llvm dev - Jan 2013 - [LLVMdev] [PATCH] parallel loop awareness to the LoopVectorizer

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

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
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

----- Original Message -----
> From: "Nadav Rotem" <nrotem at apple.com>
> To: "Pekka Jääskeläinen" <pekka.jaaskelainen at tut.fi>
> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Monday, January 28, 2013 10:45:36 AM
> Subject: Re: [LLVMdev] [PATCH] parallel loop awareness to the
LoopVectorizer
> 
> 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.
Is this sufficient to implement #pragma ivdep in clang?

 -Hal
> 
> 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
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

On 01/28/2013 06:45 PM, Nadav Rotem wrote:
> 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.
OK. Any opinions on the location of the isParallelLoop() check? Shall I
put it to Loop so it is more widely accessible? I.e. Loop->isParallel().

-- 
Pekka