Fix a error in SamplingCircularQueue found using Valgrind memcheck.

src/circular-queue.cc

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 29 matching lines @@
       chunk_size_in_bytes_(desired_chunk_size_in_bytes / record_size_in_bytes *
                         record_size_in_bytes),
       chunk_size_(chunk_size_in_bytes_ / sizeof(Cell)),
       buffer_size_(chunk_size_ * buffer_size_in_chunks),
       // The distance ensures that producer and consumer never step on
       // each other's chunks and helps eviction of produced data from
       // the CPU cache (having that chunk size is bigger than the cache.)
       producer_consumer_distance_(2 * chunk_size_),
       buffer_(NewArray<Cell>(buffer_size_ + 1)) {
   ASSERT(buffer_size_in_chunks > 2);
-  // Only need to keep the first cell of a chunk clean.
-  for (int i = 0; i < buffer_size_; i += chunk_size_) {
+  // Clean up the whole buffer to avoid encountering a random kEnd
+  // while enqueuing.
+  for (int i = 0; i < buffer_size_; ++i) {
     buffer_[i] = kClear;
   }
   buffer_[buffer_size_] = kEnd;
 
   // Layout producer and consumer position pointers each on their own
   // cache lines to avoid cache lines thrashing due to simultaneous
   // updates of positions by different processor cores.
   const int positions_size =
       RoundUp(1, kProcessorCacheLineSize) +
       RoundUp(static_cast<int>(sizeof(ProducerPosition)),
@@ skipping 50 matching lines @@
 }
 
 
 void SamplingCircularQueue::FlushResidualRecords() {
   // Eliminate producer / consumer distance.
   consumer_pos_->dequeue_chunk_poll_pos = consumer_pos_->dequeue_chunk_pos;
 }
 
 
 } }  // namespace v8::internal