Fix transferal of marking bits on array trimming.

R=vegorov@chromium.org
BUG=v8:1708
TEST=mjsunit/regress/regress-1708

Review URL: http://codereview.chromium.org/7979038

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9394 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mark-compact.cc

@@ -382,41 +382,40 @@ bool Marking::TransferMark(Address old_start, Address new_start) {
   // This is only used when resizing an object.
   ASSERT(MemoryChunk::FromAddress(old_start) ==
          MemoryChunk::FromAddress(new_start));
+
   // If the mark doesn't move, we don't check the color of the object.
   // It doesn't matter whether the object is black, since it hasn't changed
   // size, so the adjustment to the live data count will be zero anyway.
   if (old_start == new_start) return false;
 
   MarkBit new_mark_bit = MarkBitFrom(new_start);
+  MarkBit old_mark_bit = MarkBitFrom(old_start);
 
-  if (heap_->incremental_marking()->IsMarking()) {
-    MarkBit old_mark_bit = MarkBitFrom(old_start);
 #ifdef DEBUG
-    ObjectColor old_color = Color(old_mark_bit);
+  ObjectColor old_color = Color(old_mark_bit);
 #endif
-    if (Marking::IsBlack(old_mark_bit)) {
-      Marking::MarkBlack(new_mark_bit);
-      old_mark_bit.Clear();
-      return true;
-    } else if (Marking::IsGrey(old_mark_bit)) {
-      old_mark_bit.Next().Clear();
-      heap_->incremental_marking()->WhiteToGreyAndPush(
-          HeapObject::FromAddress(new_start), new_mark_bit);
-      heap_->incremental_marking()->RestartIfNotMarking();
-    }
+
+  if (Marking::IsBlack(old_mark_bit)) {
+    old_mark_bit.Clear();
+    ASSERT(IsWhite(old_mark_bit));
+    Marking::MarkBlack(new_mark_bit);
+    return true;
+  } else if (Marking::IsGrey(old_mark_bit)) {
+    ASSERT(heap_->incremental_marking()->IsMarking());
+    old_mark_bit.Clear();
+    old_mark_bit.Next().Clear();
+    ASSERT(IsWhite(old_mark_bit));
+    heap_->incremental_marking()->WhiteToGreyAndPush(
+        HeapObject::FromAddress(new_start), new_mark_bit);
+    heap_->incremental_marking()->RestartIfNotMarking();
+  }
 
 #ifdef DEBUG
-    ObjectColor new_color = Color(new_mark_bit);
-    ASSERT(new_color == old_color);
+  ObjectColor new_color = Color(new_mark_bit);
+  ASSERT(new_color == old_color);
 #endif
-    return false;
-  }
-  MarkBit old_mark_bit = MarkBitFrom(old_start);
-  if (!old_mark_bit.Get()) {
-    return false;
-  }
-  new_mark_bit.Set();
-  return true;
+
+  return false;
 }
 
 
@@ -3254,6 +3253,9 @@ static inline Address DigestFreeStart(Address approximate_free_start,
                                       uint32_t free_start_cell) {
   ASSERT(free_start_cell != 0);
 
+  // No consecutive 1 bits.
+  ASSERT((free_start_cell & (free_start_cell << 1)) == 0);
+
   int offsets[16];
   uint32_t cell = free_start_cell;
   int offset_of_last_live;
@@ -3284,6 +3286,9 @@ static inline Address DigestFreeStart(Address approximate_free_start,
 static inline Address StartOfLiveObject(Address block_address, uint32_t cell) {
   ASSERT(cell != 0);
 
+  // No consecutive 1 bits.
+  ASSERT((cell & (cell << 1)) == 0);
+
   int offsets[16];
   if (cell == 0x80000000u) {  // Avoid overflow below.
     return block_address + 31 * kPointerSize;

test/mjsunit/regress/regress-1708.js

+// Copyright 2011 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
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Regression test of a very rare corner case where left-trimming an
+// array caused invalid marking bit patterns on lazily swept pages.
+
+// Flags: --expose-gc --noincremental-marking --max-new-space-size 1000
+
+(function() {
+  var head = new Array(1);
+  var tail = head;
+
+  // Fill heap to increase old-space size and trigger lazy sweeping on
+  // some of the old-space pages.
+  for (var i = 0; i < 200; i++) {
+    tail[1] = new Array(1000);
+    tail = tail[1];
+  }
+  array = new Array(100);
+  gc(); gc();
+
+  // At this point "array" should have been promoted to old-space and be
+  // located in a lazy swept page with intact marking bits. Now shift
+  // the array to trigger left-trimming operations.
+  assertEquals(100, array.length);
+  for (var i = 0; i < 50; i++) {
+    array.shift();
+  }
+  assertEquals(50, array.length);
+
+  // At this point "array" should have been trimmed from the left with
+  // marking bits being correctly transfered to the new object start.
+  // Scavenging operations cause lazy sweeping to advance and verify
+  // that marking bit patterns are still sane.
+  for (var i = 0; i < 200; i++) {
+    tail[1] = new Array(1000);
+    tail = tail[1];
+  }
+})();