runtime: avoid unnecessary map iteration write barrier

Update #14921

Change-Id: I5c5816d0193757bf7465b1e09c27ca06897df4bf
Reviewed-on: https://go-review.googlesource.com/21814
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

diff --git a/src/runtime/hashmap.go b/src/runtime/hashmap.go
index 9e18192cd8c8d7b0be0dca19457587c947055a50..d549ce4194e6b44545e16a5c578bb4eb46218775 100644 (file)
--- a/src/runtime/hashmap.go
+++ b/src/runtime/hashmap.go
@@ -790,7 +790,9 @@ next:
                                }
                        }
                        it.bucket = bucket
-                       it.bptr = b
+                       if it.bptr != b { // avoid unnecessary write barrier; see issue 14921
+                               it.bptr = b
+                       }
                        it.i = i + 1
                        it.checkBucket = checkBucket
                        return