Implementing loop inversion

core/src/main/java/org/teavm/common/GraphUtils.java

@@ -240,8 +240,7 @@ public final class GraphUtils {
         int[] set = new int[items.length];
         int sz = 0;
         int last = -1;
-        for (int i = 0; i < items.length; ++i) {
+        for (int item : items) {
-            int item = items[i];
             if (item != last) {
                 set[sz++] = item;
                 last = item;
@@ -252,14 +251,4 @@ public final class GraphUtils {
         }
         return set;
     }
-
-    public static Graph invert(Graph graph) {
-        GraphBuilder graphBuilder = new GraphBuilder(graph.size());
-        for (int node = 0; node < graph.size(); ++node) {
-            for (int pred : graph.incomingEdges(node)) {
-                graphBuilder.addEdge(node, pred);
-            }
-        }
-        return graphBuilder.build();
-    }
 }

core/src/main/java/org/teavm/common/MutableGraphNode.java

@@ -22,14 +22,14 @@ import java.util.*;
  * @author Alexey Andreev
  */
 public class MutableGraphNode {
-    int tag;
+    private int tag;
-    Map<MutableGraphNode, MutableGraphEdge> edges = new HashMap<>();
+    final Map<MutableGraphNode, MutableGraphEdge> edges = new LinkedHashMap<>();
 
     public MutableGraphNode(int tag) {
         this.tag = tag;
     }
 
-    public MutableGraphEdge connect(MutableGraphNode other) {
+    private MutableGraphEdge connect(MutableGraphNode other) {
         MutableGraphEdge edge = edges.get(other);
         if (edge == null) {
             edge = new MutableGraphEdge();

core/src/main/java/org/teavm/model/util/InstructionVariableMapper.java

@@ -15,16 +15,43 @@
  */
 package org.teavm.model.util;
 
+import java.util.function.Function;
+import org.teavm.model.BasicBlock;
+import org.teavm.model.Incoming;
+import org.teavm.model.Instruction;
 import org.teavm.model.InvokeDynamicInstruction;
+import org.teavm.model.Phi;
+import org.teavm.model.TryCatchBlock;
 import org.teavm.model.Variable;
 import org.teavm.model.instructions.*;
 
-/**
+public class InstructionVariableMapper implements InstructionVisitor {
- *
+    private final Function<Variable, Variable> f;
- * @author Alexey Andreev
+
- */
+    public InstructionVariableMapper(Function<Variable, Variable> f) {
-public abstract class InstructionVariableMapper implements InstructionVisitor {
+        this.f = f;
-    protected abstract Variable map(Variable var);
+    }
+
+    public void apply(BasicBlock block) {
+        for (Instruction insn : block.getInstructions()) {
+            insn.acceptVisitor(this);
+        }
+        for (Phi phi : block.getPhis()) {
+            phi.setReceiver(map(phi.getReceiver()));
+            for (Incoming incoming : phi.getIncomings()) {
+                incoming.setValue(map(incoming.getValue()));
+            }
+        }
+        for (TryCatchBlock tryCatch : block.getTryCatchBlocks()) {
+            if (tryCatch.getExceptionVariable() != null) {
+                tryCatch.setExceptionVariable(map(tryCatch.getExceptionVariable()));
+            }
+        }
+    }
+
+    private Variable map(Variable var) {
+        return f.apply(var);
+    }
 
     @Override
     public void visit(EmptyInstruction insn) {
@@ -252,8 +279,4 @@ public abstract class InstructionVariableMapper implements InstructionVisitor {
     public void visit(MonitorExitInstruction insn) {
         insn.setObjectRef(map(insn.getObjectRef()));
     }
-
-
-
-
 }

core/src/main/java/org/teavm/model/util/RegisterAllocator.java

@@ -218,26 +218,11 @@ public class RegisterAllocator {
     }
 
     private void renameVariables(final Program program, final int[] varMap) {
-        InstructionVariableMapper mapper = new InstructionVariableMapper() {
+        InstructionVariableMapper mapper = new InstructionVariableMapper(var ->
-            @Override protected Variable map(Variable var) {
+                program.variableAt(varMap[var.getIndex()]));
-                return program.variableAt(varMap[var.getIndex()]);
-            }
-        };
         for (int i = 0; i < program.basicBlockCount(); ++i) {
             BasicBlock block = program.basicBlockAt(i);
-            for (Instruction insn : block.getInstructions()) {
+            mapper.apply(block);
-                insn.acceptVisitor(mapper);
-            }
-            for (Phi phi : block.getPhis()) {
-                phi.setReceiver(program.variableAt(varMap[phi.getReceiver().getIndex()]));
-                for (Incoming incoming : phi.getIncomings()) {
-                    incoming.setValue(program.variableAt(varMap[incoming.getValue().getIndex()]));
-                }
-            }
-            for (TryCatchBlock tryCatch : block.getTryCatchBlocks()) {
-                tryCatch.setExceptionVariable(program.variableAt(
-                        varMap[tryCatch.getExceptionVariable().getIndex()]));
-            }
         }
         String[][] originalNames = new String[program.variableCount()][];
         for (int i = 0; i < program.variableCount(); ++i) {

core/src/main/java/org/teavm/optimization/LoopInvariantMotion.java

@@ -29,7 +29,6 @@ import org.teavm.model.util.*;
  */
 public class LoopInvariantMotion implements MethodOptimization {
     private int[] preheaders;
-    private Instruction[] constantInstructions;
     private LoopGraph graph;
     private DominatorTree dom;
     private Program program;
@@ -45,7 +44,7 @@ public class LoopInvariantMotion implements MethodOptimization {
         IntegerStack stack = new IntegerStack(graph.size());
         int[] defLocation = new int[program.variableCount()];
         Arrays.fill(defLocation, -1);
-        constantInstructions = new Instruction[program.variableCount()];
+        Instruction[] constantInstructions = new Instruction[program.variableCount()];
         for (int i = 0; i <= method.parameterCount(); ++i) {
             defLocation[i] = 0;
         }
@@ -126,7 +125,8 @@ public class LoopInvariantMotion implements MethodOptimization {
                     }
                 }
                 if (variableMap != null) {
-                    insn.acceptVisitor(new VariableMapperImpl(variableMap));
+                    Variable[] currentVariableMap = variableMap;
+                    insn.acceptVisitor(new InstructionVariableMapper(var -> currentVariableMap[var.getIndex()]));
                 }
                 newInstructions.add(insn);
                 preheaderInstructions.addAll(preheaderInstructions.size() - 1, newInstructions);
@@ -208,21 +208,8 @@ public class LoopInvariantMotion implements MethodOptimization {
         return preheader.getIndex();
     }
 
-    private static class VariableMapperImpl extends InstructionVariableMapper {
-        private Variable[] map;
-
-        public VariableMapperImpl(Variable[] map) {
-            this.map = map;
-        }
-
-        @Override
-        protected Variable map(Variable var) {
-            return map[var.getIndex()];
-        }
-    }
-
     private static class InstructionAnalyzer implements InstructionVisitor {
-        public boolean canMove;
+        boolean canMove;
         public boolean constant;
 
         @Override

core/src/main/java/org/teavm/optimization/LoopInversionImpl.java

@@ -67,14 +67,18 @@ import org.teavm.model.util.ProgramUtils;
  * ```
  *
  * where `condition` is a part of loop that has exits and `body` has no exits.
- * More formally, we define *condition end* as a node that postdominates all loop exits.
+ * More formally, we define *body start candidate* as a node which 1) dominates all of the "tails" (i.e. nodes
- * Therefore, *condition* is a set of nodes of the loop that are postdominated by condition end and
+ * that have edges to loop header), 2) does not dominate loop exits. *Body start* is a body start candidate
- * all remaining nodes are *body*.
+ * that is not dominates by some other body start candidate. If body start does not exits, loop is
+ * not inversible.
+ *
+ * Therefore, *body* is a set of nodes of the loop that are dominated by body start and
+ * all remaining nodes are *condition*.
  */
 class LoopInversionImpl {
-    private Program program;
+    private final Program program;
     private Graph cfg;
-    private DominatorTree pdom;
+    private DominatorTree dom;
     private boolean postponed;
 
     LoopInversionImpl(Program program) {
@@ -85,7 +89,7 @@ class LoopInversionImpl {
         do {
             cfg = ProgramUtils.buildControlFlowGraph(program);
             LoopGraph loopGraph = new LoopGraph(cfg);
-            pdom = GraphUtils.buildDominatorTree(GraphUtils.invert(cfg));
+            dom = GraphUtils.buildDominatorTree(cfg);
             List<LoopWithExits> loops = getLoopsWithExits(loopGraph);
 
             postponed = false;
@@ -147,11 +151,12 @@ class LoopInversionImpl {
         final IntSet nodes = new IntOpenHashSet();
         final IntSet nodesAndCopies = new IntOpenHashSet();
         final IntSet exits = new IntOpenHashSet();
-        int conditionEnd;
+        int bodyStart;
         int copyStart;
         int headCopy;
         final IntIntMap copiedVars = new IntIntOpenHashMap();
         final IntIntMap copiedNodes = new IntIntOpenHashMap();
+        final IntIntMap varDefinitionPoints = new IntIntOpenHashMap();
         boolean shouldSkip;
 
         LoopWithExits(int head, LoopWithExits parent) {
@@ -175,8 +180,7 @@ class LoopInversionImpl {
                 return false;
             }
 
-            findCondition();
+            if (!findCondition() || bodyStart < 0) {
-            if (conditionEnd < 0 || !canInvert()) {
                 return false;
             }
 
@@ -187,37 +191,31 @@ class LoopInversionImpl {
             removeInternalPhiInputsFromCondition();
             removeExternalPhiInputsFromConditionCopy();
             putNewPhis();
+            adjustOutputPhis();
 
             return true;
         }
 
-        private void findCondition() {
+        private boolean findCondition() {
-            IntSet endNodes = new IntOpenHashSet(program.basicBlockCount());
+            IntSet tailNodes = new IntOpenHashSet(program.basicBlockCount());
-            for (int exit : exits.toArray()) {
+            for (int tailCandidate : cfg.incomingEdges(head)) {
-                for (int successor : cfg.outgoingEdges(exit)) {
+                if (nodes.contains(tailCandidate)) {
-                    if (nodes.contains(successor) && successor != head) {
+                    tailNodes.add(tailCandidate);
-                        endNodes.add(successor);
                 }
             }
-            }
-            conditionEnd = pdom.commonDominatorOf(endNodes.toArray());
-        }
 
-        /**
+            bodyStart = dom.commonDominatorOf(tailNodes.toArray());
-         * We can't invert loop if condition has back edges. Indeed, back edges from `if` statement
+            int candidate = bodyStart;
-         * must point inside loop, which makes CFG irreducible.
+            while (bodyStart != head) {
-         */
+                int currentCandidate = candidate;
-        private boolean canInvert() {
+                if (Arrays.stream(exits.toArray()).anyMatch(exit -> dom.dominates(currentCandidate, exit))) {
-            for (int node : nodes.toArray()) {
+                    break;
-                if (pdom.dominates(conditionEnd, node)) {
-                    for (int successor : cfg.outgoingEdges(node)) {
-                        if (successor == head) {
-                            return false;
                 }
+                bodyStart = candidate;
+                candidate = dom.immediateDominatorOf(candidate);
             }
-                }
+
-            }
+            return candidate != bodyStart;
-            return true;
         }
 
         private void collectNodesToCopy() {
@@ -225,7 +223,7 @@ class LoopInversionImpl {
             Arrays.sort(nodes);
             for (int node : nodes) {
                 nodesAndCopies.add(node);
-                if (pdom.dominates(conditionEnd, node)) {
+                if (node == head || (node != bodyStart && !dom.dominates(bodyStart, node))) {
                     int copy = program.createBasicBlock().getIndex();
                     if (head == node) {
                         headCopy = copy;
@@ -245,10 +243,12 @@ class LoopInversionImpl {
                     insn.acceptVisitor(definitionExtractor);
                     for (Variable var : definitionExtractor.getDefinedVariables()) {
                         varsToCopy.add(var.getIndex());
+                        varDefinitionPoints.put(var.getIndex(), node);
                     }
                 }
                 for (Phi phi : block.getPhis()) {
                     varsToCopy.add(phi.getReceiver().getIndex());
+                    varDefinitionPoints.put(phi.getReceiver().getIndex(), node);
                 }
             }
 
@@ -260,12 +260,8 @@ class LoopInversionImpl {
         }
 
         private void copyCondition() {
-            InstructionVariableMapper variableMapper = new InstructionVariableMapper() {
+            InstructionVariableMapper variableMapper = new InstructionVariableMapper(var ->
-                @Override
+                    program.variableAt(copiedVars.getOrDefault(var.getIndex(), var.getIndex())));
-                protected Variable map(Variable var) {
-                    return program.variableAt(copiedVars.getOrDefault(var.getIndex(), var.getIndex()));
-                }
-            };
             BasicBlockMapper blockMapper = new BasicBlockMapper() {
                 @Override
                 protected BasicBlock map(BasicBlock block) {
@@ -298,7 +294,7 @@ class LoopInversionImpl {
                         incomingCopy.setValue(program.variableAt(copiedVars.getOrDefault(value, value)));
                         phiCopy.getIncomings().add(incomingCopy);
                     }
-                    targetBlock.getPhis().add(phi);
+                    targetBlock.getPhis().add(phiCopy);
                 }
 
                 for (TryCatchBlock tryCatch : sourceBlock.getTryCatchBlocks()) {
@@ -358,54 +354,136 @@ class LoopInversionImpl {
             BasicBlock block = program.basicBlockAt(headCopy);
             for (Phi phi : block.getPhis()) {
                 List<Incoming> incomings = phi.getIncomings();
-                for (int i = 0; i < incomings.size(); ++i) {
+                List<Incoming> newIncomings = new ArrayList<>(incomings.size());
-                    Incoming incoming = incomings.get(i);
+                for (Incoming incoming : incomings) {
-                    if (!nodesAndCopies.contains(incoming.getSource().getIndex())) {
+                    if (nodesAndCopies.contains(incoming.getSource().getIndex())) {
-                        incomings.remove(i--);
+                        newIncomings.add(incoming);
+                    } else {
+                        for (int exit : exits.toArray()) {
+                            Incoming newIncoming = new Incoming();
+                            newIncoming.setValue(incoming.getValue());
+                            newIncoming.setSource(program.basicBlockAt(exit));
+                            newIncomings.add(newIncoming);
                         }
                     }
                 }
+                incomings.clear();
+                incomings.addAll(newIncomings);
+            }
         }
 
         /**
-         * Variables defined in condition should be converted to phis in a new loop head (i.e. condition end).
+         * Variables defined in condition should be converted to phis in a new loop head (i.e. body start).
          * Every reference to variable from old condition must be replaced by reference to corresponding phi.
          */
         private void putNewPhis() {
-            BasicBlock head = program.basicBlockAt(conditionEnd);
+            BasicBlock head = program.basicBlockAt(bodyStart);
             IntIntMap phiMap = new IntIntOpenHashMap();
 
             int[] vars = copiedVars.keys().toArray();
             Arrays.sort(vars);
+            List<Phi> phisToAdd = new ArrayList<>();
             for (int var : vars) {
+                int varCopy = copiedVars.get(var);
+
                 Phi phi = new Phi();
                 phi.setReceiver(program.createVariable());
                 phiMap.put(var, phi.getReceiver().getIndex());
-                head.getPhis().add(phi);
+                phisToAdd.add(phi);
+
+                for (int source : cfg.incomingEdges(bodyStart)) {
+                    if (!nodes.contains(source)) {
+                        continue;
+                    }
 
-                for (int source : cfg.incomingEdges(conditionEnd)) {
-                    int inputVar = copiedNodes.containsKey(source) ? var : copiedVars.get(var);
                     Incoming incoming = new Incoming();
-                    incoming.setValue(program.variableAt(inputVar));
+                    incoming.setValue(program.variableAt(var));
                     incoming.setSource(program.basicBlockAt(source));
                     phi.getIncomings().add(incoming);
+
+                    incoming = new Incoming();
+                    incoming.setValue(program.variableAt(varCopy));
+                    incoming.setSource(program.basicBlockAt(copiedNodes.get(source)));
+                    phi.getIncomings().add(incoming);
                 }
             }
 
-            InstructionVariableMapper mapper = new InstructionVariableMapper() {
+            InstructionVariableMapper mapper = new InstructionVariableMapper(var -> {
-                @Override
-                protected Variable map(Variable var) {
                 int index = var.getIndex();
                 return program.variableAt(phiMap.getOrDefault(index, index));
-                }
+            });
-            };
             for (int node : nodes.toArray()) {
-                if (copiedNodes.containsKey(node)) {
+                if (!copiedNodes.containsKey(node)) {
                     BasicBlock block = program.basicBlockAt(node);
-                    for (Instruction instruction : block.getInstructions()) {
+                    mapper.apply(block);
-                        instruction.acceptVisitor(mapper);
+                }
-                    }
+            }
-                }
+
+            head.getPhis().addAll(phisToAdd);
+        }
+
+        private void adjustOutputPhis() {
+            IntIntMap phiMap = new IntIntOpenHashMap();
+            class PhiToAdd {
+                private final Phi phi;
+                private final BasicBlock target;
+                private PhiToAdd(Phi phi, BasicBlock target) {
+                    this.phi = phi;
+                    this.target = target;
+                }
+            }
+            List<PhiToAdd> phis = new ArrayList<>();
+
+            int[] vars = copiedVars.keys().toArray();
+            Arrays.sort(vars);
+            int[] exits = this.exits.toArray();
+            Arrays.sort(exits);
+
+            for (int exit : exits) {
+                for (int var : vars) {
+                    int definedAt = varDefinitionPoints.get(var);
+                    if (!dom.dominates(definedAt, exit)) {
+                        continue;
+                    }
+
+                    int varCopy = copiedVars.get(var);
+                    int copiedAt = copiedNodes.get(definedAt);
+                    for (int successor : cfg.outgoingEdges(exit)) {
+                        if (nodes.contains(successor)) {
+                            continue;
+                        }
+
+                        Phi phi = new Phi();
+                        phi.setReceiver(program.createVariable());
+
+                        Incoming originalInput = new Incoming();
+                        originalInput.setSource(program.basicBlockAt(definedAt));
+                        originalInput.setValue(program.variableAt(var));
+                        phi.getIncomings().add(originalInput);
+
+                        Incoming copyInput = new Incoming();
+                        copyInput.setSource(program.basicBlockAt(copiedAt));
+                        copyInput.setValue(program.variableAt(varCopy));
+                        phi.getIncomings().add(copyInput);
+
+                        phis.add(new PhiToAdd(phi, program.basicBlockAt(successor)));
+                        phiMap.put(var, phi.getReceiver().getIndex());
+                    }
+                }
+            }
+
+            InstructionVariableMapper mapper = new InstructionVariableMapper(var -> {
+                int index = var.getIndex();
+                return program.variableAt(phiMap.getOrDefault(index, index));
+            });
+            for (int i = 0; i < cfg.size(); ++i) {
+                if (!nodes.contains(i)) {
+                    mapper.apply(program.basicBlockAt(i));
+                }
+            }
+
+            for (PhiToAdd phiToAdd : phis) {
+                phiToAdd.target.getPhis().add(phiToAdd.phi);
             }
         }
     }

jso/impl/src/main/java/org/teavm/jso/impl/JSClassProcessor.java

@@ -54,16 +54,13 @@ import org.teavm.model.ClassReader;
 import org.teavm.model.ClassReaderSource;
 import org.teavm.model.ElementModifier;
 import org.teavm.model.FieldHolder;
-import org.teavm.model.Incoming;
 import org.teavm.model.Instruction;
 import org.teavm.model.InstructionLocation;
 import org.teavm.model.MethodDescriptor;
 import org.teavm.model.MethodHolder;
 import org.teavm.model.MethodReader;
 import org.teavm.model.MethodReference;
-import org.teavm.model.Phi;
 import org.teavm.model.Program;
-import org.teavm.model.TryCatchBlock;
 import org.teavm.model.ValueType;
 import org.teavm.model.Variable;
 import org.teavm.model.instructions.AssignInstruction;
@@ -82,15 +79,15 @@ import org.teavm.model.util.ProgramUtils;
  * @author Alexey Andreev
  */
 class JSClassProcessor {
-    private ClassReaderSource classSource;
+    private final ClassReaderSource classSource;
-    private JSBodyRepository repository;
+    private final JSBodyRepository repository;
-    private JavaInvocationProcessor javaInvocationProcessor;
+    private final JavaInvocationProcessor javaInvocationProcessor;
     private Program program;
-    private List<Instruction> replacement = new ArrayList<>();
+    private final List<Instruction> replacement = new ArrayList<>();
-    private JSTypeHelper typeHelper;
+    private final JSTypeHelper typeHelper;
-    private Diagnostics diagnostics;
+    private final Diagnostics diagnostics;
     private int methodIndexGenerator;
-    private Map<MethodReference, MethodReader> overridenMethodCache = new HashMap<>();
+    private final Map<MethodReference, MethodReader> overridenMethodCache = new HashMap<>();
 
     public JSClassProcessor(ClassReaderSource classSource, JSBodyRepository repository, Diagnostics diagnostics) {
         this.classSource = classSource;
@@ -170,32 +167,15 @@ class JSClassProcessor {
                 callerMethod.getModifiers().add(ElementModifier.STATIC);
                 final Program program = ProgramUtils.copy(method.getProgram());
                 program.createVariable();
-                InstructionVariableMapper variableMapper = new InstructionVariableMapper() {
+                InstructionVariableMapper variableMapper = new InstructionVariableMapper(var ->
-                    @Override protected Variable map(Variable var) {
+                         program.variableAt(var.getIndex() + 1));
-                        return program.variableAt(var.getIndex() + 1);
-                    }
-                };
                 for (int i = program.variableCount() - 1; i > 0; --i) {
                     program.variableAt(i).getDebugNames().addAll(program.variableAt(i - 1).getDebugNames());
                     program.variableAt(i - 1).getDebugNames().clear();
                 }
                 for (int i = 0; i < program.basicBlockCount(); ++i) {
                     BasicBlock block = program.basicBlockAt(i);
-                    for (Instruction insn : block.getInstructions()) {
+                    variableMapper.apply(block);
-                        insn.acceptVisitor(variableMapper);
-                    }
-                    for (Phi phi : block.getPhis()) {
-                        phi.setReceiver(program.variableAt(phi.getReceiver().getIndex() + 1));
-                        for (Incoming incoming : phi.getIncomings()) {
-                            incoming.setValue(program.variableAt(incoming.getValue().getIndex() + 1));
-                        }
-                    }
-                    for (TryCatchBlock tryCatch : block.getTryCatchBlocks()) {
-                        if (tryCatch.getExceptionVariable() != null) {
-                            tryCatch.setExceptionVariable(program.variableAt(
-                                    tryCatch.getExceptionVariable().getIndex() + 1));
-                        }
-                    }
                 }
                 callerMethod.setProgram(program);
                 ModelUtils.copyAnnotations(method.getAnnotations(), callerMethod.getAnnotations());
@@ -277,7 +257,7 @@ class JSClassProcessor {
         }
     }
 
-    static ValueType[] getStaticSignature(MethodReference method) {
+    private static ValueType[] getStaticSignature(MethodReference method) {
         ValueType[] signature = method.getSignature();
         ValueType[] staticSignature = new ValueType[signature.length + 1];
         for (int i = 0; i < signature.length; ++i) {
@@ -287,7 +267,7 @@ class JSClassProcessor {
         return staticSignature;
     }
 
-    public void processProgram(MethodHolder methodToProcess) {
+    void processProgram(MethodHolder methodToProcess) {
         program = methodToProcess.getProgram();
         for (int i = 0; i < program.basicBlockCount(); ++i) {
             BasicBlock block = program.basicBlockAt(i);

tests/src/test/java/org/teavm/optimizations/LoopInversionTest.java

@@ -0,0 +1,36 @@
+/*
+ *  Copyright 2016 Alexey Andreev.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.teavm.optimizations;
+
+import static org.junit.Assert.assertEquals;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.teavm.junit.TeaVMTestRunner;
+
+@RunWith(TeaVMTestRunner.class)
+public class LoopInversionTest {
+    @Test
+    public void respectsLoopOutput() {
+        int a = 0;
+        int b = 1;
+        for (int i = 0; i < 10; ++i) {
+            int c = a + b;
+            a = b;
+            b = c;
+        }
+        assertEquals(55, a);
+    }
+}