Superfast

public unsafe int FreeFasterSimplifiedAsmAlignedNonTemporalUnrolledUnsafePin(T obj)
        {
            var items = _items;
            var length = items.Length;
            var nullVector = Vector256<long>.Zero;
            fixed (long* addrPtr = &itemsRef[0])
            {
                var aligned = (long*)(((ulong)addrPtr + 31UL) & ~31UL);
                var pos = (int)(aligned - addrPtr);
                for (int w = 0; w < pos; w++)
                    if (addrPtr[w] == 0)
                    {
                        items[w].Value = obj;
                        return w;
                    }

                int i = 0;
                const int vSizeLong = 4;
                const int bSizeLong = 8;
                var lengthWithoutBlockSize = length - bSizeLong;
                for (; i <= lengthWithoutBlockSize; i += bSizeLong)
                {
                    var v2i = i + vSizeLong;
                    var v21 = Avx2.LoadAlignedVector256NonTemporal(aligned + i);
                    var v22 = Avx2.LoadAlignedVector256NonTemporal(aligned + v2i);
                    var temp1 = Avx2.CompareEqual(v21, nullVector).AsDouble();
                    var m1 = Avx.MoveMask(temp1);
                    var temp2 = Avx2.CompareEqual(v22, nullVector).AsDouble();
                    var m2 = Avx.MoveMask(temp2);
                    if (m1 + m2 == 0)
                        continue;
                    if (m1 == 0)
                    {
                        i += vSizeLong;
                    }
                    goto nullOrFound;
                }
            nullOrFound:
                for (int x = i + pos; x < length; x++)
                {
                    if (addrPtr[x] == 0)
                    {
                        return x;
                    }
                }
            }
            return -1;
        }