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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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