--- /dev/null	2017-01-22 10:16:57.869617664 -0800
+++ new/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.core.common/src/org/graalvm/compiler/core/common/alloc/BiDirectionalTraceBuilder.java	2017-02-15 16:57:00.032421404 -0800
@@ -0,0 +1,165 @@
+/*
+ * Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package org.graalvm.compiler.core.common.alloc;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.BitSet;
+import java.util.Collection;
+import java.util.Deque;
+
+import org.graalvm.compiler.core.common.alloc.TraceBuilderResult.TrivialTracePredicate;
+import org.graalvm.compiler.core.common.cfg.AbstractBlockBase;
+import org.graalvm.compiler.debug.Debug;
+import org.graalvm.compiler.debug.Indent;
+
+/**
+ * Computes traces by selecting the unhandled block with the highest execution frequency and going
+ * in both directions, up and down, as long as possible.
+ */
+public final class BiDirectionalTraceBuilder {
+
+    public static TraceBuilderResult computeTraces(AbstractBlockBase<?> startBlock, AbstractBlockBase<?>[] blocks, TrivialTracePredicate pred) {
+        return new BiDirectionalTraceBuilder(blocks).build(startBlock, blocks, pred);
+    }
+
+    private final Deque<AbstractBlockBase<?>> worklist;
+    private final BitSet processed;
+    private final Trace[] blockToTrace;
+
+    private BiDirectionalTraceBuilder(AbstractBlockBase<?>[] blocks) {
+        processed = new BitSet(blocks.length);
+        worklist = createQueue(blocks);
+        blockToTrace = new Trace[blocks.length];
+    }
+
+    private static Deque<AbstractBlockBase<?>> createQueue(AbstractBlockBase<?>[] blocks) {
+        ArrayList<AbstractBlockBase<?>> queue = new ArrayList<>(Arrays.asList(blocks));
+        queue.sort(BiDirectionalTraceBuilder::compare);
+        return new ArrayDeque<>(queue);
+    }
+
+    private static int compare(AbstractBlockBase<?> a, AbstractBlockBase<?> b) {
+        return Double.compare(b.probability(), a.probability());
+    }
+
+    private boolean processed(AbstractBlockBase<?> b) {
+        return processed.get(b.getId());
+    }
+
+    @SuppressWarnings("try")
+    private TraceBuilderResult build(AbstractBlockBase<?> startBlock, AbstractBlockBase<?>[] blocks, TrivialTracePredicate pred) {
+        try (Indent indent = Debug.logAndIndent("BiDirectionalTraceBuilder: start trace building")) {
+            ArrayList<Trace> traces = buildTraces();
+            assert traces.get(0).getBlocks()[0].equals(startBlock) : "The first traces always contains the start block";
+            return TraceBuilderResult.create(blocks, traces, blockToTrace, pred);
+        }
+    }
+
+    protected ArrayList<Trace> buildTraces() {
+        ArrayList<Trace> traces = new ArrayList<>();
+        // process worklist
+        while (!worklist.isEmpty()) {
+            AbstractBlockBase<?> block = worklist.pollFirst();
+            assert block != null;
+            if (!processed(block)) {
+                Trace trace = new Trace(startTrace(block));
+                for (AbstractBlockBase<?> traceBlock : trace.getBlocks()) {
+                    blockToTrace[traceBlock.getId()] = trace;
+                }
+                trace.setId(traces.size());
+                traces.add(trace);
+            }
+        }
+        return traces;
+    }
+
+    /**
+     * Build a new trace starting at {@code block}.
+     */
+    @SuppressWarnings("try")
+    private Collection<AbstractBlockBase<?>> startTrace(AbstractBlockBase<?> block) {
+        ArrayDeque<AbstractBlockBase<?>> trace = new ArrayDeque<>();
+        try (Indent i = Debug.logAndIndent("StartTrace: %s", block)) {
+            try (Indent indentFront = Debug.logAndIndent("Head:")) {
+                for (AbstractBlockBase<?> currentBlock = block; currentBlock != null; currentBlock = selectPredecessor(currentBlock)) {
+                    addBlockToTrace(currentBlock);
+                    trace.addFirst(currentBlock);
+                }
+            }
+            /* Number head blocks. Can not do this in the loop as we go backwards. */
+            int blockNr = 0;
+            for (AbstractBlockBase<?> b : trace) {
+                b.setLinearScanNumber(blockNr++);
+            }
+
+            try (Indent indentBack = Debug.logAndIndent("Tail:")) {
+                for (AbstractBlockBase<?> currentBlock = selectSuccessor(block); currentBlock != null; currentBlock = selectSuccessor(currentBlock)) {
+                    addBlockToTrace(currentBlock);
+                    trace.addLast(currentBlock);
+                    /* This time we can number the blocks immediately as we go forwards. */
+                    currentBlock.setLinearScanNumber(blockNr++);
+                }
+            }
+        }
+        Debug.log("Trace: %s", trace);
+        return trace;
+    }
+
+    private void addBlockToTrace(AbstractBlockBase<?> currentBlock) {
+        Debug.log("add %s (prob: %f)", currentBlock, currentBlock.probability());
+        processed.set(currentBlock.getId());
+    }
+
+    /**
+     * @return The unprocessed predecessor with the highest probability, or {@code null}.
+     */
+    private AbstractBlockBase<?> selectPredecessor(AbstractBlockBase<?> currentBlock) {
+        AbstractBlockBase<?> next = null;
+        for (AbstractBlockBase<?> pred : currentBlock.getPredecessors()) {
+            if (!processed(pred) && !isBackEdge(pred, currentBlock) && (next == null || pred.probability() > next.probability())) {
+                next = pred;
+            }
+        }
+        return next;
+    }
+
+    private static boolean isBackEdge(AbstractBlockBase<?> from, AbstractBlockBase<?> to) {
+        assert Arrays.asList(from.getSuccessors()).contains(to) : "No edge from " + from + " to " + to;
+        return from.isLoopEnd() && to.isLoopHeader() && from.getLoop().equals(to.getLoop());
+    }
+
+    /**
+     * @return The unprocessed successor with the highest probability, or {@code null}.
+     */
+    private AbstractBlockBase<?> selectSuccessor(AbstractBlockBase<?> currentBlock) {
+        AbstractBlockBase<?> next = null;
+        for (AbstractBlockBase<?> succ : currentBlock.getSuccessors()) {
+            if (!processed(succ) && (next == null || succ.probability() > next.probability())) {
+                next = succ;
+            }
+        }
+        return next;
+    }
+}