--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/graal/com.oracle.graal.compiler.common/src/com/oracle/graal/compiler/common/alloc/BiDirectionalTraceBuilder.java	Wed Jan 20 12:03:05 2016 +0100
@@ -0,0 +1,161 @@
+/*
+ * 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 com.oracle.graal.compiler.common.alloc;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.BitSet;
+import java.util.Collection;
+import java.util.Deque;
+import java.util.List;
+
+import com.oracle.graal.compiler.common.cfg.AbstractBlockBase;
+import com.oracle.graal.debug.Debug;
+import com.oracle.graal.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<T extends AbstractBlockBase<T>> {
+
+    public static <T extends AbstractBlockBase<T>> TraceBuilderResult<T> computeTraces(T startBlock, List<T> blocks) {
+        return new BiDirectionalTraceBuilder<>(blocks).build(startBlock);
+    }
+
+    private final Deque<T> worklist;
+    private final BitSet processed;
+    private final int[] blockToTrace;
+
+    private BiDirectionalTraceBuilder(List<T> blocks) {
+        processed = new BitSet(blocks.size());
+        worklist = createQueue(blocks);
+        blockToTrace = new int[blocks.size()];
+    }
+
+    private static <T extends AbstractBlockBase<T>> Deque<T> createQueue(List<T> blocks) {
+        ArrayList<T> queue = new ArrayList<>(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(T b) {
+        return processed.get(b.getId());
+    }
+
+    @SuppressWarnings("try")
+    private TraceBuilderResult<T> build(T startBlock) {
+        try (Indent indent = Debug.logAndIndent("start trace building: " + startBlock)) {
+            ArrayList<Trace<T>> traces = buildTraces(startBlock);
+            return new TraceBuilderResult<>(traces, blockToTrace);
+        }
+    }
+
+    protected ArrayList<Trace<T>> buildTraces(T startBlock) {
+        ArrayList<Trace<T>> traces = new ArrayList<>();
+        // add start block
+        worklist.add(startBlock);
+        // process worklist
+        while (!worklist.isEmpty()) {
+            T block = worklist.pollFirst();
+            assert block != null;
+            if (!processed(block)) {
+                traces.add(new Trace<>(startTrace(block, traces.size())));
+            }
+        }
+        return traces;
+    }
+
+    /**
+     * Build a new trace starting at {@code block}.
+     */
+    @SuppressWarnings("try")
+    private Collection<T> startTrace(T block, int traceNumber) {
+        ArrayDeque<T> trace = new ArrayDeque<>();
+        try (Indent i = Debug.logAndIndent("StartTrace: " + block)) {
+            try (Indent indentFront = Debug.logAndIndent("Head:")) {
+                for (T currentBlock = block; currentBlock != null; currentBlock = selectPredecessor(currentBlock)) {
+                    addBlockToTrace(currentBlock, traceNumber);
+                    trace.addFirst(currentBlock);
+                }
+            }
+            /* Number head blocks. Can not do this in the loop as we go backwards. */
+            int blockNr = 0;
+            for (T b : trace) {
+                b.setLinearScanNumber(blockNr++);
+            }
+
+            try (Indent indentBack = Debug.logAndIndent("Tail:")) {
+                for (T currentBlock = selectSuccessor(block); currentBlock != null; currentBlock = selectSuccessor(currentBlock)) {
+                    addBlockToTrace(currentBlock, traceNumber);
+                    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(T currentBlock, int traceNumber) {
+        Debug.log("add %s (prob: %f)", currentBlock, currentBlock.probability());
+        processed.set(currentBlock.getId());
+        blockToTrace[currentBlock.getId()] = traceNumber;
+    }
+
+    /**
+     * @return The unprocessed predecessor with the highest probability, or {@code null}.
+     */
+    private T selectPredecessor(T currentBlock) {
+        T next = null;
+        for (T pred : currentBlock.getPredecessors()) {
+            if (!processed(pred) && !isBackEdge(pred, currentBlock) && (next == null || pred.probability() > next.probability())) {
+                next = pred;
+            }
+        }
+        return next;
+    }
+
+    private boolean isBackEdge(T from, T to) {
+        assert 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 T selectSuccessor(T currentBlock) {
+        T next = null;
+        for (T succ : currentBlock.getSuccessors()) {
+            if (!processed(succ) && (next == null || succ.probability() > next.probability())) {
+                next = succ;
+            }
+        }
+        return next;
+    }
+}

--- a/graal/com.oracle.graal.lir/src/com/oracle/graal/lir/alloc/trace/TraceBuilderPhase.java	Wed Jan 20 12:12:37 2016 +0100
+++ b/graal/com.oracle.graal.lir/src/com/oracle/graal/lir/alloc/trace/TraceBuilderPhase.java	Wed Jan 20 12:03:05 2016 +0100
@@ -24,17 +24,28 @@
 
 import java.util.List;
 
+import com.oracle.graal.compiler.common.alloc.UniDirectionalTraceBuilder;
+import com.oracle.graal.compiler.common.alloc.BiDirectionalTraceBuilder;
 import com.oracle.graal.compiler.common.alloc.TraceBuilderResult;
-import com.oracle.graal.compiler.common.alloc.UniDirectionalTraceBuilder;
 import com.oracle.graal.compiler.common.cfg.AbstractBlockBase;
 import com.oracle.graal.lir.LIR;
 import com.oracle.graal.lir.gen.LIRGenerationResult;
 import com.oracle.graal.lir.phases.LIRPhase;
+import com.oracle.graal.options.Option;
+import com.oracle.graal.options.OptionType;
+import com.oracle.graal.options.OptionValue;
 
 import jdk.vm.ci.code.TargetDescription;
 
 public class TraceBuilderPhase extends LIRPhase<TraceBuilderPhase.TraceBuilderContext> {
 
+    public static class Options {
+        // @formatter:off
+        @Option(help = "", type = OptionType.Debug)
+        public static final OptionValue<Boolean> TraceRAbiDirectionalTraceBuilder = new OptionValue<>(false);
+        // @formatter:on
+    }
+
     public static final class TraceBuilderContext {
         public TraceBuilderResult<?> traceBuilderResult;
 
@@ -45,7 +56,11 @@
         B startBlock = linearScanOrder.get(0);
         LIR lir = lirGenRes.getLIR();
         assert startBlock.equals(lir.getControlFlowGraph().getStartBlock());
-        context.traceBuilderResult = UniDirectionalTraceBuilder.computeTraces(startBlock, linearScanOrder);
+        if (Options.TraceRAbiDirectionalTraceBuilder.getValue()) {
+            context.traceBuilderResult = BiDirectionalTraceBuilder.computeTraces(startBlock, linearScanOrder);
+        } else {
+            context.traceBuilderResult = UniDirectionalTraceBuilder.computeTraces(startBlock, linearScanOrder);
+        }
 
     }