[llvm] r200294 - [vectorizer] Completely disable the block frequency guidance of the loop

[Tobias Grosser]

On 01/28/2014 04:10 AM, Chandler Carruth wrote:
> Author: chandlerc
> Date: Tue Jan 28 03:10:41 2014
> New Revision: 200294
>
> URL: http://llvm.org/viewvc/llvm-project?rev=200294&view=rev
> Log:
> [vectorizer] Completely disable the block frequency guidance of the loop
> vectorizer, placing it behind an off-by-default flag.
>
> It turns out that block frequency isn't what we want at all, here or
> elsewhere. This has been I think a nagging feeling for several of us
> working with it, but Arnold has given some really nice simple examples
> where the results are so comprehensively wrong that they aren't useful.
>
> I'm planning to email the dev list with a summary of why its not really
> useful and a couple of ideas about how to better structure these types
> of heuristics.

Hi Chandler,

just for your info this causes compile time increases from 10 - 15% for 
almost all TSVC kernels which has not been recovered since this commit.

http://llvm.org/perf/db_default/v4/nts/21436?num_comparison_runs=10&test_filter=&test_min_value_filter=&aggregation_fn=median&compare_to=21433&submit=Update

I do not see any benefits in compile time or execution time. So it seems 
even if the block frequency was not what we wanted, it still did 
something good (by accident?).

Cheers,
Tobias