linux-kernel - [PATCH 2/2] lib/vsprintf.c: Further simplify uuid

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Message-ID: <20150511195501.3772.qmail@ns.horizon.com>
Date:	11 May 2015 15:55:01 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	joe@...ches.com, linux@...musvillemoes.dk
Cc:	linux-kernel@...r.kernel.org, linux@...izon.com
Subject: [PATCH 2/2] lib/vsprintf.c: Further simplify uuid_string()

Make the endianness permutation table do double duty by having it
list not source offsets, but destination offsets.  Thus, it both puts
the bytes in the right order and skips the hyphens.

This further shrinks the code from 256 to 214 bytes.  Eliminating
erratic branches probably helps speed, too.

This has been verified to produce the same output as the older code using
a user-space test harness.

Signed-off-by: George Spelvin <linux@...izon.com>
---
 lib/vsprintf.c | 25 +++++++++----------------
 1 file changed, 9 insertions(+), 16 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 4c4f9055..38c1d87e 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1265,10 +1265,9 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
 		  struct printf_spec spec, const char *fmt)
 {
 	char uuid[sizeof("xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx")];
-	char *p = uuid;
 	int i;
-	static const u8 be[16] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
-	static const u8 le[16] = {3,2,1,0,5,4,7,6,8,9,10,11,12,13,14,15};
+	static const u8 be[16] = {0,2,4,6,9,11,14,16,19,21,24,26,28,30,32,34};
+	static const u8 le[16] = {6,4,2,0,11,9,16,14,19,21,24,26,28,30,32,34};
 	const u8 *index = be;
 	const char *hex = hex_asc;
 
@@ -1284,20 +1283,14 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
 	}
 
 	for (i = 0; i < 16; i++) {
-		u8 byte = addr[index[i]];
-		*p++ = hex[byte >> 4];
-		*p++ = hex[byte & 0x0f];
-		switch (i) {
-		case 3:
-		case 5:
-		case 7:
-		case 9:
-			*p++ = '-';
-			break;
-		}
-	}
+		u8 byte = addr[i];
+		char *p = uuid + index[i];
 
-	*p = 0;
+		p[0] = hex[byte >> 4];
+		p[1] = hex[byte & 0x0f];
+	}
+	uuid[23] = uuid[18] = uuid[13] = uuid[8] = '-';
+	uuid[36] = 0;
 
 	return string(buf, end, uuid, spec);
 }
-- 
2.1.4

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