heap/MarkStack.cpp

   1 /*
   2  * Copyright (C) 2009, 2011 Apple Inc. All rights reserved.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions
   6  * are met:
   7  * 1. Redistributions of source code must retain the above copyright
   8  *    notice, this list of conditions and the following disclaimer.
   9  * 2. Redistributions in binary form must reproduce the above copyright
  10  *    notice, this list of conditions and the following disclaimer in the
  11  *    documentation and/or other materials provided with the distribution.
  12  *
  13  * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
  14  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  15  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  16  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
  17  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  18  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  19  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  20  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  21  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  22  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  23  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  24  */
  25
  26 #include "config.h"
  27 #include "MarkStack.h"
  28
  29 #include "JSCInlines.h"
  30
  31 namespace JSC {
  32
  33 MarkStackArray::MarkStackArray(BlockAllocator& blockAllocator)
  34     : GCSegmentedArray<const JSCell*>(blockAllocator)
  35 {
  36 }
  37
  38 void MarkStackArray::donateSomeCellsTo(MarkStackArray& other)
  39 {
  40     // Try to donate about 1 / 2 of our cells. To reduce copying costs,
  41     // we prefer donating whole segments over donating individual cells,
  42     // even if this skews away from our 1 / 2 target.
  43
  44     size_t segmentsToDonate = m_numberOfSegments / 2; // If we only have one segment (our head) we don't donate any segments.
  45
  46     if (!segmentsToDonate) {
  47         size_t cellsToDonate = m_top / 2; // Round down to donate 0 / 1 cells.
  48         while (cellsToDonate--) {
  49             ASSERT(m_top);
  50             other.append(removeLast());
  51         }
  52         return;
  53     }
  54
  55     validatePrevious();
  56     other.validatePrevious();
  57
  58     // Remove our head and the head of the other list before we start moving segments around.
  59     // We'll add them back on once we're done donating.
  60     GCArraySegment<const JSCell*>* myHead = m_segments.removeHead();
  61     GCArraySegment<const JSCell*>* otherHead = other.m_segments.removeHead();
  62
  63     while (segmentsToDonate--) {
  64         GCArraySegment<const JSCell*>* current = m_segments.removeHead();
  65         ASSERT(current);
  66         ASSERT(m_numberOfSegments > 1);
  67         other.m_segments.push(current);
  68         m_numberOfSegments--;
  69         other.m_numberOfSegments++;
  70     }
  71
  72     // Put the original heads back in their places.
  73     m_segments.push(myHead);
  74     other.m_segments.push(otherHead);
  75
  76     validatePrevious();
  77     other.validatePrevious();
  78 }
  79
  80 void MarkStackArray::stealSomeCellsFrom(MarkStackArray& other, size_t idleThreadCount)
  81 {
  82     // Try to steal 1 / Nth of the shared array, where N is the number of idle threads.
  83     // To reduce copying costs, we prefer stealing a whole segment over stealing
  84     // individual cells, even if this skews away from our 1 / N target.
  85
  86     validatePrevious();
  87     other.validatePrevious();
  88
  89     // If other has an entire segment, steal it and return.
  90     if (other.m_numberOfSegments > 1) {
  91         // Move the heads of the lists aside. We'll push them back on after.
  92         GCArraySegment<const JSCell*>* otherHead = other.m_segments.removeHead();
  93         GCArraySegment<const JSCell*>* myHead = m_segments.removeHead();
  94
  95         ASSERT(other.m_segments.head()->m_top == s_segmentCapacity);
  96
  97         m_segments.push(other.m_segments.removeHead());
  98
  99         m_numberOfSegments++;
 100         other.m_numberOfSegments--;
 101
 102         m_segments.push(myHead);
 103         other.m_segments.push(otherHead);
 104
 105         validatePrevious();
 106         other.validatePrevious();
 107         return;
 108     }
 109
 110     size_t numberOfCellsToSteal = (other.size() + idleThreadCount - 1) / idleThreadCount; // Round up to steal 1 / 1.
 111     while (numberOfCellsToSteal-- > 0 && other.canRemoveLast())
 112         append(other.removeLast());
 113 }
 114
 115 } // namespace JSC