[MOD] XQuery: Tail-call optimization optimizations. #2314

basex-core/src/main/java/org/basex/query/QueryContext.java

@@ -95,11 +95,8 @@ public final class QueryContext extends Job implements Closeable {
 
   /** Available collations. */
   public TokenObjMap<Collation> collations;
-
-  /** Number of successive tail calls. */
-  public int tailCalls;
-  /** Maximum number of successive tail calls (will be set before compilation). */
-  public int maxCalls;
+  /** Perform tail-call optimizations. */
+  public boolean tco;
 
   /** Function for the next tail call. */
   private XQFunction tcFunc;
@@ -275,7 +272,7 @@ public void compile() throws QueryException {
     run(info.compiling, () -> {
       // assign tail call option after compiling options
       options.compile();
-      maxCalls = context.options.get(MainOptions.TAILCALLS);
+      tco = context.options.get(MainOptions.TAILCALLS) >= 0;
 
       // bind external variables
       if(parent == null) {
@@ -546,11 +543,11 @@ public void set(final Var var, final Value value) throws QueryException {
   /**
    * Registers a tail-called function and its arguments to this query context.
    * @param fn function to call
-   * @param arg arguments to pass to {@code fn}
+   * @param args arguments to pass to {@code fn}
    */
-  public void registerTailCall(final XQFunction fn, final Value[] arg) {
+  public void registerTailCall(final XQFunction fn, final Value[] args) {
     tcFunc = fn;
-    tcArgs = arg;
+    tcArgs = args;
   }
 
   /**
@@ -568,9 +565,9 @@ public XQFunction pollTailCall() {
    * @return argument values if a tail call was registered, {@code null} otherwise
    */
   public Value[] pollTailArgs() {
-    final Value[] as = tcArgs;
+    final Value[] args = tcArgs;
     tcArgs = null;
-    return as;
+    return args;
   }
 
   /**

basex-core/src/main/java/org/basex/query/func/XQFunction.java

@@ -24,17 +24,17 @@ public interface XQFunction extends XQFunctionExpr {
   default Value invoke(final QueryContext qc, final InputInfo info, final Value... args)
       throws QueryException {
 
-    XQFunction fn = this;
-    Value[] values = args;
-    final int fp = qc.stack.enterFrame(fn.stackFrameSize());
+    final int fp = qc.stack.enterFrame(stackFrameSize());
     try {
+      XQFunction fn = this;
+      Value[] values = args;
       while(true) {
         qc.checkStop();
         final Value value = fn.invokeInternal(qc, info, values);
         fn = qc.pollTailCall();
         if(fn == null) return value;
-        qc.stack.reuseFrame(fn.stackFrameSize());
         values = qc.pollTailArgs();
+        qc.stack.reuseFrame(fn.stackFrameSize());
       }
     } catch(final QueryException ex) {
       throw ex.add(info);
@@ -56,21 +56,19 @@ default Value invokeTail(final QueryContext qc, final InputInfo info, final Valu
       throws QueryException {
 
     qc.checkStop();
-    final int calls = qc.tailCalls, max = qc.maxCalls;
-    if(max >= 0 && calls >= max) {
+    final int size = stackFrameSize();
+    if(qc.tco && qc.stack.tco(size)) {
       // too many tail calls on the stack, eliminate them
       qc.registerTailCall(this, args);
       return Empty.VALUE;
     }
 
-    qc.tailCalls++;
-    final int fp = qc.stack.enterFrame(stackFrameSize());
+    final int fp = qc.stack.enterFrame(size);
     try {
       return invokeInternal(qc, info, args);
     } catch(final QueryException ex) {
       throw ex.add(info);
     } finally {
-      qc.tailCalls = calls;
       qc.stack.exitFrame(fp);
     }
   }

basex-core/src/main/java/org/basex/query/var/QueryStack.java

@@ -12,7 +12,7 @@
  */
 public final class QueryStack {
   /** Initial stack size. */
-  private static final int INIT = 1 << 3;
+  private static final int INIT = 1 << 5;
   /** The currently assigned values. */
   private Value[] stack = new Value[INIT];
   /** The currently assigned variables. */
@@ -50,21 +50,30 @@ public void reuseFrame(final int size) {
 
   /**
    * Exits a stack frame and makes all bound variables eligible for garbage collection.
-   * @param frame frame pointer of the underlying stack frame
+   * @param fp frame pointer of the underlying stack frame
    */
-  public void exitFrame(final int frame) {
+  public void exitFrame(final int fp) {
     final int s = start;
     final Value[] stck = stack;
     for(int en = end; --en >= s;) stck[en] = null;
     end = s;
-    start = frame;
+    start = fp;
 
     final int sl = stck.length;
     int len = sl;
     while(len > INIT && sl <= len >> 2) len >>= 1;
     if(len != sl) resize(len);
   }
 
+  /**
+   * Checks if tail calls should be eliminated.
+   * @param size new frame size
+   * @return result of check
+   */
+  public boolean tco(final int size) {
+    return end + (size << 1) > INIT;
+  }
+
   /**
    * Ensures that the query stack has at least the given size.
    * @param size required stack size