From f1b183123befc1a3f096ba36b1c3834b4c67d3ec Mon Sep 17 00:00:00 2001
From: David Rowley <dgrowley@gmail.com>
Date: Thu, 17 Nov 2022 13:24:40 +1300
Subject: [PATCH v2 3/5] Unroll loop in heapgetpage

---
 src/backend/access/heap/heapam.c | 48 ++++++++++++++++++++++++++++----
 1 file changed, 42 insertions(+), 6 deletions(-)

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 81c7f69644..8fe4f4c837 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -484,15 +484,51 @@ heapgetpage(TableScanDesc sscan, BlockNumber block)
 		loctup.t_tableOid = RelationGetRelid(scan->rs_base.rs_rd);
 
 		/*
-		 * Iterate forward over line items, they're laid out in increasing
-		 * order in memory. Doing this separately allows to benefit from
-		 * out-of-order capabilities of the CPU and simplifies the next loop.
+		 * Iterate forward over line items processing 16 at a time (this
+		 * assumes there will be 16 ItemIds per CPU cacheline.
 		 */
 		for (lineoff = FirstOffsetNumber, lpp = PageGetItemId(page, lineoff);
-			 lineoff <= lines;
-			 lineoff++, lpp++)
+			 lineoff <= lines - 15;
+			 lineoff += 16, lpp += 16)
+		{
+			pg_prefetch_mem(PageGetItemId(page, lineoff + 16));
+			if (ItemIdIsNormal(lpp))
+				normoffsets[normcount++] = lineoff;
+			if (ItemIdIsNormal(lpp + 1))
+				normoffsets[normcount++] = lineoff + 1;
+			if (ItemIdIsNormal(lpp + 2))
+				normoffsets[normcount++] = lineoff + 2;
+			if (ItemIdIsNormal(lpp + 3))
+				normoffsets[normcount++] = lineoff + 3;
+			if (ItemIdIsNormal(lpp + 4))
+				normoffsets[normcount++] = lineoff + 4;
+			if (ItemIdIsNormal(lpp + 5))
+				normoffsets[normcount++] = lineoff + 5;
+			if (ItemIdIsNormal(lpp + 6))
+				normoffsets[normcount++] = lineoff + 6;
+			if (ItemIdIsNormal(lpp + 7))
+				normoffsets[normcount++] = lineoff + 7;
+			if (ItemIdIsNormal(lpp + 8))
+				normoffsets[normcount++] = lineoff + 8;
+			if (ItemIdIsNormal(lpp + 9))
+				normoffsets[normcount++] = lineoff + 9;
+			if (ItemIdIsNormal(lpp + 10))
+				normoffsets[normcount++] = lineoff + 10;
+			if (ItemIdIsNormal(lpp + 11))
+				normoffsets[normcount++] = lineoff + 11;
+			if (ItemIdIsNormal(lpp + 12))
+				normoffsets[normcount++] = lineoff + 12;
+			if (ItemIdIsNormal(lpp + 13))
+				normoffsets[normcount++] = lineoff + 13;
+			if (ItemIdIsNormal(lpp + 14))
+				normoffsets[normcount++] = lineoff + 14;
+			if (ItemIdIsNormal(lpp + 15))
+				normoffsets[normcount++] = lineoff + 15;
+		}
+
+		/* get remainder */
+		for (; lineoff <= lines; lineoff++, lpp++)
 		{
-			pg_prefetch_mem(PageGetItemId(page, lineoff+5));
 			if (ItemIdIsNormal(lpp))
 				normoffsets[normcount++] = lineoff;
 		}
-- 
2.35.1.windows.2