Re: Linux doesn't follow x86/x86-64 ABI wrt direction flag

Chris Lattner
2008-03-07T04:56:24Z

On Mar 6, 2008, at 2:06 PM, Andi Kleen wrote:

> On Thu, Mar 06, 2008 at 12:56:16PM -0800, H. Peter Anvin wrote:
>> Richard Guenther wrote:
>>>
>>> A patched GCC IMHO makes only sense if it is always-on, yet
>>> another option
>>> won't help in corner cases. And corner cases is exactly what
>>> people seem
>>> to care about. For this reason that we have this single release,
>>> 4.3.0,
>>> that
>>> behaves "bad" is already a problem.
>>>
>>
>> The option will help embedded vendors who can guarantee that it's
>> not a
>> problem.
>
> For very very low values of "help".
>
> To be realistic it is very unlikely anybody will measure a difference
> from a few more or a few less clds in a program. It's not that they're
> expensive instructions

They aren't? According to http://www.agner.org/optimize/instruction_tables.pdf
, they have a latency of 52 cycles on at least one popular x86 chip.

-Chris