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

[Tim Bird]

On 2/8/2012 9:15 PM, Dima Zavin wrote:
> 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;
That's a good idea.  I was looking for a way to use a byte array that
wouldn't get misaligned if the function declaration changed.  I'll
test this out and see what it looks like.
> > -       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.
Yeah.  I keep omitting spaces.  I'll fix this, and the ones
mentioned on patch 1/5.
 -- Tim

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