linux-kernel - Re: [PATCH 2/5 v2] logger: simplify and optimize get_entry

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Date:	Wed, 8 Feb 2012 21:58:28 -0800
From:	Tim Bird <tim.bird@...sony.com>
To:	Dima Zavin <dima@...roid.com>
CC:	Greg KH <gregkh@...uxfoundation.org>,
	linux-embedded <linux-embedded@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Brian Swetland <swetland@...gle.com>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH 2/5 v2] logger: simplify and optimize get_entry_len

On 2/8/2012 9:15 PM, Dima Zavin wrote:
> On Wed, Feb 8, 2012 at 10:37 AM, Tim Bird<tim.bird@...sony.com>  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@...sony.com>
>> ---
>>   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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