Improve the performance of bit set searches with a compiler intrinsic.

runtime/vm/freelist.cc

Index: runtime/vm/freelist.cc
diff --git a/runtime/vm/freelist.cc b/runtime/vm/freelist.cc
index 391dd22bcde0decd7b8f4f013a7e948e66aed057..ffc88aeaf5a4d441f8f51cf996b55c5cb6a89cd4 100644
--- a/runtime/vm/freelist.cc
+++ b/runtime/vm/freelist.cc
@@ -54,17 +54,17 @@ uword FreeList::TryAllocate(intptr_t size) {
     return reinterpret_cast<uword>(DequeueElement(index));
   }
 
-  if (index < kNumLists) {
-    index++;
-    while (index < kNumLists) {
-      if (free_map_.Test(index)) {
-        // Dequeue an element from the list, split and enqueue the remainder in
-        // the appropriate list.
-        FreeListElement* element = DequeueElement(index);
-        SplitElementAfterAndEnqueue(element, size);
-        return reinterpret_cast<uword>(element);
-      }
-      index++;
+  if ((index + 1) < kNumLists) {
+    intptr_t next_index = free_map_.Next(index + 1);
+    if (next_index == kNumLists) {
+      next_index = -1;
+    }

[siva, 2012/07/24 01:07:43]

per offline discussion this check could go away if you set up the freelist bit
set to be just 128 long.

+    if (next_index != -1) {
+      // Dequeue an element from the list, split and enqueue the remainder in
+      // the appropriate list.
+      FreeListElement* element = DequeueElement(next_index);
+      SplitElementAfterAndEnqueue(element, size);
+      return reinterpret_cast<uword>(element);
     }
   }