[PATCH 1/4] lib: vsprintf: Optimize division by 10 for small integers.

[George Spelvin <linux@xxxxxxxxxxx>]

Shrink the reciprocal approximations used in put_dec_full4
based on the comments in put_dec_full9.

Signed-off-by: George Spelvin <linux@xxxxxxxxxxx>
Cc: Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx>
Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
---
 lib/vsprintf.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

I was looking over the code and noticed that the constants could be smaller.

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index c3f36d41..2f32fe8 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -243,13 +243,14 @@ char *put_dec(char *buf, unsigned long long n)
 
 /* Second algorithm: valid only for 64-bit long longs */
 
+/* See comment in put_dec_full9 for choice of constants */
 static noinline_for_stack
 char *put_dec_full4(char *buf, unsigned q)
 {
 	unsigned r;
-	r      = (q * 0xcccd) >> 19;
+	r      = (q * 0xccd) >> 15;
 	*buf++ = (q - 10 * r) + '0';
-	q      = (r * 0x199a) >> 16;
+	q      = (r * 0xcd) >> 11;
 	*buf++ = (r - 10 * q)  + '0';
 	r      = (q * 0xcd) >> 11;
 	*buf++ = (q - 10 * r)  + '0';
-- 
1.7.10.4

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/