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

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Message-ID: <20171009183055.567190e7@gandalf.local.home>
Date:   Mon, 9 Oct 2017 18:30:55 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Michael Sartain <mikesart@...tmail.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] trace-cmd: Use unsigned values in Hsieh's
 trace_hash fast hash function

On Sat, 12 Aug 2017 11:30:46 -0600
Michael Sartain <mikesart@...tmail.com> wrote:

> 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.
> 

Heh, nice catch. Although the hash was never used for anything too
important. Mostly just colors of the graph.

> 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;

Why not just have val be unsigned int, and not do the typecast?

-- Steve

>  	hash = (hash << 16) ^ tmp;
>  	hash += hash >> 11;
>