lkml.org 
[lkml]   [2011]   [Mar]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 2/6] x86-64: Don't generate cmov in vread_tsc
On Tue, Mar 29, 2011 at 2:15 AM, Ingo Molnar <mingo@elte.hu> wrote:
>
> * Andy Lutomirski <luto@MIT.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
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2011-03-29 13:55    [from the cache]
©2003-2014 Jasper Spaans. hosted at Digital Ocean