[PATCH net-next v2 2/3] 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 <redacted>
---

Changes for v2:
	- Remember to zero next_free_slot and next_cmd
	  (they are removed by patch 3/3).
	- Update commit message to mention that zeroing the union isn't
	  really necessary.

A further patch could delete the assignments to 'zero-all'.
If the wrong sized union members are used that would expose
bugs on LE systems that currently only affect BE systems.

 include/net/sctp/command.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/net/sctp/command.h b/include/net/sctp/command.h
index 020eb95..a4f76d3 100644
--- a/include/net/sctp/command.h
+++ b/include/net/sctp/command.h

@@ -118,6 +118,7 @@ typedef enum {
 #define SCTP_MAX_NUM_COMMANDS 18
 
 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.  */
 }
 

-- 
1.8.1.2