On Wed, Feb 8, 2012 at 10:37 AM, Tim Bird <tim.bird@xxxxxxxxxxx> wrote:
> Make this code slightly easier to read, and eliminate calls
> to sub-routines. Some of these were previously optimized away
> by the compiler, but one memcpy was not.
>
> In my testing, this makes the code about 20% smaller, and
> has no sub-routine calls and no branches (on ARM).
>
> v2 of this patch is, IMHO, easier to read than v1. Compared to
> that patch it uses __u8 instead of unsigned char, for
> consistency with the __u16 val data type, simplifies the
> conditional expression, adds a another comment, and
> moves a common statement out of the if.
>
> Signed-off-by: Tim Bird <tim.bird@xxxxxxxxxxx>
> ---
> drivers/staging/android/logger.c | 20 ++++++++++++--------
> 1 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/staging/android/logger.c b/drivers/staging/android/logger.c
> index 92456d7..3475cb7 100644
> --- a/drivers/staging/android/logger.c
> +++ b/drivers/staging/android/logger.c
> @@ -93,20 +93,24 @@ static inline struct logger_log *file_get_log(struct file *file)
> * get_entry_len - Grabs the length of the payload of the next entry starting
> * from 'off'.
> *
> + * An entry length is 2 bytes (16 bits) in host endian order.
> + * In the log, the length does not include the size of the log entry structure.
> + * This function returns the size including the log entry structure.
> + *
> * Caller needs to hold log->mutex.
> */
> static __u32 get_entry_len(struct logger_log *log, size_t off)
> {
> __u16 val;
Could using a union here make things look a little cleaner in the meat
of the function? Something like
union {
__u16 s;
__u8 b[2];
} val;
>
> - switch (log->size - off) {
> - case 1:
> - memcpy(&val, log->buffer + off, 1);
> - memcpy(((char *) &val) + 1, log->buffer, 1);
> - break;
> - default:
> - memcpy(&val, log->buffer + off, 2);
> - }
> + /* copy 2 bytes from buffer, in memcpy order, */
> + /* handling possible wrap at end of buffer */
> +
> + ((__u8 *)&val)[0] = log->buffer[off];
> + if (likely(off+1 < log->size))
> + ((__u8 *)&val)[1] = log->buffer[off+1];
spaces around the + in 'off+1' in the above two lines.
> + else
> + ((__u8 *)&val)[1] = log->buffer[0];
>
> return sizeof(struct logger_entry) + val;
> }
> --
> 1.7.2.3
>
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