
+              if( f.lub() != f )
+                 return false;
+        return true;
+    }
+
+
+    // Is forward-reference
+    @Override public boolean isFRef() { return _fields==null; }
+    // All fields are final
+    @Override public boolean isFinal() {
+        if( _fields==null ) return true;
+        for( Field fld : _fields )
+            if( !fld.isFinal() )
+                return false;
+        return true;
+    }
 
     @Override
     boolean eq(Type t) {
@@ -161,10 +199,8 @@ public class TypeStruct extends Type {
         sb.append(_name);
         if( _fields == null ) return sb; // Forward reference struct, just print the name
         sb.append(" {");
-        for( Field f : _fields ) {
-            sb.append("  ").append(f._fname).append(" :");
-            f._type.print(sb).append("; ");
-        }
+        for( Field f : _fields )
+            f._type.print(sb).append(f._final ? " " : " !").append(f._fname).append("; ");
         return sb.append("}");
     }