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Commit b9420e1c authored by David Laight's avatar David Laight Committed by David S. Miller
Browse files

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: default avatarDavid Laight <david.laight@aculab.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent be1f4f48

include/net/sctp/command.h

+5 −3
Original line number Diff line number Diff line
@@ -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.  */

}