linux-kernel - [PATCH 3/4] lib: vsprintf: Optimize put_dec

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Date:	Fri,  3 Aug 2012 01:21:10 -0400
From:	George Spelvin <linux@...izon.com>
To:	vda.linux@...glemail.com, mina86@...a86.com
Cc:	hughd@...gle.com, linux-kernel@...r.kernel.org, linux@...izon.com
Subject: [PATCH 3/4] lib: vsprintf: Optimize put_dec_trunc8

If you're going to have a conditional branch after
each 32x32->64-bit multiply, might as well shrink the code
and make it a loop.

This also avoids using the long multiply for small integers.

(This leaves the comments in a confusing state, but that's a separate
patch to make review easier.)

Signed-off-by: George Spelvin <linux@...izon.com>
---
 lib/vsprintf.c |   20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index a8e7392..3ca77b8 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -174,20 +174,12 @@ char *put_dec_trunc8(char *buf, unsigned r)
 	unsigned q;
 
 	/* Copy of previous function's body with added early returns */
-	q      = (r * (uint64_t)0x1999999a) >> 32;
-	*buf++ = (r - 10 * q) + '0'; /* 2 */
-	if (q == 0)
-		return buf;
-	r      = (q * (uint64_t)0x1999999a) >> 32;
-	*buf++ = (q - 10 * r) + '0'; /* 3 */
-	if (r == 0)
-		return buf;
-	q      = (r * (uint64_t)0x1999999a) >> 32;
-	*buf++ = (r - 10 * q) + '0'; /* 4 */
-	if (q == 0)
-		return buf;
-	r      = (q * (uint64_t)0x1999999a) >> 32;
-	*buf++ = (q - 10 * r) + '0'; /* 5 */
+	while (r >= 10000) {
+		q = r + '0';
+		r  = (r * (uint64_t)0x1999999a) >> 32;
+		*buf++ = q - 10*r;
+	}
+
 	if (r == 0)
 		return buf;
 	q      = (r * 0x199a) >> 16;
-- 
1.7.10.4

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