[PATCH v2] libuuid: improve uuid_unparse() performance

* [PATCH v2] libuuid: improve uuid_unparse() performance
@ 2020-03-26 14:38 Aurelien LAJOIE
  2020-03-31 13:30 ` Karel Zak
  2020-04-01  8:31 ` Karel Zak
  0 siblings, 2 replies; 7+ messages in thread
From: Aurelien LAJOIE @ 2020-03-26 14:38 UTC (permalink / raw)
  To: util-linux; +Cc: Aurelien LAJOIE

There is 2 improvements:

 * remove useless uuid_unpack,
 * directly print the hexa format from memory without using printf
   we can do this as the bytes order is the network byte order
   https://tools.ietf.org/html/rfc4122#section-4.1.2
   even the spatially unique node identifier(the last 6 bytes)

The improvement is important, some results for 1000000 uuid_unparse calls:

Little Endian Ubuntu:
before took 382623 us
after  took  36740 us

Big Endian OpenBSD:
before took 3138172 us
after  took  180116 us

Signed-off-by: Aurelien LAJOIE <orel@melix.net>
---
 libuuid/src/unparse.c | 45 ++++++++++++++++++++-----------------------
 1 file changed, 21 insertions(+), 24 deletions(-)

diff --git a/libuuid/src/unparse.c b/libuuid/src/unparse.c
index a95bbb042..0e7e8eae4 100644
--- a/libuuid/src/unparse.c
+++ b/libuuid/src/unparse.c
@@ -36,41 +36,38 @@
 
 #include "uuidP.h"
 
-static const char *fmt_lower =
-	"%08x-%04x-%04x-%02x%02x-%02x%02x%02x%02x%02x%02x";
+static char const hexdigits_lower[16] = "0123456789abcdef";
+static char const hexdigits_upper[16] = "0123456789ABCDEF";
 
-static const char *fmt_upper =
-	"%08X-%04X-%04X-%02X%02X-%02X%02X%02X%02X%02X%02X";
-
-#ifdef UUID_UNPARSE_DEFAULT_UPPER
-#define FMT_DEFAULT fmt_upper
-#else
-#define FMT_DEFAULT fmt_lower
-#endif
-
-static void uuid_unparse_x(const uuid_t uu, char *out, const char *fmt)
+static void uuid_fmt(const uuid_t uuid, char *buf, char const fmt[restrict])
 {
-	struct uuid uuid;
+	char *p = buf;
 
-	uuid_unpack(uu, &uuid);
-	sprintf(out, fmt,
-		uuid.time_low, uuid.time_mid, uuid.time_hi_and_version,
-		uuid.clock_seq >> 8, uuid.clock_seq & 0xFF,
-		uuid.node[0], uuid.node[1], uuid.node[2],
-		uuid.node[3], uuid.node[4], uuid.node[5]);
+	for (int i = 0; i < 16; i++) {
+		if (i == 4 || i == 6 || i == 8 || i == 10) {
+			*p++ = '-';
+		}
+		size_t tmp = uuid[i];
+		*p++ = fmt[tmp >> 4];
+		*p++ = fmt[tmp & 15];
+	}
+	*p = '\0';
 }
 
 void uuid_unparse_lower(const uuid_t uu, char *out)
 {
-	uuid_unparse_x(uu, out,	fmt_lower);
+	uuid_fmt(uu, out, hexdigits_lower);
 }
 
 void uuid_unparse_upper(const uuid_t uu, char *out)
 {
-	uuid_unparse_x(uu, out,	fmt_upper);
+	uuid_fmt(uu, out, hexdigits_upper);
 }
 
+#ifdef UUID_UNPARSE_DEFAULT_UPPER
 void uuid_unparse(const uuid_t uu, char *out)
-{
-	uuid_unparse_x(uu, out, FMT_DEFAULT);
-}
+	__attribute__((alias("uuid_unparse_upper")));
+#else
+void uuid_unparse(const uuid_t uu, char *out)
+	__attribute__((alias("uuid_unparse_lower")));
+#endif
-- 
2.20.1


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