likely/unlikely

likely/unlikely

[David Schwartz]

	By the way, has anybody actually benchmarked these macros to see if they 
make any difference. I did a few quick inexact benchmarks on test code and 
found that in most cases there was no difference and in some cases code was 
made worse.

-- 
David Schwartz
<davids@webmaster.com>

Re: likely/unlikely

[Jeff Garzik]

David Schwartz wrote:
> 
>         By the way, has anybody actually benchmarked these macros to see if they
> make any difference. I did a few quick inexact benchmarks on test code and
> found that in most cases there was no difference and in some cases code was
> made worse.

likely/unlikely set the branch prediction values to 99% or 1%
respectively.  If this causes the code generated to perform less
optimally than without, I'm sure the gcc guys would be -very- interested
to hear that...

-- 
Jeff Garzik      | Alternate titles for LOTR:
Building 1024    | Fast Times at Uruk-Hai
MandrakeSoft     | The Took, the Elf, His Daughter and Her Lover
                 | Samwise Gamgee: International Hobbit of Mystery

Re: likely/unlikely

[Anton Blanchard]

 
> likely/unlikely set the branch prediction values to 99% or 1%
> respectively.  If this causes the code generated to perform less
> optimally than without, I'm sure the gcc guys would be -very- interested
> to hear that...

On some ppc64 the branch prediction is quite good and static prediction
will override the dynamic prediction. I think we avoid predicting a
branch unless we are quite sure (95%/5%).

So if likely/unlikely is overused (on more marginal conditionals) then
it could be a performance loss.

Anton

Re: likely/unlikely

[Jeff Garzik]

Anton Blanchard wrote:
> 
> 
> > likely/unlikely set the branch prediction values to 99% or 1%
> > respectively.  If this causes the code generated to perform less
> > optimally than without, I'm sure the gcc guys would be -very- interested
> > to hear that...
> 
> On some ppc64 the branch prediction is quite good and static prediction
> will override the dynamic prediction. I think we avoid predicting a
> branch unless we are quite sure (95%/5%).
> 
> So if likely/unlikely is overused (on more marginal conditionals) then
> it could be a performance loss.

oh agreed... but marginal conditionals should not be getting
likely()/unlikely() as you are then lying to the compiler about the true
branch predictability...

	Jeff


-- 
Jeff Garzik      | Alternate titles for LOTR:
Building 1024    | Fast Times at Uruk-Hai
MandrakeSoft     | The Took, the Elf, His Daughter and Her Lover
                 | Samwise Gamgee: International Hobbit of Mystery

Re: likely/unlikely

[Craig Christophel]

> likely/unlikely set the branch prediction values to 99% or 1%


	So all of the BUG() routines in the kernel would benifit greatly from this.



Craig.

Re: likely/unlikely

[Adrian Bunk]

On Wed, 16 Jan 2002, Craig Christophel wrote:

> > likely/unlikely set the branch prediction values to 99% or 1%
>
>
> 	So all of the BUG() routines in the kernel would benifit greatly from this.

In the 2.5 kernels this is done when you use the BUG_ON macro that is
defined as follows:

#define BUG_ON(condition) do { if (unlikely((condition)!=0)) BUG(); } while(0)


BTW: Note that likely/unlikely doesn't has any effect for most of us
     because __builtin_expect is only available in gcc >= 2.96.


> Craig.

cu
Adrian

Re: likely/unlikely

[Jeff Garzik]

Craig Christophel wrote:
> 
> > likely/unlikely set the branch prediction values to 99% or 1%
> 
>         So all of the BUG() routines in the kernel would benifit greatly from this.


It's likely :)  I would put one unlikely() in the definition of BUG_ON,
rather rather touching all the code that calls BUG(), though.

	Jeff



-- 
Jeff Garzik      | Alternate titles for LOTR:
Building 1024    | Fast Times at Uruk-Hai
MandrakeSoft     | The Took, the Elf, His Daughter and Her Lover
                 | Samwise Gamgee: International Hobbit of Mystery

Re: likely/unlikely

[Jens Axboe]

On Wed, Jan 16 2002, Jeff Garzik wrote:
> Craig Christophel wrote:
> > 
> > > likely/unlikely set the branch prediction values to 99% or 1%
> > 
> >         So all of the BUG() routines in the kernel would benifit greatly from this.
> 
> 
> It's likely :)  I would put one unlikely() in the definition of BUG_ON,
> rather rather touching all the code that calls BUG(), though.

BUG_ON has had unlikely since the very beginning :-)

-- 
Jens Axboe