linux-kernel - Re: [PATCH 2/6] x86-64: Don't generate cmov in vread

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Message-ID: <20110329061546.GA27398@elte.hu>
Date:	Tue, 29 Mar 2011 08:15:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andy Lutomirski <luto@....EDU>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/6] x86-64: Don't generate cmov in vread_tsc


* Andy Lutomirski <luto@....EDU> wrote:

> -	/* This doesn't multiply 'zero' by anything, which *should*
> -	 * generate nicer code, except that gcc cleverly embeds the
> -	 * dereference into the cmp and the cmovae.  Oh, well.
> +	/* This doesn't multiply 'zero' by anything, which generates
> +	 * very slightly nicer code than multiplying it by 8.
>  	 */
>  	last = *( (cycle_t *)
>  		  ((char *)&__vsyscall_gtod_data.clock.cycle_last + zero) );
>  
> -	return ret >= last ? ret : last;
> +	if (likely(ret >= last))
> +		return ret;
> +
> +	/* GCC likes to generate cmov here, but this branch is extremely
> +	   predictable (it's just a funciton of time and the likely is
> +	   very likely) and there's a data dependence, so force GCC
> +	   to generate a branch instead. */
> +	asm volatile ("");

barrier() would do the same, right?

Also, a nit, please use the customary (multi-line) comment style:

  /*
   * Comment .....
   * ...... goes here.
   */

specified in Documentation/CodingStyle.

Thanks,

        Ingo
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