Blackfin: optimize strncpy a bit

Add a little strncpy optimization which can easily cut boot time by 20%.

When the kernel is booting with initramfs, it builds up the filesystem
from a cpio archive by calling strncpy_from_user() via fs/namei.c's
do_getname() on every file in the archive (which can be lots) with a
length of PATH_MAX (1024).  This causes the dest of the strncpy to be
padded with many NUL bytes.

This optimization mostly causes these NUL bytes to be padded with a call
to memset() which is already optimized for filling memory quickly, but
the hardware loop helps a little bit as well.

Boot time measured with 'loglevel=0' so UART speed doesn't get in the way.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

arch/blackfin/lib/memset.S

@@ -20,6 +20,7 @@
  * R1 = filler byte
  * R2 = count
  * Favours word aligned data.
+ * The strncpy assumes that I0 and I1 are not used in this function
  */
 
 ENTRY(_memset)

arch/blackfin/lib/strncpy.S

@@ -5,12 +5,14 @@
  */
 
 #include <linux/linkage.h>
+#include <asm/context.S>
 
 /* void *strncpy(char *dest, const char *src, size_t n);
  * R0 = address (dest)
  * R1 = address (src)
  * R2 = size
- * Returns a pointer to the destination string dest
+ * Returns a pointer (R0) to the destination string dest
+ *  we do this by not changing R0
  */
 
 #ifdef CONFIG_STRNCPY_L1
@@ -24,29 +26,60 @@
 ENTRY(_strncpy)
 	CC = R2 == 0;
 	if CC JUMP 4f;
+
+	P2 = R2 ;       /* size */
 	P0 = R0 ;       /* dst*/
 	P1 = R1 ;       /* src*/
 
+	LSETUP (1f, 2f) LC0 = P2;
 1:
 	R1 = B [P1++] (Z);
 	B [P0++] = R1;
-	CC = R1;
-	if ! cc jump 2f;
-	R2 += -1;
-	CC = R2 == 0;
-	if ! cc jump 1b (bp);
-	jump 4f;
+	CC = R1 == 0;
 2:
-	/* if src is shorter than n, we need to null pad bytes in dest */
-	R1 = 0;
+	if CC jump 3f;
+
+	RTS;
+
+	/* if src is shorter than n, we need to null pad bytes in dest
+	 * but, we can get here when the last byte is zero, and we don't
+	 * want to copy an extra byte at the end, so we need to check
+	 */
 3:
+	R2 = LC0;
+	CC = R2
+	if ! CC jump 6f;
+
+	/* if the required null padded portion is small, do it here, rather than
+	 * handling the overhead of memset (which is OK when things are big).
+	 */
+	R3 = 0x20;
+	CC = R2 < R3;
+	IF CC jump 4f;
+
 	R2 += -1;
-	CC = R2 == 0;
-	if cc jump 4f;
-	B [P0++] = R1;
-	jump 3b;
+
+	/* Set things up for memset
+	 * R0 = address
+	 * R1 = filler byte (this case it's zero, set above)
+	 * R2 = count (set above)
+	 */
+
+	I1 = R0;
+	R0 = RETS;
+	I0 = R0;
+	R0 = P0;
+	pseudo_long_call _memset, p0;
+	R0 = I0;
+	RETS = R0;
+	R0 = I1;
+	RTS;
 
 4:
+	LSETUP(5f, 5f) LC0;
+5:
+	B [P0++] = R1;
+6:
 	RTS;
 
 ENDPROC(_strncpy)