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

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Date:	Wed, 8 Feb 2012 10:37:57 -0800
From:	Tim Bird <tim.bird@...sony.com>
To:	Greg KH <gregkh@...uxfoundation.org>,
	linux-embedded <linux-embedded@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Brian Swetland <swetland@...gle.com>,
	Dima Zavin <dima@...roid.com>, Andrew Morton <akpm@...l.org>
Subject: [PATCH 2/5 v2] logger: simplify and optimize get_entry_len

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;

-	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];
+	else
+		((__u8 *)&val)[1] = log->buffer[0];

 	return sizeof(struct logger_entry) + val;
 }
-- 
1.7.2.3

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