protected DictionaryImpl(int capacity, DictionaryImpl <TKey, TKeyStore, TValue> other)
{
capacity = AlignToPowerOfTwo(capacity);
this._entries = new Entry[capacity];
this._size = other._size;
this._topDict = other._topDict;
this._keyComparer = other._keyComparer;
}
public Snapshot(DictionaryImpl <TKey, TKeyStore, TValue> dict)
{
this._table = dict;
// linearization point.
// if table is quiescent and has no copy in progress,
// we can simply iterate over its table.
while (true)
{
if (_table._newTable == null)
{
break;
}
// there is a copy in progress, finish it and try again
_table.HelpCopyImpl(copy_all: true);
this._table = (DictionaryImpl <TKey, TKeyStore, TValue>)(this._table._topDict._table);
}
// Warm-up the iterator
MoveNext();
}
public SnapshotIDict(DictionaryImpl <TKey, TKeyStore, TValue> dict) : base(dict)
{
}
// Copy one K/V pair from old table to new table.
// Returns true if we actually did the copy.
// Regardless, once this returns, the copy is available in the new table and
// slot in the old table is no longer usable.
private static bool CopySlot(ref Entry oldEntry, DictionaryImpl <TKey, TKeyStore, TValue> newTable)
{
Debug.Assert(newTable != null);
// Blindly set the hash from 0 to TOMBPRIMEHASH, to eagerly stop
// fresh put's from claiming new slots in the old table when the old
// table is mid-resize.
var hash = oldEntry.hash;
if (hash == 0)
{
hash = Interlocked.CompareExchange(ref oldEntry.hash, TOMBPRIMEHASH, 0);
if (hash == 0)
{
// slot was not claimed, copy is done here
return(true);
}
}
if (hash == TOMBPRIMEHASH)
{
// slot was trivially copied, but not by us
return(false);
}
// Prevent new values from appearing in the old table.
// Box what we see in the old table, to prevent further updates.
// NOTE: Read of the value below must happen before reading of the key,
// however this read does not need to be volatile since we will have
// some fences in between reads.
object oldval = oldEntry.value;
// already boxed?
Prime box = oldval as Prime;
if (box != null)
{
// volatile read here since we need to make sure
// that the key read below happens after we have read oldval above
Volatile.Read(ref box.originalValue);
}
else
{
do
{
box = EntryValueNullOrDead(oldval) ?
TOMBPRIME :
new Prime(oldval);
// CAS down a box'd version of oldval
// also works as a complete fence between reading the value and the key
object prev = Interlocked.CompareExchange(ref oldEntry.value, box, oldval);
if (prev == oldval)
{
// If we made the Value slot hold a TOMBPRIME, then we both
// prevented further updates here but also the (absent)
// oldval is vacuously available in the new table. We
// return with true here: any thread looking for a value for
// this key can correctly go straight to the new table and
// skip looking in the old table.
if (box == TOMBPRIME)
{
return(true);
}
// Break loop; oldval is now boxed by us
// it still needs to be copied into the new table.
break;
}
oldval = prev;
box = oldval as Prime;
}while (box == null);
}
if (box == TOMBPRIME)
{
// Copy already complete here, but not by us.
return(false);
}
// Copy the value into the new table, but only if we overwrite a null.
// If another value is already in the new table, then somebody else
// wrote something there and that write is happens-after any value that
// appears in the old table. If putIfMatch does not find a null in the
// new table - somebody else should have recorded the null-not_null
// transition in this copy.
object originalValue = box.originalValue;
Debug.Assert(originalValue != TOMBSTONE);
// since we have a real value, there must be a nontrivial key in the table
// regular read is ok because value is always CASed down after the key
// and we ensured that we read the key after the value with fences above
var key = oldEntry.key;
bool copiedIntoNew = newTable.PutSlotCopy(key, originalValue, hash);
// Finally, now that any old value is exposed in the new table, we can
// forever hide the old-table value by gently inserting TOMBPRIME value.
// This will stop other threads from uselessly attempting to copy this slot
// (i.e., it's a speed optimization not a correctness issue).
// Check if we are not too late though, to not pay for MESI RFO and
// GC fence needlessly.
if (oldEntry.value != TOMBPRIME)
{
oldEntry.value = TOMBPRIME;
}
// if we failed to copy, it means something has already appeared in
// the new table and old value should have been copied before that (not by us).
return(copiedIntoNew);
}
