linux-kernel - [patch 2/3] ipc: use shifts to extract seq/idx

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Date:	Thu, 20 May 2010 17:00:04 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Manfred Spraul <manfred@...orfullife.com>,
	linux-kernel@...r.kernel.org
Subject: [patch 2/3] ipc: use shifts to extract seq/idx


All the ipc ids and sequences are signed integers, so power of 2
sequence multiplier does not work so well. Convert it to use shifts,
which improves generated code particularly in ipc_lock/ipc_lock_check
fast paths.

Index: linux-2.6/ipc/util.c
===================================================================
--- linux-2.6.orig/ipc/util.c
+++ linux-2.6/ipc/util.c
@@ -123,7 +123,7 @@ void ipc_init_ids(struct ipc_ids *ids)
 	ids->in_use = 0;
 	ids->seq = 0;
 	{
-		int seq_limit = INT_MAX/SEQ_MULTIPLIER;
+		int seq_limit = INT_MAX >> SEQ_SHIFT;
 		if (seq_limit > USHORT_MAX)
 			ids->seq_max = USHORT_MAX;
 		 else
Index: linux-2.6/ipc/util.h
===================================================================
--- linux-2.6.orig/ipc/util.h
+++ linux-2.6/ipc/util.h
@@ -13,9 +13,11 @@
 #include <linux/unistd.h>
 #include <linux/err.h>
 
-#define IPCMNI_MAX 32768  /* <= MAX_INT absolute limit for ipc arrays */
+/* IPCMNI_MAX should be <= MAX_INT, absolute limit for ipc arrays */
+#define IPCMNI_MAX_SHIFT	15
+#define IPCMNI_MAX		(1 << IPCMNI_MAX_SHIFT)
 
-#define SEQ_MULTIPLIER	(IPCMNI_MAX)
+#define SEQ_SHIFT		IPCMNI_MAX_SHIFT
 
 void sem_init (void);
 void msg_init (void);
@@ -93,7 +95,7 @@ void __init ipc_init_proc_interface(cons
 #define IPC_MSG_IDS	1
 #define IPC_SHM_IDS	2
 
-#define ipcid_to_idx(id) ((id) % SEQ_MULTIPLIER)
+#define ipcid_to_idx(id) ((id) & ((1UL << SEQ_SHIFT) - 1))
 
 /* must be called with ids->rw_mutex acquired for writing */
 int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);
@@ -146,7 +148,7 @@ extern void recompute_msgmni(struct ipc_
 
 static inline int ipc_buildid(int id, int seq)
 {
-	return SEQ_MULTIPLIER * seq + id;
+	return (seq << SEQ_SHIFT) + id;
 }
 
 /*
@@ -154,7 +156,7 @@ static inline int ipc_buildid(int id, in
  */
 static inline int ipc_checkid(struct kern_ipc_perm *ipcp, int uid)
 {
-	if (uid / SEQ_MULTIPLIER != ipcp->seq)
+	if ((uid >> SEQ_SHIFT) != ipcp->seq)
 		return 1;
 	return 0;
 }
--
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