linux-kernel - Re: [RFT/PATCH v2 3/6] x86-64: Don't generate cmov in vread

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Message-ID: <20110407075456.GC24879@elte.hu>
Date:	Thu, 7 Apr 2011 09:54:56 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andy Lutomirski <luto@....edu>
Cc:	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFT/PATCH v2 3/6] x86-64: Don't generate cmov in vread_tsc


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

> vread_tsc checks whether rdtsc returns something less than
> cycle_last, which is an extremely predictable branch.  GCC likes
> to generate a cmov anyway, which is several cycles slower than
> a predicted branch.  This saves a couple of nanoseconds.
> 
> Signed-off-by: Andy Lutomirski <luto@....edu>
> ---
>  arch/x86/kernel/tsc.c |   19 +++++++++++++++----
>  1 files changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 858c084..69ff619 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -794,14 +794,25 @@ static cycle_t __vsyscall_fn vread_tsc(void)
>  	 */
>  
>  	/*
> -	 * 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 *)&VVAR(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.  I don't barrier() because
> +	 * we don't actually need a barrier, and if this function
> +	 * ever gets inlined it will generate worse code.
> +	 */
> +	asm volatile ("");

Hm, you have not addressed the review feedback i gave in:

  Message-ID: <20110329061546.GA27398@...e.hu>

Thanks,

	Ingo
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