Fix store buffer ensure space heuristics.

src/store-buffer.cc

Index: src/store-buffer.cc
diff --git a/src/store-buffer.cc b/src/store-buffer.cc
index 7d73dd5ed1be27464640752efc5eb5990a749b4f..1137efb54efe78993c316fd8971705c95e1aacc8 100644
--- a/src/store-buffer.cc
+++ b/src/store-buffer.cc
@@ -142,6 +142,11 @@ void StoreBuffer::Uniq() {
 }
 
 
+bool StoreBuffer::SpaceAvailable(intptr_t space_needed) {
+  return old_limit_ - old_top_ >= space_needed;
+}
+
+
 void StoreBuffer::EnsureSpace(intptr_t space_needed) {
   while (old_limit_ - old_top_ < space_needed &&
          old_limit_ < old_reserved_limit_) {
@@ -152,7 +157,7 @@ void StoreBuffer::EnsureSpace(intptr_t space_needed) {
     old_limit_ += grow;
   }
 
-  if (old_limit_ - old_top_ >= space_needed) return;
+  if (SpaceAvailable(space_needed)) return;
 
   if (old_buffer_is_filtered_) return;
   ASSERT(may_move_store_buffer_entries_);
@@ -171,10 +176,7 @@ void StoreBuffer::EnsureSpace(intptr_t space_needed) {
     Filter(MemoryChunk::SCAN_ON_SCAVENGE);
   }
 
-  // If filtering out the entries from scan_on_scavenge pages got us down to
-  // less than half full, then we are satisfied with that.
-  if (old_limit_ - old_top_ > old_top_ - old_start_) return;
-
+  if (SpaceAvailable(space_needed)) return;
   // Sample 1 entry in 97 and filter out the pages where we estimate that more
   // than 1 in 8 pointers are to new space.
   static const int kSampleFinenesses = 5;
@@ -188,11 +190,11 @@ void StoreBuffer::EnsureSpace(intptr_t space_needed) {
     { 3, ((Page::kPageSize / kPointerSize) / 3) / 256 },
     { 1, 0}
   };
-  for (int i = kSampleFinenesses - 1; i >= 0; i--) {
+  for (int i = 0; i < kSampleFinenesses; i++) {
     ExemptPopularPages(samples[i].prime_sample_step, samples[i].threshold);
     // As a last resort we mark all pages as being exempt from the store buffer.
     ASSERT(i != 0 || old_top_ == old_start_);
-    if (old_limit_ - old_top_ > old_top_ - old_start_) return;
+    if (SpaceAvailable(space_needed)) return;
   }
   UNREACHABLE();
 }