From: Stefan Metzmacher <metze@samba.org>
Date: Mon, 30 Jan 2023 15:10:07 +0000 (+0100)
Subject: tevent: introduce DLIST_DEMOTE_SHORT()
X-Git-Tag: tevent-0.16.0~9
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=30d22631a6b2e9d5a8df17e7f3f231cc6020cb30;p=thirdparty%2Fsamba.git

tevent: introduce DLIST_DEMOTE_SHORT()

It turns out that the overhead of DLIST_DEMOTE() implemented
as DLIST_REMOVE();DLIST_ADD_END(), is very high if the list
contains only 1 or 2 elements.

The next commits will make use of DLIST_DEMOTE_SHORT() for
multiplexing multiple tevent_fd structures for a single fd
and the most important and common case is a list with just
one element.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
---

diff --git a/lib/tevent/tevent_dlinklist.h b/lib/tevent/tevent_dlinklist.h
index a775e8dcdc1..49a135a23bd 100644
--- a/lib/tevent/tevent_dlinklist.h
+++ b/lib/tevent/tevent_dlinklist.h
@@ -156,6 +156,27 @@ do { \
 	DLIST_ADD_END(list, p); \
 } while (0)
 
+/*
+ * like DLIST_DEMOTE(), but optimized
+ * for short lists with 0, 1 or 2 elements
+ */
+#define DLIST_DEMOTE_SHORT(list, p) \
+do { \
+	if ((list) == NULL) { \
+		/* no reason to demote, just add */ \
+		DLIST_ADD(list, p); \
+	} else if ((list)->prev == (p)) { \
+		/* optimize if p is last */ \
+	} else if ((list) == (p)) { \
+		/* optimize if p is first */ \
+		(list)->prev->next = (p); \
+		(list) = (p)->next; \
+		(p)->next = NULL; \
+	} else { \
+		DLIST_DEMOTE(list, p); \
+	} \
+} while (0)
+
 /*
    concatenate two lists - putting all elements of the 2nd list at the
    end of the first list.