llvm dev - Apr 2015 - [LLVMdev] Loss of precision with very large branch weights

On Fri, Apr 24, 2015 at 2:32 PM, Xinliang David Li <davidxl at google.com>
wrote:
> Since we have decided to capture the global hotness using function
> entry count, it is ok to CAP the max weight as of today. I think the
> remaining bug in PR22718 is that when capping happens, the scaling is
> not done. That needs to be fixed.
>
The problem here is that the two loops show up equally hot. So the function
entry count for bar() won't really help since it seems like both loops call
bar() equally frequently.

> The efficiency of the interface is a different issue.
>
>
Yeah. I'm looking at the hotness calculation first.


Diego.
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On Fri, Apr 24, 2015 at 11:36 AM, Diego Novillo <dnovillo at google.com>
wrote:
>
>
> On Fri, Apr 24, 2015 at 2:32 PM, Xinliang David Li <davidxl at
google.com>
> wrote:
>>
>> Since we have decided to capture the global hotness using function
>> entry count, it is ok to CAP the max weight as of today. I think the
>> remaining bug in PR22718 is that when capping happens, the scaling is
>> not done. That needs to be fixed.
>
>
> The problem here is that the two loops show up equally hot. So the function
> entry count for bar() won't really help since it seems like both loops
call
> bar() equally frequently.
>
Isn't that the direct result of  the branch weights not being scaled
(after reaching the cap) -- thus leading to wrong branch probability
(computed from weights)? Wrong probability leads to wrong Frequency
propagation.

David
>>
>> The efficiency of the interface is a different issue.
>>
>
> Yeah. I'm looking at the hotness calculation first.
>
>
> Diego.

On 04/24/15 14:44, Xinliang David Li wrote:
>
> Isn't that the direct result of  the branch weights not being scaled
> (after reaching the cap) -- thus leading to wrong branch probability
> (computed from weights)? Wrong probability leads to wrong Frequency
> propagation.
Yup, I'm trying to see if we couldn't just use 64bit values all over to 
make things easier. The drawback would be that we are just punting the 
problem to a different scale of values (but, it may be enough).


Diego.