--- /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/UniDirectionalTraceBuilder.java	2017-02-15 16:57:02.623533028 -0800
@@ -0,0 +1,160 @@
+/*
+ * Copyright (c) 2015, 2015, 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.ArrayList;
+import java.util.BitSet;
+import java.util.List;
+import java.util.PriorityQueue;
+
+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 starting at a trace head and keep adding predecessors as long as possible.
+ */
+public final class UniDirectionalTraceBuilder {
+
+    public static TraceBuilderResult computeTraces(AbstractBlockBase<?> startBlock, AbstractBlockBase<?>[] blocks, TrivialTracePredicate pred) {
+        return new UniDirectionalTraceBuilder(blocks).build(startBlock, blocks, pred);
+    }
+
+    private final PriorityQueue<AbstractBlockBase<?>> worklist;
+    private final BitSet processed;
+    /**
+     * Contains the number of unprocessed predecessors for every {@link AbstractBlockBase#getId()
+     * block}.
+     */
+    private final int[] blocked;
+    private final Trace[] blockToTrace;
+
+    private UniDirectionalTraceBuilder(AbstractBlockBase<?>[] blocks) {
+        processed = new BitSet(blocks.length);
+        worklist = new PriorityQueue<>(UniDirectionalTraceBuilder::compare);
+        assert (worklist != null);
+
+        blocked = new int[blocks.length];
+        blockToTrace = new Trace[blocks.length];
+        for (AbstractBlockBase<?> block : blocks) {
+            blocked[block.getId()] = block.getPredecessorCount();
+        }
+    }
+
+    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("UniDirectionalTraceBuilder: start trace building: %s", startBlock)) {
+            ArrayList<Trace> traces = buildTraces(startBlock);
+            return TraceBuilderResult.create(blocks, traces, blockToTrace, pred);
+        }
+    }
+
+    protected ArrayList<Trace> buildTraces(AbstractBlockBase<?> startBlock) {
+        ArrayList<Trace> traces = new ArrayList<>();
+        // add start block
+        worklist.add(startBlock);
+        // process worklist
+        while (!worklist.isEmpty()) {
+            AbstractBlockBase<?> block = worklist.poll();
+            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 List<AbstractBlockBase<?>> startTrace(AbstractBlockBase<?> block) {
+        assert checkPredecessorsProcessed(block);
+        ArrayList<AbstractBlockBase<?>> trace = new ArrayList<>();
+        int blockNumber = 0;
+        try (Indent i = Debug.logAndIndent("StartTrace: %s", block)) {
+            for (AbstractBlockBase<?> currentBlock = block; currentBlock != null; currentBlock = selectNext(currentBlock)) {
+                Debug.log("add %s (prob: %f)", currentBlock, currentBlock.probability());
+                processed.set(currentBlock.getId());
+                trace.add(currentBlock);
+                unblock(currentBlock);
+                currentBlock.setLinearScanNumber(blockNumber++);
+            }
+        }
+        return trace;
+    }
+
+    private boolean checkPredecessorsProcessed(AbstractBlockBase<?> block) {
+        for (AbstractBlockBase<?> pred : block.getPredecessors()) {
+            if (!processed(pred)) {
+                assert false : "Predecessor unscheduled: " + pred;
+                return false;
+            }
+
+        }
+        return true;
+    }
+
+    /**
+     * Decrease the {@link #blocked} count for all predecessors and add them to the worklist once
+     * the count reaches 0.
+     */
+    private void unblock(AbstractBlockBase<?> currentBlock) {
+        for (AbstractBlockBase<?> successor : currentBlock.getSuccessors()) {
+            if (!processed(successor)) {
+                int blockCount = --blocked[successor.getId()];
+                assert blockCount >= 0;
+                if (blockCount == 0) {
+                    worklist.add(successor);
+                }
+            }
+        }
+    }
+
+    /**
+     * @return The unprocessed predecessor with the highest probability, or {@code null}.
+     */
+    private AbstractBlockBase<?> selectNext(AbstractBlockBase<?> currentBlock) {
+        AbstractBlockBase<?> next = null;
+        for (AbstractBlockBase<?> succ : currentBlock.getSuccessors()) {
+            if (!processed(succ) && (next == null || succ.probability() > next.probability())) {
+                next = succ;
+            }
+        }
+        return next;
+    }
+}