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net: sctp: Optimise the way 'sctp_arg_t' values are initialised. 

Even if memset() is inlined (as on x86) using it to zero the union
generates a memory word write of zero, followed by a write of the
smaller field, and then a read of the word.
As well as being a lot of instructions the sequence is unlikely to
be optimised by the store-load forward hardware so will be slow.

Instead allocate a field of the union that is the same size as the
entire union and write a zero value to it. The compiler will then
generate the required value in a register.

Zeroing the union shouldn't be necessary, but this patch series isn't
intended to have a behavioural change.

Signed-off-by: David Laight <david.laight@aculab.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
David Laight 2014-07-04 14:35:51 +00:00 committed by David S. Miller
parent be1f4f48ce
commit b9420e1c87
1 changed files with 5 additions and 3 deletions
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8
include/net/sctp/command.h
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@ -118,6 +118,7 @@ typedef enum {
#define SCTP_MAX_NUM_COMMANDS 14
typedef union {
void *zero_all; /* Set to NULL to clear the entire union */
__s32 i32;
__u32 u32;
__be32 be32;
@ -154,7 +155,7 @@ typedef union {
static inline sctp_arg_t \
SCTP_## name (type arg) \
{ sctp_arg_t retval;\
memset(&retval, 0, sizeof(sctp_arg_t));\
retval.zero_all = NULL;\
retval.elt = arg;\
return retval;\
}
@ -191,7 +192,7 @@ static inline sctp_arg_t SCTP_NOFORCE(void)
static inline sctp_arg_t SCTP_NULL(void)
{
sctp_arg_t retval;
memset(&retval, 0, sizeof(sctp_arg_t));
retval.zero_all = NULL;
return retval;
}
@ -212,7 +213,8 @@ typedef struct {
*/
static inline int sctp_init_cmd_seq(sctp_cmd_seq_t *seq)
{
memset(seq, 0, sizeof(sctp_cmd_seq_t));
seq->next_free_slot = 0;
seq->next_cmd = 0;
return 1; /* We always succeed. */
}