void CopyAndSortWithBitonic(uint cachedLength)
{
var start = _startPtr;
var tmp = _tempStart;
var byteCount = cachedLength * sizeof(int);
var adjustedLength = cachedLength & ~0b111;
Store(tmp + adjustedLength, Vector256.Create(int.MaxValue));
Unsafe.CopyBlockUnaligned(tmp, start, byteCount);
BitonicSort <int> .Sort(tmp, (int)Math.Min(adjustedLength + 8, BitonicSort <int> .MaxBitonicSortSize));
Unsafe.CopyBlockUnaligned(start, tmp, byteCount);
}
internal void HybridSort(int *left, int *right, long realignHint, int depthLimit)
{
Debug.Assert(left <= right);
var length = (int)(right - left + 1);
int *mid;
switch (length)
{
case 0:
case 1:
return;
case 2:
SwapIfGreater(left, right);
return;
case 3:
mid = right - 1;
SwapIfGreater(left, mid);
SwapIfGreater(left, right);
SwapIfGreater(mid, right);
return;
}
_depth++;
// SMALL_SORT_THRESHOLD_ELEMENTS is guaranteed (and asserted) to be a multiple of 8
// So we can check if length is strictly smaller, knowing that we will round up to
// SMALL_SORT_THRESHOLD_ELEMENTS exactly and no more
// This is kind of critical given that we only limited # of implementation of
// vectorized bitonic sort
if (length < SMALL_SORT_THRESHOLD_ELEMENTS)
{
var nextLength = (length & 7) > 0 ? (length + V256_N) & ~7: length;
Debug.Assert(nextLength <= BitonicSort <int> .MaxBitonicSortSize);
var extraSpaceNeeded = nextLength - length;
var fakeLeft = left - extraSpaceNeeded;
if (fakeLeft >= _startPtr)
{
BitonicSort <int> .Sort(fakeLeft, nextLength);
}
else
{
InsertionSort(left, right);
}
_depth--;
return;
}
// Detect a whole bunch of bad cases where partitioning
// will not do well:
// 1. Reverse sorted array
// 2. High degree of repeated values (dutch flag problem, one value)
if (depthLimit == 0)
{
HeapSort(new Span <int>(left, (int)(right - left + 1)));
_depth--;
return;
}
depthLimit--;
// This is going to be a bit weird:
// Pre/Post alignment calculations happen here: we prepare hints to the
// partition function of how much to align and in which direction (pre/post).
// The motivation to do these calculations here and the actual alignment inside the partitioning code is
// that here, we can cache those calculations.
// As we recurse to the left we can reuse the left cached calculation, And when we recurse
// to the right we reuse the right calculation, so we can avoid re-calculating the same aligned addresses
// throughout the recursion, at the cost of a minor code complexity
// Since we branch on the magi values REALIGN_LEFT & REALIGN_RIGHT its safe to assume
// the we are not torturing the branch predictor.'
// We use a long as a "struct" to pass on alignment hints to the partitioning
// By packing 2 32 bit elements into it, as the JIT seem to not do this.
// In reality we need more like 2x 4bits for each side, but I don't think
// there is a real difference'
var preAlignedLeft = (int *)((ulong)left & ~ALIGN_MASK);
var cannotPreAlignLeft = (preAlignedLeft - _startPtr) >> 63;
var preAlignLeftOffset = (preAlignedLeft - left) + (V256_N & cannotPreAlignLeft);
if ((realignHint & REALIGN_LEFT) != 0)
{
// Alignment flow:
// * Calculate pre-alignment on the left
// * See it would cause us an out-of bounds read
// * Since we'd like to avoid that, we adjust for post-alignment
// * There are no branches since we do branch->arithmetic
realignHint &= unchecked ((long)0xFFFFFFFF00000000UL);
realignHint |= preAlignLeftOffset;
}
var preAlignedRight = (int *)(((ulong)right - 1 & ~ALIGN_MASK) + ALIGN);
var cannotPreAlignRight = (_endPtr - preAlignedRight) >> 63;
var preAlignRightOffset = (preAlignedRight - right - (V256_N & cannotPreAlignRight));
if ((realignHint & REALIGN_RIGHT) != 0)
{
// right is pointing just PAST the last element we intend to partition (where we also store the pivot)
// So we calculate alignment based on right - 1, and YES: I am casting to ulong before doing the -1, this
// is intentional since the whole thing is either aligned to 32 bytes or not, so decrementing the POINTER value
// by 1 is sufficient for the alignment, an the JIT sucks at this anyway
realignHint &= 0xFFFFFFFF;
realignHint |= preAlignRightOffset << 32;
}
Debug.Assert(((ulong)(left + (realignHint & 0xFFFFFFFF)) & ALIGN_MASK) == 0);
Debug.Assert(((ulong)(right + (realignHint >> 32)) & ALIGN_MASK) == 0);
// Compute median-of-three, of:
// the first, mid and one before last elements
mid = left + (right - left) / 2;
SwapIfGreater(left, mid);
SwapIfGreater(left, right - 1);
SwapIfGreater(mid, right - 1);
// Pivot is mid, place it in the right hand side
Swap(mid, right);
var sep = length < PARTITION_TMP_SIZE_IN_ELEMENTS?
Partition1VectorInPlace(left, right, realignHint) :
Partition8VectorsInPlace(left, right, realignHint);
HybridSort(left, sep - 2, realignHint | REALIGN_RIGHT, depthLimit);
HybridSort(sep, right, realignHint | REALIGN_LEFT, depthLimit);
_depth--;
}