linux-kernel - Re: [RFC] microoptimizing hlist_add

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Date:   Sat, 21 Sep 2019 04:11:17 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC] microoptimizing hlist_add_{before,behind}

On Sat, Sep 21, 2019 at 12:12:33AM +0100, Al Viro wrote:
> 	Neither hlist_add_before() nor hlist_add_behind() should ever
> be called with both arguments pointing to the same hlist_node.
> However, gcc doesn't know that, so it ends up with pointless reloads.
> AFAICS, the following generates better code, is obviously equivalent
> in case when arguments are different and actually even in case when
> they are same, the end result is identical (if the hlist hadn't been
> corrupted even earlier than that).
> 
> 	Objections?
> 
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>

*gyah*

git diff >/tmp/y1
<build>
<fix a braino>
<test>
scp-out /tmp/y1
<send mail with the original diff>
<several hours later: reread the sent mail>

My apologies ;-/  Correct diff follows:

diff --git a/include/linux/list.h b/include/linux/list.h
index 85c92555e31f..5c84383675bc 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -793,21 +793,21 @@ static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h)
 static inline void hlist_add_before(struct hlist_node *n,
 					struct hlist_node *next)
 {
-	n->pprev = next->pprev;
+	struct hlist_node **p = n->pprev = next->pprev;
 	n->next = next;
 	next->pprev = &n->next;
-	WRITE_ONCE(*(n->pprev), n);
+	WRITE_ONCE(*p, n);
 }
 
 static inline void hlist_add_behind(struct hlist_node *n,
 				    struct hlist_node *prev)
 {
-	n->next = prev->next;
+	struct hlist_node *p = n->next = prev->next;
 	prev->next = n;
 	n->pprev = &prev->next;
 
-	if (n->next)
-		n->next->pprev  = &n->next;
+	if (p)
+		p->pprev  = &n->next;
 }
 
 /* after that we'll appear to be on some hlist and hlist_del will work */