linux-kernel - [PATCH 4/5] trace-cmd: Use unsigned values in Hsieh's trace

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Message-Id: <20170812173047.840-5-mikesart@fastmail.com>
Date:   Sat, 12 Aug 2017 11:30:46 -0600
From:   Michael Sartain <mikesart@...tmail.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Michael Sartain <mikesart@...tmail.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 4/5] trace-cmd: Use unsigned values in Hsieh's trace_hash fast hash function

Signed int values were being used where the original code used uint32_t types:

  http://www.azillionmonkeys.com/qed/hash.html

Right shifting negative int values has implementation-defined and left shifting
has undefined behavior.

On my platform (x86_64) right shifting was doing sign extension and filling
high bits with 1s, which is different than the original algorithm.

Signed-off-by: Michael Sartain <mikesart@...tmail.com>
---
 trace-hash-local.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/trace-hash-local.h b/trace-hash-local.h
index b2a1002..b3f9b06 100644
--- a/trace-hash-local.h
+++ b/trace-hash-local.h
@@ -22,7 +22,7 @@
 
 static inline unsigned int trace_hash(int val)
 {
-	int hash, tmp;
+	unsigned int hash, tmp;
 
 	hash = 12546869;	/* random prime */
 
@@ -34,7 +34,7 @@ static inline unsigned int trace_hash(int val)
 	 */
 
 	hash +=	(val & 0xffff);
-	tmp = (val >> 16) ^ hash;
+	tmp = ((unsigned int)val >> 16) ^ hash;
 	hash = (hash << 16) ^ tmp;
 	hash += hash >> 11;
 
-- 
2.13.2