# HG changeset patch
# User Christian Wimmer <christian.wimmer@oracle.com>
# Date 1395348119 25200
# Node ID 07ce8ff3603d66d2fd9de3a75f8dcd5f003ccf08
# Parent  ab8a5b82fe7335e95905a0ed5922b020db143af7
Reduce unncessary list allocations in register allocator

diff -r ab8a5b82fe73 -r 07ce8ff3603d graal/com.oracle.graal.compiler/src/com/oracle/graal/compiler/alloc/LinearScanWalker.java
--- a/graal/com.oracle.graal.compiler/src/com/oracle/graal/compiler/alloc/LinearScanWalker.java	Thu Mar 20 20:40:11 2014 +0100
+++ b/graal/com.oracle.graal.compiler/src/com/oracle/graal/compiler/alloc/LinearScanWalker.java	Thu Mar 20 13:41:59 2014 -0700
@@ -53,6 +53,14 @@
 
     private MoveResolver moveResolver; // for ordering spill moves
 
+    /**
+     * Only 10% of the lists in {@link #spillIntervals} are actually used. But when they are used,
+     * they can grow quite long. The maximum length observed was 45 (all numbers taken from a
+     * bootstrap run of Graal). Therefore, we initialize {@link #spillIntervals} with this marker
+     * value, and allocate a "real" list only on demand in {@link #setUsePos}.
+     */
+    private static final List<Interval> EMPTY_LIST = new ArrayList<>(0);
+
     // accessors mapped to same functions in class LinearScan
     int blockCount() {
         return allocator.blockCount();
@@ -76,7 +84,7 @@
         moveResolver = new MoveResolver(allocator);
         spillIntervals = Util.uncheckedCast(new List[allocator.registers.length]);
         for (int i = 0; i < allocator.registers.length; i++) {
-            spillIntervals[i] = new ArrayList<>(2);
+            spillIntervals[i] = EMPTY_LIST;
         }
         usePos = new int[allocator.registers.length];
         blockPos = new int[allocator.registers.length];
@@ -111,7 +119,12 @@
                     this.usePos[i] = usePos;
                 }
                 if (!onlyProcessUsePos) {
-                    spillIntervals[i].add(interval);
+                    List<Interval> list = spillIntervals[i];
+                    if (list == EMPTY_LIST) {
+                        list = new ArrayList<>(2);
+                        spillIntervals[i] = list;
+                    }
+                    list.add(interval);
                 }
             }
         }