RE: [PATCH net-next 2/3] net: sctp: Optimise the way 'sctp_arg_t' values are initialised
From: David Laight <hidden>
Date: 2014-07-01 13:15:35
Also in:
linux-sctp
From: David Laight
quoted hunk ↗ jump to hunk
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. Signed-off-by: David Laight <redacted> --- include/net/sctp/command.h | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)diff --git a/include/net/sctp/command.h b/include/net/sctp/command.h index 020eb95..5fcd1a7 100644 --- a/include/net/sctp/command.h +++ b/include/net/sctp/command.h
...
quoted hunk ↗ jump to hunk
@@ -212,7 +213,6 @@ typedef struct { */ static inline int sctp_init_cmd_seq(sctp_cmd_seq_t *seq) { - memset(seq, 0, sizeof(sctp_cmd_seq_t)); return 1; /* We always succeed. */ }
I've realised that this version of this chunk leaves seq->next_free_slot and seq->next_cmd uninitialized (they need to be zero). The next patch in the series contains different initialisers - which are correct. I guess this needs fixing for 'git bisect' ? David