jdk-24/hotspot/src/share/vm/gc/shared/taskqueue.inline.hpp

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#ifndef SHARE_VM_GC_SHARED_TASKQUEUE_INLINE_HPP
#define SHARE_VM_GC_SHARED_TASKQUEUE_INLINE_HPP

#include "gc/shared/taskqueue.hpp"
#include "memory/allocation.inline.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/atomic.inline.hpp"
#include "runtime/orderAccess.inline.hpp"
#include "utilities/debug.hpp"
#include "utilities/stack.inline.hpp"

template <class T, MEMFLAGS F>
inline GenericTaskQueueSet<T, F>::GenericTaskQueueSet(int n) : _n(n) {
  typedef T* GenericTaskQueuePtr;
  _queues = NEW_C_HEAP_ARRAY(GenericTaskQueuePtr, n, F);
  for (int i = 0; i < n; i++) {
    _queues[i] = NULL;
  }
}

template<class E, MEMFLAGS F, unsigned int N>
inline void GenericTaskQueue<E, F, N>::initialize() {
  _elems = _array_allocator.allocate(N);
}

template<class E, MEMFLAGS F, unsigned int N>
inline GenericTaskQueue<E, F, N>::~GenericTaskQueue() {
  FREE_C_HEAP_ARRAY(E, _elems);
}

template<class E, MEMFLAGS F, unsigned int N>
bool GenericTaskQueue<E, F, N>::push_slow(E t, uint dirty_n_elems) {
  if (dirty_n_elems == N - 1) {
    // Actually means 0, so do the push.
    uint localBot = _bottom;
    // g++ complains if the volatile result of the assignment is
    // unused, so we cast the volatile away.  We cannot cast directly
    // to void, because gcc treats that as not using the result of the
    // assignment.  However, casting to E& means that we trigger an
    // unused-value warning.  So, we cast the E& to void.
    (void)const_cast<E&>(_elems[localBot] = t);
    OrderAccess::release_store(&_bottom, increment_index(localBot));
    TASKQUEUE_STATS_ONLY(stats.record_push());
    return true;
  }
  return false;
}

template<class E, MEMFLAGS F, unsigned int N> inline bool
GenericTaskQueue<E, F, N>::push(E t) {
  uint localBot = _bottom;
  assert(localBot < N, "_bottom out of range.");
  idx_t top = _age.top();
  uint dirty_n_elems = dirty_size(localBot, top);
  assert(dirty_n_elems < N, "n_elems out of range.");
  if (dirty_n_elems < max_elems()) {
    // g++ complains if the volatile result of the assignment is
    // unused, so we cast the volatile away.  We cannot cast directly
    // to void, because gcc treats that as not using the result of the
    // assignment.  However, casting to E& means that we trigger an
    // unused-value warning.  So, we cast the E& to void.
    (void) const_cast<E&>(_elems[localBot] = t);
    OrderAccess::release_store(&_bottom, increment_index(localBot));
    TASKQUEUE_STATS_ONLY(stats.record_push());
    return true;
  } else {
    return push_slow(t, dirty_n_elems);
  }
}

template <class E, MEMFLAGS F, unsigned int N>
inline bool OverflowTaskQueue<E, F, N>::push(E t)
{
  if (!taskqueue_t::push(t)) {
    overflow_stack()->push(t);
    TASKQUEUE_STATS_ONLY(stats.record_overflow(overflow_stack()->size()));
  }
  return true;
}

// pop_local_slow() is done by the owning thread and is trying to
// get the last task in the queue.  It will compete with pop_global()
// that will be used by other threads.  The tag age is incremented
// whenever the queue goes empty which it will do here if this thread
// gets the last task or in pop_global() if the queue wraps (top == 0
// and pop_global() succeeds, see pop_global()).
template<class E, MEMFLAGS F, unsigned int N>
bool GenericTaskQueue<E, F, N>::pop_local_slow(uint localBot, Age oldAge) {
  // This queue was observed to contain exactly one element; either this
  // thread will claim it, or a competing "pop_global".  In either case,
  // the queue will be logically empty afterwards.  Create a new Age value
  // that represents the empty queue for the given value of "_bottom".  (We
  // must also increment "tag" because of the case where "bottom == 1",
  // "top == 0".  A pop_global could read the queue element in that case,
  // then have the owner thread do a pop followed by another push.  Without
  // the incrementing of "tag", the pop_global's CAS could succeed,
  // allowing it to believe it has claimed the stale element.)
  Age newAge((idx_t)localBot, oldAge.tag() + 1);
  // Perhaps a competing pop_global has already incremented "top", in which
  // case it wins the element.
  if (localBot == oldAge.top()) {
    // No competing pop_global has yet incremented "top"; we'll try to
    // install new_age, thus claiming the element.
    Age tempAge = _age.cmpxchg(newAge, oldAge);
    if (tempAge == oldAge) {
      // We win.
      assert(dirty_size(localBot, _age.top()) != N - 1, "sanity");
      TASKQUEUE_STATS_ONLY(stats.record_pop_slow());
      return true;
    }
  }
  // We lose; a completing pop_global gets the element.  But the queue is empty
  // and top is greater than bottom.  Fix this representation of the empty queue
  // to become the canonical one.
  _age.set(newAge);
  assert(dirty_size(localBot, _age.top()) != N - 1, "sanity");
  return false;
}

template<class E, MEMFLAGS F, unsigned int N> inline bool
GenericTaskQueue<E, F, N>::pop_local(volatile E& t) {
  uint localBot = _bottom;
  // This value cannot be N-1.  That can only occur as a result of
  // the assignment to bottom in this method.  If it does, this method
  // resets the size to 0 before the next call (which is sequential,
  // since this is pop_local.)
  uint dirty_n_elems = dirty_size(localBot, _age.top());
  assert(dirty_n_elems != N - 1, "Shouldn't be possible...");
  if (dirty_n_elems == 0) return false;
  localBot = decrement_index(localBot);
  _bottom = localBot;
  // This is necessary to prevent any read below from being reordered
  // before the store just above.
  OrderAccess::fence();
  // g++ complains if the volatile result of the assignment is
  // unused, so we cast the volatile away.  We cannot cast directly
  // to void, because gcc treats that as not using the result of the
  // assignment.  However, casting to E& means that we trigger an
  // unused-value warning.  So, we cast the E& to void.
  (void) const_cast<E&>(t = _elems[localBot]);
  // This is a second read of "age"; the "size()" above is the first.
  // If there's still at least one element in the queue, based on the
  // "_bottom" and "age" we've read, then there can be no interference with
  // a "pop_global" operation, and we're done.
  idx_t tp = _age.top();    // XXX
  if (size(localBot, tp) > 0) {
    assert(dirty_size(localBot, tp) != N - 1, "sanity");
    TASKQUEUE_STATS_ONLY(stats.record_pop());
    return true;
  } else {
    // Otherwise, the queue contained exactly one element; we take the slow
    // path.
    return pop_local_slow(localBot, _age.get());
  }
}

template <class E, MEMFLAGS F, unsigned int N>
bool OverflowTaskQueue<E, F, N>::pop_overflow(E& t)
{
  if (overflow_empty()) return false;
  t = overflow_stack()->pop();
  return true;
}

template<class E, MEMFLAGS F, unsigned int N>
bool GenericTaskQueue<E, F, N>::pop_global(volatile E& t) {
  Age oldAge = _age.get();
  // Architectures with weak memory model require a barrier here
  // to guarantee that bottom is not older than age,
  // which is crucial for the correctness of the algorithm.
#if !(defined SPARC || defined IA32 || defined AMD64)
  OrderAccess::fence();
#endif
  uint localBot = OrderAccess::load_acquire((volatile juint*)&_bottom);
  uint n_elems = size(localBot, oldAge.top());
  if (n_elems == 0) {
    return false;
  }

  // g++ complains if the volatile result of the assignment is
  // unused, so we cast the volatile away.  We cannot cast directly
  // to void, because gcc treats that as not using the result of the
  // assignment.  However, casting to E& means that we trigger an
  // unused-value warning.  So, we cast the E& to void.
  (void) const_cast<E&>(t = _elems[oldAge.top()]);
  Age newAge(oldAge);
  newAge.increment();
  Age resAge = _age.cmpxchg(newAge, oldAge);

  // Note that using "_bottom" here might fail, since a pop_local might
  // have decremented it.
  assert(dirty_size(localBot, newAge.top()) != N - 1, "sanity");
  return resAge == oldAge;
}

template<class T, MEMFLAGS F> bool
GenericTaskQueueSet<T, F>::steal_best_of_2(uint queue_num, int* seed, E& t) {
  if (_n > 2) {
    uint k1 = queue_num;
    while (k1 == queue_num) k1 = TaskQueueSetSuper::randomParkAndMiller(seed) % _n;
    uint k2 = queue_num;
    while (k2 == queue_num || k2 == k1) k2 = TaskQueueSetSuper::randomParkAndMiller(seed) % _n;
    // Sample both and try the larger.
    uint sz1 = _queues[k1]->size();
    uint sz2 = _queues[k2]->size();
    if (sz2 > sz1) return _queues[k2]->pop_global(t);
    else return _queues[k1]->pop_global(t);
  } else if (_n == 2) {
    // Just try the other one.
    uint k = (queue_num + 1) % 2;
    return _queues[k]->pop_global(t);
  } else {
    assert(_n == 1, "can't be zero.");
    return false;
  }
}

template<class T, MEMFLAGS F> bool
GenericTaskQueueSet<T, F>::steal(uint queue_num, int* seed, E& t) {
  for (uint i = 0; i < 2 * _n; i++) {
    if (steal_best_of_2(queue_num, seed, t)) {
      TASKQUEUE_STATS_ONLY(queue(queue_num)->stats.record_steal(true));
      return true;
    }
  }
  TASKQUEUE_STATS_ONLY(queue(queue_num)->stats.record_steal(false));
  return false;
}

template <unsigned int N, MEMFLAGS F>
inline typename TaskQueueSuper<N, F>::Age TaskQueueSuper<N, F>::Age::cmpxchg(const Age new_age, const Age old_age) volatile {
  return (size_t) Atomic::cmpxchg_ptr((intptr_t)new_age._data,
                                      (volatile intptr_t *)&_data,
                                      (intptr_t)old_age._data);
}

template<class E, MEMFLAGS F, unsigned int N>
template<class Fn>
inline void GenericTaskQueue<E, F, N>::iterate(Fn fn) {
  uint iters = size();
  uint index = _bottom;
  for (uint i = 0; i < iters; ++i) {
    index = decrement_index(index);
    fn(const_cast<E&>(_elems[index])); // cast away volatility
  }
}


#endif // SHARE_VM_GC_SHARED_TASKQUEUE_INLINE_HPP