[LLVMdev] [PATCH] parallel loop awareness to the LoopVectorizer

[Nadav Rotem]

Hi Pekka, 

I am okay with this patch, assuming that you follow the review of Tobias and Renato and provide a separate patch for the min-iter-count and a few test cases.

I think that it would be a good idea to start a new thread and to discuss the best way to annotate loops in LLVM. 

Thanks,
Nadav

On Jan 28, 2013, at 5:49 AM, Pekka Jääskeläinen <pekka.jaaskelainen at tut.fi> wrote:

> Hi Renato,
> 
> On 01/28/2013 03:22 PM, Renato Golin wrote:
>> This seems an awfully specific check on a generic part of the code... If
> 
> True. Perhaps the check is better encapsulated, e.g., in the Loop class?
> Or, if there's such thing as a loop-carried data dependency analyzer,
> the correct place could be there, as a trivial "no deps" analysis.
> 
> > this metadata standard in any form? If this OpenCL specific? Does all
> 
> This metadata is not standard in any form. Therefore the request
> for comments. However, its meaning is generic, not OpenCL
> specific at all. It specifies that the loop iterations can be
> treated as independent, regardless of the memory operations the
> body contains. Thus, the potential cross-iteration memory dependencies
> can be considered a programming error.
> 
> > OpenCL front-ends generate the same meta-data in that way? Etc...
> 
> I have no knowledge of other OpenCL implementations than
> pocl as I haven't seen their code.
> 
>>    It also converts the "min iteration count to vectorize" to a
>>    parameter so
>>    this can be controlled from the command line.
>> 
>> 
>> Is this really necessary? Do you have use cases where this would make sense?
> 
> Where a lower threshold could be useful? At least with loops having long
> bodies and loops with outer loops that iterate the inner loop many
> times.
> 
> In fact, shouldn't the default minimum be the minimum vector width of the
> machine? The cost estimation routine should take care of the actual
> profitability estimate?
> 
>> I think you should send a test case with this patch, not separate.
> 
> As soon as there's a consensus on the metadata format and where
> the check shall reside in, I'll prepare a proper patch with
> a vectorizer test case.
> 
> Thanks for the comments so far,
> -- 
> Pekka
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