Reduce branches in heapgetpage()'s per-tuple loop

Until now, heapgetpage()'s loop over all tuples performed some conditional
checks for each tuple, even though condition did not change across the loop.

This commit fixes that by moving the loop into an inline function. By calling
it with different constant arguments, the compiler can generate an optimized
loop for the different conditions, at the price of two per-page checks.

For cases of all-visible tables and an isolation level other than
serializable, speedups of up to 25% have been measured.

Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Reviewed-by: Zhang Mingli <zmlpostgres@gmail.com>
Tested-by: Quan Zongliang <quanzongliang@yeah.net>
Discussion: https://postgr.es/m/20230716015656.xjvemfbp5fysjiea@awork3.anarazel.de
Discussion: https://postgr.es/m/2ef7ff1b-3d18-2283-61b1-bbd25fc6c7ce@yeah.net

src/backend/access/heap/heapam.c

@@ -364,6 +364,56 @@ heap_setscanlimits(TableScanDesc sscan, BlockNumber startBlk, BlockNumber numBlk
 	scan->rs_numblocks = numBlks;
 }
 
+/*
+ * Per-tuple loop for heapgetpage() in pagemode. Pulled out so it can be
+ * called multiple times, with constant arguments for all_visible,
+ * check_serializable.
+ */
+pg_attribute_always_inline
+static int
+heapgetpage_collect(HeapScanDesc scan, Snapshot snapshot,
+					Page page, Buffer buffer,
+					BlockNumber block, int lines,
+					bool all_visible, bool check_serializable)
+{
+	int			ntup = 0;
+	OffsetNumber lineoff;
+
+	for (lineoff = FirstOffsetNumber; lineoff <= lines; lineoff++)
+	{
+		ItemId		lpp = PageGetItemId(page, lineoff);
+		HeapTupleData loctup;
+		bool		valid;
+
+		if (!ItemIdIsNormal(lpp))
+			continue;
+
+		loctup.t_data = (HeapTupleHeader) PageGetItem(page, lpp);
+		loctup.t_len = ItemIdGetLength(lpp);
+		loctup.t_tableOid = RelationGetRelid(scan->rs_base.rs_rd);
+		ItemPointerSet(&(loctup.t_self), block, lineoff);
+
+		if (all_visible)
+			valid = true;
+		else
+			valid = HeapTupleSatisfiesVisibility(&loctup, snapshot, buffer);
+
+		if (check_serializable)
+			HeapCheckForSerializableConflictOut(valid, scan->rs_base.rs_rd,
+												&loctup, buffer, snapshot);
+
+		if (valid)
+		{
+			scan->rs_vistuples[ntup] = lineoff;
+			ntup++;
+		}
+	}
+
+	Assert(ntup <= MaxHeapTuplesPerPage);
+
+	return ntup;
+}
+
 /*
  * heap_prepare_pagescan - Prepare current scan page to be scanned in pagemode
  *
@@ -379,9 +429,8 @@ heap_prepare_pagescan(TableScanDesc sscan)
 	Snapshot	snapshot;
 	Page		page;
 	int			lines;
-	int			ntup;
-	OffsetNumber lineoff;
 	bool		all_visible;
+	bool		check_serializable;
 
 	Assert(BufferGetBlockNumber(buffer) == block);
 
@@ -403,7 +452,6 @@ heap_prepare_pagescan(TableScanDesc sscan)
 
 	page = BufferGetPage(buffer);
 	lines = PageGetMaxOffsetNumber(page);
-	ntup = 0;
 
 	/*
 	 * If the all-visible flag indicates that all tuples on the page are
@@ -426,37 +474,35 @@ heap_prepare_pagescan(TableScanDesc sscan)
 	 * tuple for visibility the hard way.
 	 */
 	all_visible = PageIsAllVisible(page) && !snapshot->takenDuringRecovery;
+	check_serializable =
+		CheckForSerializableConflictOutNeeded(scan->rs_base.rs_rd, snapshot);
 
-	for (lineoff = FirstOffsetNumber; lineoff <= lines; lineoff++)
+	/*
+	 * We call heapgetpage_collect() with constant arguments, to get the
+	 * compiler to constant fold the constant arguments. Separate calls with
+	 * constant arguments, rather than variables, are needed on several
+	 * compilers to actually perform constant folding.
+	 */
+	if (likely(all_visible))
 	{
-		ItemId		lpp = PageGetItemId(page, lineoff);
-		HeapTupleData loctup;
-		bool		valid;
-
-		if (!ItemIdIsNormal(lpp))
-			continue;
-
-		loctup.t_tableOid = RelationGetRelid(scan->rs_base.rs_rd);
-		loctup.t_data = (HeapTupleHeader) PageGetItem(page, lpp);
-		loctup.t_len = ItemIdGetLength(lpp);
-		ItemPointerSet(&(loctup.t_self), block, lineoff);
-
-		if (all_visible)
-			valid = true;
+		if (likely(!check_serializable))
+			scan->rs_ntuples = heapgetpage_collect(scan, snapshot, page, buffer,
+												   block, lines, true, false);
 		else
-			valid = HeapTupleSatisfiesVisibility(&loctup, snapshot, buffer);
-
-		HeapCheckForSerializableConflictOut(valid, scan->rs_base.rs_rd,
-											&loctup, buffer, snapshot);
-
-		if (valid)
-			scan->rs_vistuples[ntup++] = lineoff;
+			scan->rs_ntuples = heapgetpage_collect(scan, snapshot, page, buffer,
+												   block, lines, true, true);
+	}
+	else
+	{
+		if (likely(!check_serializable))
+			scan->rs_ntuples = heapgetpage_collect(scan, snapshot, page, buffer,
+												   block, lines, false, false);
+		else
+			scan->rs_ntuples = heapgetpage_collect(scan, snapshot, page, buffer,
+												   block, lines, false, true);
 	}
 
 	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
-
-	Assert(ntup <= MaxHeapTuplesPerPage);
-	scan->rs_ntuples = ntup;
 }
 
 /*