Improve sift up/down code in binaryheap.c and logtape.c.

tglsfdc · tglsfdc · commit a2ff18e89ff8 · 2021-12-14T13:35:22.000-05:00
Borrow the logic that's long been used in tuplesort.c: instead of physically swapping the data in two heap entries, keep the value that's being sifted up or down in a local variable, and just move the other values as necessary. This makes the code shorter as well as faster. It's not clear that any current callers are really time-critical enough to notice, but we might as well code heap maintenance the same way everywhere. Ma Liangzhu and Tom Lane Discussion: https://postgr.es/m/17336-fc4e522d26a750fd@postgresql.org
diff --git a/src/backend/lib/binaryheap.c b/src/backend/lib/binaryheap.c
@@ -19,7 +19,6 @@
 
 static void sift_down(binaryheap *heap, int node_off);
 static void sift_up(binaryheap *heap, int node_off);
-static inline void swap_nodes(binaryheap *heap, int a, int b);
 
 /*
  * binaryheap_allocate
@@ -173,24 +172,28 @@ binaryheap_first(binaryheap *heap)
 Datum
 binaryheap_remove_first(binaryheap *heap)
 {
+	Datum		result;
+
 	Assert(!binaryheap_empty(heap) && heap->bh_has_heap_property);
 
+	/* extract the root node, which will be the result */
+	result = heap->bh_nodes[0];
+
+	/* easy if heap contains one element */
 	if (heap->bh_size == 1)
 	{
 		heap->bh_size--;
-		return heap->bh_nodes[0];
+		return result;
 	}
 
 	/*
-	 * Swap the root and last nodes, decrease the size of the heap (i.e.
-	 * remove the former root node) and sift the new root node down to its
-	 * correct position.
+	 * Remove the last node, placing it in the vacated root entry, and sift
+	 * the new root node down to its correct position.
 	 */
-	swap_nodes(heap, 0, heap->bh_size - 1);
-	heap->bh_size--;
+	heap->bh_nodes[0] = heap->bh_nodes[--heap->bh_size];
 	sift_down(heap, 0);
 
-	return heap->bh_nodes[heap->bh_size];
+	return result;
 }
 
 /*
@@ -211,49 +214,47 @@ binaryheap_replace_first(binaryheap *heap, Datum d)
 		sift_down(heap, 0);
 }
 
-/*
- * Swap the contents of two nodes.
- */
-static inline void
-swap_nodes(binaryheap *heap, int a, int b)
-{
-	Datum		swap;
-
-	swap = heap->bh_nodes[a];
-	heap->bh_nodes[a] = heap->bh_nodes[b];
-	heap->bh_nodes[b] = swap;
-}
-
 /*
  * Sift a node up to the highest position it can hold according to the
  * comparator.
  */
 static void
 sift_up(binaryheap *heap, int node_off)
 {
+	Datum		node_val = heap->bh_nodes[node_off];
+
+	/*
+	 * Within the loop, the node_off'th array entry is a "hole" that
+	 * notionally holds node_val, but we don't actually store node_val there
+	 * till the end, saving some unnecessary data copying steps.
+	 */
 	while (node_off != 0)
 	{
 		int			cmp;
 		int			parent_off;
+		Datum		parent_val;
 
 		/*
 		 * If this node is smaller than its parent, the heap condition is
 		 * satisfied, and we're done.
 		 */
 		parent_off = parent_offset(node_off);
-		cmp = heap->bh_compare(heap->bh_nodes[node_off],
-							   heap->bh_nodes[parent_off],
+		parent_val = heap->bh_nodes[parent_off];
+		cmp = heap->bh_compare(node_val,
+							   parent_val,
 							   heap->bh_arg);
 		if (cmp <= 0)
 			break;
 
 		/*
-		 * Otherwise, swap the node and its parent and go on to check the
-		 * node's new parent.
+		 * Otherwise, swap the parent value with the hole, and go on to check
+		 * the node's new parent.
 		 */
-		swap_nodes(heap, node_off, parent_off);
+		heap->bh_nodes[node_off] = parent_val;
 		node_off = parent_off;
 	}
+	/* Re-fill the hole */
+	heap->bh_nodes[node_off] = node_val;
 }
 
 /*
@@ -263,6 +264,13 @@ sift_up(binaryheap *heap, int node_off)
 static void
 sift_down(binaryheap *heap, int node_off)
 {
+	Datum		node_val = heap->bh_nodes[node_off];
+
+	/*
+	 * Within the loop, the node_off'th array entry is a "hole" that
+	 * notionally holds node_val, but we don't actually store node_val there
+	 * till the end, saving some unnecessary data copying steps.
+	 */
 	while (true)
 	{
 		int			left_off = left_offset(node_off);
@@ -271,14 +279,14 @@ sift_down(binaryheap *heap, int node_off)
 
 		/* Is the left child larger than the parent? */
 		if (left_off < heap->bh_size &&
-			heap->bh_compare(heap->bh_nodes[node_off],
+			heap->bh_compare(node_val,
 							 heap->bh_nodes[left_off],
 							 heap->bh_arg) < 0)
 			swap_off = left_off;
 
 		/* Is the right child larger than the parent? */
 		if (right_off < heap->bh_size &&
-			heap->bh_compare(heap->bh_nodes[node_off],
+			heap->bh_compare(node_val,
 							 heap->bh_nodes[right_off],
 							 heap->bh_arg) < 0)
 		{
@@ -298,10 +306,12 @@ sift_down(binaryheap *heap, int node_off)
 			break;
 
 		/*
-		 * Otherwise, swap the node with the child that violates the heap
+		 * Otherwise, swap the hole with the child that violates the heap
 		 * property; then go on to check its children.
 		 */
-		swap_nodes(heap, swap_off, node_off);
+		heap->bh_nodes[node_off] = heap->bh_nodes[swap_off];
 		node_off = swap_off;
 	}
+	/* Re-fill the hole */
+	heap->bh_nodes[node_off] = node_val;
 }
diff --git a/src/backend/utils/sort/logtape.c b/src/backend/utils/sort/logtape.c
@@ -340,16 +340,6 @@ ltsReadFillBuffer(LogicalTape *lt)
 	return (lt->nbytes > 0);
 }
 
-static inline void
-swap_nodes(long *heap, unsigned long a, unsigned long b)
-{
-	long		swap;
-
-	swap = heap[a];
-	heap[a] = heap[b];
-	heap[b] = swap;
-}
-
 static inline unsigned long
 left_offset(unsigned long i)
 {
@@ -390,31 +380,33 @@ ltsGetFreeBlock(LogicalTapeSet *lts)
 	long	   *heap = lts->freeBlocks;
 	long		blocknum;
 	int			heapsize;
-	unsigned long pos;
+	long		holeval;
+	unsigned long holepos;
 
 	/* freelist empty; allocate a new block */
 	if (lts->nFreeBlocks == 0)
 		return lts->nBlocksAllocated++;
 
+	/* easy if heap contains one element */
 	if (lts->nFreeBlocks == 1)
 	{
 		lts->nFreeBlocks--;
 		return lts->freeBlocks[0];
 	}
 
-	/* take top of minheap */
+	/* remove top of minheap */
 	blocknum = heap[0];
 
-	/* replace with end of minheap array */
-	heap[0] = heap[--lts->nFreeBlocks];
+	/* we'll replace it with end of minheap array */
+	holeval = heap[--lts->nFreeBlocks];
 
 	/* sift down */
-	pos = 0;
+	holepos = 0;				/* holepos is where the "hole" is */
 	heapsize = lts->nFreeBlocks;
 	while (true)
 	{
-		unsigned long left = left_offset(pos);
-		unsigned long right = right_offset(pos);
+		unsigned long left = left_offset(holepos);
+		unsigned long right = right_offset(holepos);
 		unsigned long min_child;
 
 		if (left < heapsize && right < heapsize)
@@ -426,12 +418,13 @@ ltsGetFreeBlock(LogicalTapeSet *lts)
 		else
 			break;
 
-		if (heap[min_child] >= heap[pos])
+		if (heap[min_child] >= holeval)
 			break;
 
-		swap_nodes(heap, min_child, pos);
-		pos = min_child;
+		heap[holepos] = heap[min_child];
+		holepos = min_child;
 	}
+	heap[holepos] = holeval;
 
 	return blocknum;
 }
@@ -483,7 +476,7 @@ static void
 ltsReleaseBlock(LogicalTapeSet *lts, long blocknum)
 {
 	long	   *heap;
-	unsigned long pos;
+	unsigned long holepos;
 
 	/*
 	 * Do nothing if we're no longer interested in remembering free space.
@@ -508,24 +501,23 @@ ltsReleaseBlock(LogicalTapeSet *lts, long blocknum)
 											lts->freeBlocksLen * sizeof(long));
 	}
 
+	/* create a "hole" at end of minheap array */
 	heap = lts->freeBlocks;
-	pos = lts->nFreeBlocks;
-
-	/* place entry at end of minheap array */
-	heap[pos] = blocknum;
+	holepos = lts->nFreeBlocks;
 	lts->nFreeBlocks++;
 
-	/* sift up */
-	while (pos != 0)
+	/* sift up to insert blocknum */
+	while (holepos != 0)
 	{
-		unsigned long parent = parent_offset(pos);
+		unsigned long parent = parent_offset(holepos);
 
-		if (heap[parent] < heap[pos])
+		if (heap[parent] < blocknum)
 			break;
 
-		swap_nodes(heap, parent, pos);
-		pos = parent;
+		heap[holepos] = heap[parent];
+		holepos = parent;
 	}
+	heap[holepos] = blocknum;
 }
 
 /*
