netdev - [PATCH net v1] netlink: optimize the NMLSG

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Message-ID: <20240912024018.8117-1-atlas.yu@canonical.com>
Date: Thu, 12 Sep 2024 10:40:18 +0800
From: Atlas Yu <atlas.yu@...onical.com>
To: edumazet@...gle.com,
	davem@...emloft.net,
	kuba@...nel.org,
	pabeni@...hat.com,
	stephen@...workplumber.org
Cc: netdev@...r.kernel.org,
	Atlas Yu <atlas.yu@...onical.com>
Subject: [PATCH net v1] netlink: optimize the NMLSG_OK macro

When nlmsg_len >= sizeof(hdr) and nlmsg_len <= len are true, we can set
up an inequation: len >= nlmsg_len >= sizeof(hdr), which makes checking
len >= sizeof(hdr) useless.

gcc -O3 cannot optimize ok1 to generate the same instructions as ok2 on
x86_64 (not investigated on other architectures).
  int ok1(int a, int b, int c) { return a >= b && c >= b && c <= a; }
  int ok2(int a, int b, int c) { return c >= b && c <= a; }

Signed-off-by: Atlas Yu <atlas.yu@...onical.com>
---
 include/uapi/linux/netlink.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index f87aaf28a649..85dcfa6b33af 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -104,8 +104,7 @@ struct nlmsghdr {
 #define NLMSG_NEXT(nlh,len)	 ((len) -= NLMSG_ALIGN((nlh)->nlmsg_len), \
 				  (struct nlmsghdr *)(((char *)(nlh)) + \
 				  NLMSG_ALIGN((nlh)->nlmsg_len)))
-#define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
-			   (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
+#define NLMSG_OK(nlh,len) ((nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
 			   (nlh)->nlmsg_len <= (len))
 #define NLMSG_PAYLOAD(nlh,len) ((nlh)->nlmsg_len - NLMSG_SPACE((len)))
 
-- 
2.43.0