--- jsr166/src/main/java/util/ArrayDeque.java 2016/11/05 17:45:44 1.110 +++ jsr166/src/main/java/util/ArrayDeque.java 2016/11/20 07:24:11 1.118 @@ -68,13 +68,15 @@ public class ArrayDeque extends Abstr * * Because in a circular array, elements are in general stored in * two disjoint such slices, we help the VM by writing unusual - * nested loops for all traversals over the elements. + * nested loops for all traversals over the elements. Having only + * one hot inner loop body instead of two or three eases human + * maintenance and encourages VM loop inlining into the caller. */ /** * The array in which the elements of the deque are stored. - * We guarantee that all array cells not holding deque elements - * are always null. + * All array cells not holding deque elements are always null. + * The array always has at least one null slot (at tail). */ transient Object[] elements; @@ -88,7 +90,8 @@ public class ArrayDeque extends Abstr /** * The index at which the next element would be added to the tail - * of the deque (via addLast(E), add(E), or push(E)). + * of the deque (via addLast(E), add(E), or push(E)); + * elements[tail] is always null. */ transient int tail; @@ -187,7 +190,10 @@ public class ArrayDeque extends Abstr * @param numElements lower bound on initial capacity of the deque */ public ArrayDeque(int numElements) { - elements = new Object[Math.max(1, numElements + 1)]; + elements = + new Object[(numElements < 1) ? 1 : + (numElements == Integer.MAX_VALUE) ? Integer.MAX_VALUE : + numElements + 1]; } /** @@ -201,7 +207,7 @@ public class ArrayDeque extends Abstr * @throws NullPointerException if the specified collection is null */ public ArrayDeque(Collection c) { - elements = new Object[c.size() + 1]; + this(c.size()); addAll(c); } @@ -224,19 +230,21 @@ public class ArrayDeque extends Abstr } /** - * Adds i and j, mod modulus. - * Precondition and postcondition: 0 <= i < modulus, 0 <= j <= modulus. + * Circularly adds the given distance to index i, mod modulus. + * Precondition: 0 <= i < modulus, 0 <= distance <= modulus. + * @return index 0 <= i < modulus */ - static final int add(int i, int j, int modulus) { - if ((i += j) - modulus >= 0) i -= modulus; + static final int add(int i, int distance, int modulus) { + if ((i += distance) - modulus >= 0) distance -= modulus; return i; } /** * Subtracts j from i, mod modulus. - * Index i must be logically ahead of j. - * Returns the "circular distance" from j to i. - * Precondition and postcondition: 0 <= i < modulus, 0 <= j < modulus. + * Index i must be logically ahead of index j. + * Precondition: 0 <= i < modulus, 0 <= j < modulus. + * @return the "circular distance" from j to i; corner case i == j + * is diambiguated to "empty", returning 0. */ static final int sub(int i, int j, int modulus) { if ((i -= j) < 0) i += modulus; @@ -314,10 +322,10 @@ public class ArrayDeque extends Abstr * of its elements are null */ public boolean addAll(Collection c) { - final int s = size(), needed; - if ((needed = s + c.size() - elements.length + 1) > 0) + final int s, needed; + if ((needed = (s = size()) + c.size() + 1 - elements.length) > 0) grow(needed); - c.forEach((e) -> addLast(e)); + c.forEach(this::addLast); // checkInvariants(); return size() > s; } @@ -608,10 +616,11 @@ public class ArrayDeque extends Abstr // checkInvariants(); final Object[] es = elements; final int capacity = es.length; - final int h = head; + final int h, t; // number of elements before to-be-deleted elt - final int front = sub(i, h, capacity); - final int back = size() - front - 1; // number of elements after + final int front = sub(i, h = head, capacity); + // number of elements after to-be-deleted elt + final int back = sub(t = tail, i, capacity) - 1; if (front < back) { // move front elements forwards if (h <= i) { @@ -627,14 +636,13 @@ public class ArrayDeque extends Abstr return false; } else { // move back elements backwards - tail = dec(tail, capacity); + tail = dec(t, capacity); if (i <= tail) { System.arraycopy(es, i + 1, es, i, back); } else { // Wrap around - int firstLeg = capacity - (i + 1); - System.arraycopy(es, i + 1, es, i, firstLeg); + System.arraycopy(es, i + 1, es, i, capacity - (i + 1)); es[capacity - 1] = es[0]; - System.arraycopy(es, 1, es, 0, back - firstLeg - 1); + System.arraycopy(es, 1, es, 0, t - 1); } es[tail] = null; // checkInvariants(); @@ -779,7 +787,7 @@ public class ArrayDeque extends Abstr if (to == end) { if (end != head) throw new ConcurrentModificationException(); - lastRet = head; + lastRet = end; break; } } @@ -858,13 +866,12 @@ public class ArrayDeque extends Abstr public boolean tryAdvance(Consumer action) { if (action == null) throw new NullPointerException(); - int t, i; - if ((t = fence) < 0) t = getFence(); - if (t == (i = cursor)) + final int t, i; + if ((t = getFence()) == (i = cursor)) return false; - final Object[] es; - action.accept(nonNullElementAt(es = elements, i)); + final Object[] es = elements; cursor = inc(i, es.length); + action.accept(nonNullElementAt(es, i)); return true; } @@ -950,7 +957,7 @@ public class ArrayDeque extends Abstr ; i = 0, to = end) { for (; i < to; i++) if (filter.test(elementAt(es, i))) - return bulkRemoveModified(filter, i, to); + return bulkRemoveModified(filter, i); if (to == end) { if (end != tail) throw new ConcurrentModificationException(); break; @@ -959,45 +966,62 @@ public class ArrayDeque extends Abstr return false; } + // A tiny bit set implementation + + private static long[] nBits(int n) { + return new long[((n - 1) >> 6) + 1]; + } + private static void setBit(long[] bits, int i) { + bits[i >> 6] |= 1L << i; + } + private static boolean isClear(long[] bits, int i) { + return (bits[i >> 6] & (1L << i)) == 0; + } + /** * Helper for bulkRemove, in case of at least one deletion. - * @param i valid index of first element to be deleted + * Tolerate predicates that reentrantly access the collection for + * read (but writers still get CME), so traverse once to find + * elements to delete, a second pass to physically expunge. + * + * @param beg valid index of first element to be deleted */ private boolean bulkRemoveModified( - Predicate filter, int i, int to) { + Predicate filter, final int beg) { final Object[] es = elements; final int capacity = es.length; - // a two-finger algorithm, with hare i reading, tortoise j writing - int j = i++; final int end = tail; - try { - for (;; j = 0) { // j rejoins i on second leg - E e; - // In this loop, i and j are on the same leg, with i > j - for (; i < to; i++) - if (!filter.test(e = elementAt(es, i))) - es[j++] = e; - if (to == end) break; - // In this loop, j is on the first leg, i on the second - for (i = 0, to = end; i < to && j < capacity; i++) - if (!filter.test(e = elementAt(es, i))) - es[j++] = e; - if (i >= to) { - if (j == capacity) j = 0; // "corner" case - break; - } + final long[] deathRow = nBits(sub(end, beg, capacity)); + deathRow[0] = 1L; // set bit 0 + for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg; + ; i = 0, to = end, k -= capacity) { + for (; i < to; i++) + if (filter.test(elementAt(es, i))) + setBit(deathRow, i - k); + if (to == end) break; + } + // a two-finger traversal, with hare i reading, tortoise w writing + int w = beg; + for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg; + ; w = 0) { // w rejoins i on second leg + // In this loop, i and w are on the same leg, with i > w + for (; i < to; i++) + if (isClear(deathRow, i - k)) + es[w++] = es[i]; + if (to == end) break; + // In this loop, w is on the first leg, i on the second + for (i = 0, to = end, k -= capacity; i < to && w < capacity; i++) + if (isClear(deathRow, i - k)) + es[w++] = es[i]; + if (i >= to) { + if (w == capacity) w = 0; // "corner" case + break; } - return true; - } catch (Throwable ex) { - // copy remaining elements - for (; i != end; i = inc(i, capacity), j = inc(j, capacity)) - es[j] = es[i]; - throw ex; - } finally { - if (end != tail) throw new ConcurrentModificationException(); - circularClear(es, tail = j, end); - // checkInvariants(); } + if (end != tail) throw new ConcurrentModificationException(); + circularClear(es, tail = w, end); + // checkInvariants(); + return true; } /** @@ -1215,6 +1239,8 @@ public class ArrayDeque extends Abstr /** debugging */ void checkInvariants() { + // Use head and tail fields with empty slot at tail strategy. + // head == tail disambiguates to "empty". try { int capacity = elements.length; // assert head >= 0 && head < capacity;