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

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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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