llvm dev - Mar 2016 - [GSoC'16] Need details on New Transformations and Analyses

> On Mar 22, 2016, at 5:13 PM, Philip Reames <listmail at philipreames.com
<mailto:listmail at philipreames.com>> wrote:
> 
> 
> 
> On 03/20/2016 05:38 AM, Aries Gunawan via llvm-dev wrote:
>> Hi everyone,
>>  
>> I am very interested in contributing to LLVM project in this year’s
GSoC. I am new with LLVM, but this is used in the compiler course in my
university. So, I am thinking to involve in LLVM development to have a better
knowledge of the system. Currently, I am preparing the proposal.
>>  
>> One of the project that caught my eyes is “New Transformations and
Analysis”. Several code transformations and analyses have been introduced in the
compiler course that I am currently taking. That’s why I am thinking to involve
in writing some new transformations and code analyses. But the list of
transformations in the LLVM Open Projects web page seems too brief for me and I
need more details on those stuffs.
>>  
>> Loop Dependence Analysis Infrastructure. I have looked in the source
codes repo and I saw that there is a file named “DependenceAnalysis.cpp”. So,
does that mean this analysis has been implemented?
> I believe major progress has been made it, but haven't been following
it closely.  I'd suggest talking to committers active in this file in the
recent past to determine what useful work might be left of appropriate scope.  \
We actually have two DA frameworks at the moment.  The file you mention is only
used currently by the LoopInterchange pass that is off by default.  There is
also the other framework that I’ve been working on called LoopAccessAnalysis
that’s currently used by the LoopVectorizer, LoopLoadElimination,
LoopDistribution and LICMLoopVersioning (the latter two are off by default).
>>  
>> Value range propagation pass. There was a discussion about this topic
(https://groups.google.com/forum/#!topic/llvm-dev/XXqfemtDX74/discussion
<https://groups.google.com/forum/#%21topic/llvm-dev/XXqfemtDX74/discussion>).
Someone already proposed to do this pass for several years ago GSoC. But I can’t
find the progress of the work. If no progress, then does it mean that the VRP
based on Patterson’s paper need to be implemented although range analysis has
been implemented?
> This is largely stalled.  The key problem is that between LazyValueInfo
(constant ranges) and SCEV (symbolic ranges in loops), there's fairly little
profit to be had and range analysis is relatively expensive.  I'd strongly
discourage you from implementing a traditional range analysis for LLVM without
deeply understanding the history here.
>>  
>> Predictive Commoning. The presentation side by Arie Tal seems provide
quite clear explanation and examples of the algorithm. I guess the
implementation should be straightforward, isn’t it?
> The closest I know of to this in tree is LoadLoadElimination.cpp and (in
some cases) the PRE code inside GVN.cpp.  Building something like this on top of
SCEV could be quite interesting.  You should definitely talk to Adam Nemet
(CC'd) about this
Yes, I think the memory operations can be handled in LoopLoadElimination.  The
algorithm is different from the above paper though.  We use the loop-carried
dependences to find opportunities to reuse loaded values from earlier
iterations.  Thus our approach is more similar to Loop Scalar Replacement by
Steve Carr et al.  It may be possible to extend this to expressions that are
derived from these loads to also cover the additions in the mgrid example cited
by the slides.

Adam
> .
>>  
>> Type Inference (aka. Devirtualization) and Value assertions.
>> Can I get more details of these topics? Does the type inference mean
the translation of auto keyword or something else? For value assertions,
“unreachable” intrinsic seems has been implemented cause I can find the usage in
some of the testcases.
> I believe the "value assertions" link may be stale.   If I'm
reading that correctly, it looks like the motivation for @llvm.assume.
>>  
>> Finally, for this project, must I propose to do all of these analyses
and transformations in my GSoC proposal or can I just propose some of them? In
addition, I am also looking for a mentor for guidance?
> If you want further ideas, consider the list I just sent to llvm-dev a few
moments ago titled "A couple ideas for possible GSoC projects".
>>  
>> Looking forward for your comments and feedbacks.
>> Thank you.
>>  
>>  
>> Best regards,
>>  
>>  
>> Aries Thio.
>>  
>> 
>> 
>> _______________________________________________
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>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> 
> 
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Hi Adam,

On Wed, Mar 23, 2016 at 1:50 PM, Adam Nemet via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> On Mar 22, 2016, at 5:13 PM, Philip Reames <listmail at
philipreames.com>
> wrote:
>
>
>
> On 03/20/2016 05:38 AM, Aries Gunawan via llvm-dev wrote:
>
>
> *Loop Dependence Analysis Infrastructure. *I have looked in the source
> codes repo and I saw that there is a file named “DependenceAnalysis.cpp”.
> So, does that mean this analysis has been implemented?
>
> I believe major progress has been made it, but haven't been following
it
> closely.  I'd suggest talking to committers active in this file in the
> recent past to determine what useful work might be left of appropriate
> scope.  \
>
>
> We actually have two DA frameworks at the moment.  The file you mention is
> only used currently by the LoopInterchange pass that is off by default.
> There is also the other framework that I’ve been working on called
> LoopAccessAnalysis that’s currently used by the LoopVectorizer,
> LoopLoadElimination, LoopDistribution and LICMLoopVersioning (the latter
> two are off by default).
>
Do you think it is reasonable and feasible to provide a common interface
for different loop dependence analyses like alias analysis, such that
passes like LoopVectorizer/LoopLoadElimination/LoopDistribution etc. can
query loop dependency information from different implementations for
precision/compile-time trade off?

I ask because Utpal proposed to introduce dependency information calculated
by Polly to LoopVectorizer. And I am interested in building a common
interface for loop dependence analyses.


Thanks
Hongbin

>
>
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Hi Hongbin,
> On Mar 28, 2016, at 7:12 AM, Hongbin Zheng <etherzhhb at gmail.com>
wrote:
> 
> Hi Adam,
> 
> On Wed, Mar 23, 2016 at 1:50 PM, Adam Nemet via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> 
>> On Mar 22, 2016, at 5:13 PM, Philip Reames <listmail at
philipreames.com <mailto:listmail at philipreames.com>> wrote:
>> 
>> 
>> 
>> On 03/20/2016 05:38 AM, Aries Gunawan via llvm-dev wrote:
>>>  
>>> Loop Dependence Analysis Infrastructure. I have looked in the
source codes repo and I saw that there is a file named “DependenceAnalysis.cpp”.
So, does that mean this analysis has been implemented?
>> I believe major progress has been made it, but haven't been
following it closely.  I'd suggest talking to committers active in this file
in the recent past to determine what useful work might be left of appropriate
scope.  \
> 
> We actually have two DA frameworks at the moment.  The file you mention is
only used currently by the LoopInterchange pass that is off by default.  There
is also the other framework that I’ve been working on called LoopAccessAnalysis
that’s currently used by the LoopVectorizer, LoopLoadElimination,
LoopDistribution and LICMLoopVersioning (the latter two are off by default).
> 
> Do you think it is reasonable and feasible to provide a common interface
for different loop dependence analyses like alias analysis, such that passes
like LoopVectorizer/LoopLoadElimination/LoopDistribution etc. can query loop
dependency information from different implementations for precision/compile-time
trade off?
> 
Yes, that sounds reasonable.  It will probably take some careful generalization
of the DA within LAA (mostly the MemoryDepChecker class).

Adam 
> I ask because Utpal proposed to introduce dependency information calculated
by Polly to LoopVectorizer. And I am interested in building a common interface
for loop dependence analyses.
> 
>  
> Thanks
> Hongbin
>  
> 
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