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

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Date:	Tue, 29 Mar 2011 07:52:52 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Ingo Molnar <mingo@...e.hu>
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

On Tue, Mar 29, 2011 at 2:15 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * 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?

Yes.  It's overkill (the memory clobber is unnecessary) but should be harmless.

I'll take a look at folding this and [1/6] together and sticking them
in a .S.  The down side is that rdtsc_barrier() expends to two
alternatives, and it has other callers.  I'll see what it looks like.

>
> Also, a nit, please use the customary (multi-line) comment style:
>
>  /*
>   * Comment .....
>   * ...... goes here.
>   */
>
> specified in Documentation/CodingStyle.

I was hoping checkpatch would warn me :)

--Andy

>
> Thanks,
>
>        Ingo
>
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