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

[Philip Reames via llvm-dev]

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.
>
> **
>
> *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.
>
> **
>
> *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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