Re: [PATCH 2/5 v2] logger: simplify and optimize get_entry_len

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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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