[llvm-dev] GSoC Opportunity
Nicholas Krause via llvm-dev
llvm-dev at lists.llvm.org
Tue Mar 17 06:35:34 PDT 2020
On 3/17/20 9:23 AM, Stefanos Baziotis wrote:
> > Yes that's correct.
> Well, now that I saw the LoopNestAnalysis* files, they try to do sth
> similar. So, I hope it helped.
>
> > My idea was similar but using the call graph directly
>
> Personally I don't see how the call graph can help you, since well...
> it's a call graph. :)
Not directly but you can could implement a call graph for the loops
internal to a function and walk up
it backwards. In addition you could make it possible to implement this
call graph to know the number
of loops nested and pop out to the outermost as a function. Basically
SCC for loops themselves rather
than functions.
That's probably beyond the scope of the project so your right it doesn't
matter for this,
Nick
> You care about loops in a specific function. What can help you is the
> Control-Flow graph, which is basically what LoopInfo uses to identify
> loops in a function.
> But because of that, loop identification is not your problem, loop
> traversing is, if I understand it correctly.
> Although you have to do things similar to loop identification (i.e.
> what LoopInfo does) when trying to
> decide for perfect nestings etc.
>
> Best,
> Stefanos
>
>
> Στις Τρί, 17 Μαρ 2020 στις 3:08 μ.μ., ο/η Nicholas Krause
> <xerofoify at gmail.com <mailto:xerofoify at gmail.com>> έγραψε:
>
>
>
> On 3/16/20 11:41 PM, Stefanos Baziotis wrote:
>> Hi everyone,
>>
>> > I probably do not have the time to get a patch through.
>> IMHO, you do. :)
>>
>> First of all, @Benson sorry but I'm not at all familiar with LLDB
>> so I can't help there.
>>
>> Other than that, I'll also disappoint you both probably
>> because I'm not that familiar with the creation of passes and the
>> problem at hand. I'll try to help as I can.
>>
>> > Is there a specific section of the dragon book that I should
>> read so that I can at least understand theoretically what it
>> means to create a LoopNestPass? As I can understand, no because
>> it's more of a structural, LLVM-specific problem than a generic,
>> compiler optimization problem. > Stefanos can speak to this more
>> but in order to create a LoopNestPass after reading what they are
>> talking about requires information from the call graph
>> > for a function or the loop hierarchy in LLVM IR. I'm not sure
>> of the internal classes for this so Stefanos is there a way
>> currently to get the info in IR about
>> > the outer loop or from the call graph? That seems to be the
>> biggest problem getting the outer loop in the IR or the call
>> graph. After that you would
>> > basically check if the loop is the outer loop and if so you can
>> add dynamically to the pipeline.
>>
>> I'm not sure I followed you here. First of all, if you create a
>> regular LoopPass, you'll visit loops from the innermost to the
>> outermost. In the loop nest pass
>> you want the outermost though, so you'll have to visit them all
>> until you there. Now if you do it in a function pass, you lose
>> the ability to put loops
>> back into the pipeline, as this is how the function pass works.
>> So, the way I understand it, to solve that problem, one would
>> create something like a function
>> pass, figure out the loops there (i.e. with LoopInfo), then
>> convert it to LoopPass so that you can run loop passes over the
>> loops.
>> I think this can happen already, but right now, loops are going
>> in reverse order:
>> https://github.com/llvm/llvm-project/blob/master/llvm/include/llvm/Transforms/Scalar/LoopPassManager.h#L230
>> So, maybe if you could modify that to something like
>> FunctionToLestNestPassAdaptor, it would work? I don't know that's
>> just an idea, let me not confuse you more.
>>
>> Best,
>> Stefanos
> Yes that's correct. My idea was similar but using the call graph
> directly. The other problem is how to keep LCSSA form for all the
> loops as well and I'm
> aware that function passes don't care about that. So you can't
> really convert to a function pass itself but something similar.
>
> Nick
>> Στις Δευ, 16 Μαρ 2020 στις 5:53 π.μ., ο/η Nicholas Krause
>> <xerofoify at gmail.com <mailto:xerofoify at gmail.com>> έγραψε:
>>
>>
>>
>> On 3/15/20 11:12 PM, Benson Bin Bin Li via llvm-dev wrote:
>>> Hi Stefanos,
>>>
>>> First, thanks a lot for the very detailed response! I
>>> watched both of the videos, and I seem to have a rough idea
>>> now of how each of the different pieces of software maps
>>> onto the compilation process. Though I found blogs such as
>>> these two:
>>> https://jonasdevlieghere.com/understanding-the-clang-ast/,
>>> https://releases.llvm.org/2.6/docs/tutorial/JITTutorial1.html
>>> to be better for a more in-depth understanding. Anyways, in
>>> response to your answers:
>>>
>>> The latter can even be problematic if you start applying
>>> C++ craziness while the first is pretty much always
>>> needed when working in a team project.
>>>
>>> Ok, that makes sense as you would want the style to be
>>> consistent throughout.
>>>
>>> running the LLVM suite is super easy
>>>
>>> Yeah, everything went fine from following your instructions.
>>> I do have a question though: How do I diagnose failed tests?
>>> I found the files that correspond to them, and they seem to
>>> be 1 line scripts rather than "code" per say.
>>>
>>> But I think every good GSoC proposal includes a
>>> biography-like section
>>>
>>> Then, try to study it, understand the context and the
>>> problem.
>>>
>>> But because submitting good patches is one of the best
>>> indicators
>>>
>>> Ok, so for the application process, basically try to get
>>> more info on the projects I am interested in and from there
>>> submit a proposal? Given the whole coronavirus situation and
>>> the time remaining for the application, I probably do not
>>> have the time to get a patch through. Regarding the projects
>>> I am interested in, I have narrowed it down to two(mostly
>>> because I don't think I have the ability to tackle
>>> PostDominatorTree project as of now), and have the following
>>> questions about them:
>>>
>>> LLVM Pass
>>>
>>> 1.
>>>
>>> I am following the guide to create a LLVM pass following
>>> this
>>> guide(https://llvm.org/docs/WritingAnLLVMPass.html), but
>>> it appears “add_llvm_library” is a macro and not a
>>> built-in command. So I have two questions. 1) In
>>> comparing the online repo I found this macro in and my
>>> local, it appears I don’t have the file. Do I need to
>>> build it then? 2) How do I tell CMake to look for this
>>> macro?
>>>
>>> 2.
>>>
>>> Is there a specific section of the dragon book that I
>>> should read so that I can at least understand
>>> theoretically what it means to create a LoopNestPass?
>>>
>> Stefanos can speak to this more but in order to create a
>> LoopNestPass after reading what they are talking about
>> requires information from the call graph
>> for a function or the loop hierarchy in LLVM IR. I'm not sure
>> of the internal classes for this so Stefanos is there a way
>> currently to get the info in IR about
>> the outer loop or from the call graph? That seems to be the
>> biggest problem getting the outer loop in the IR or the call
>> graph. After that you would
>> basically check if the loop is the outer loop and if so you
>> can add dynamically to the pipeline.
>>
>> Sorry if I'm not much help as I'm not sure if the call graph
>> API supports this but I'm pretty sure LLVM IR doesn't make
>> this easy,
>> Nick
>>>
>>> LLDB Tab Completion
>>>
>>> 1.
>>>
>>> Is there any resource I can read that explains how lldb
>>> is able to “pause” the executable and map it to a
>>> certain line in the source file/in general how lldb
>>> represents the state of the executable?
>>>
>>> 2.
>>>
>>> Where in the source code can I go to see how existing
>>> tab completions are implemented?
>>>
>>> 3. I built lldb and check-lldb, but it seems that the call
>>> path to clang got messed up, as it is trying to call
>>> "Example=Code/llvm-project" rather than my actual name
>>> for the directory "Example-Code/llvm-project". Should I
>>> just clone the repo into a parent directory that doesn't
>>> use hyphen?
>>>
>>>
>>> (Would it be better if I posted this on the forum?)
>>>
>>> Best regards,
>>> Benson
>>>
>>>
>>> On Sat, Mar 14, 2020 at 11:10 AM Stefanos Baziotis
>>> <stefanos.baziotis at gmail.com
>>> <mailto:stefanos.baziotis at gmail.com>> wrote:
>>>
>>> Hi Benson,
>>>
>>> You're welcome to the LLVM community!
>>>
>>> I'll try to help but note that I'm no formal position to
>>> talk about how LLVM decides about GSoC (I'm a LLVM
>>> newcomer anyway).
>>> With that said, the rest is _my_ opinion which is
>>> partially formed from my experience as a GSoC student.
>>>
>>> > But to be up front about this, I have not done any
>>> full scale C++ project
>>> Depending on how you define "full-scale", a lot of
>>> amazing LLVM contributors have not done a full-scale
>>> C++. So, I think no problem there, it's just good to
>>> have a relatively good knowledge of C++.
>>> Talking about C++ skills, I think they're more important
>>> if you want to contribute to Clang than say LLVM middle
>>> or back-end. Because for Clang, you have to know a lot
>>> of details of the language
>>> in order to parse it, type-check it and generate LLVM
>>> IR. In most other parts of LLVM, you're only using the
>>> language.
>>> As a matter of fact, if you have a good knowledge of
>>> C++, I believe it's more important to be able to
>>> understand and adapt to "nearby" code, than to be an
>>> expert in C++.
>>> The latter can even be problematic if you start applying
>>> C++ craziness while the first is pretty much always
>>> needed when working in a team project.
>>>
>>> > 1. Do I need to submit a resume/screening/patches?
>>> As far as the resume, in the way that you may usually
>>> apply to jobs, no. But I think every good GSoC proposal
>>> includes a biography-like section
>>> where you basically tell your story in programming and
>>> how you fit into the project (in our case, LLVM).
>>>
>>> I'm not sure what you mean by screening.
>>>
>>> As for patches, I don't think they're required but
>>> they're super useful. Not because they're some part of
>>> unrelated logistics (like "you have to have X patches to
>>> be considered").
>>> But because submitting good patches is one of the best
>>> indicators (if not the best) that you are able to do
>>> useful work in this project. :)
>>> And they don't only show your technical skills. But also
>>> communication skills, independence etc.
>>>
>>> > 2. Although I do have interests in certain projects
>>> posted on you website(Implement missing tab completion,
>>> createLoopPass, and PostDominatorTree), I am uncertain
>>> if I have enough expertise to decide what would be an
>>> appropriate project to contribute to given my current
>>> knowledge and experience.
>>> This is kind of a generic sentence.
>>> I'd say, start with finding a project that you're truly
>>> interested in. Then, try to study it, understand the
>>> context and the problem.
>>> You don't need to get very far, that's totally ok. You
>>> can then do a post (either here or on Discourse:
>>> https://llvm.discourse.group/c/community/gsoc/32)
>>> for this specific project (you can do posts for multiple
>>> projects).
>>> Hopefully, by discussing with people (and mentors) and
>>> understanding what the project is asking better,
>>> you can find if you want to do it or not. Certainly, the
>>> mentors of the project can guide you through.
>>>
>>> 3. The GCC GSoC website suggested checking out their
>>> source code, compiling and running their test suite. Can
>>> I do something similar for LLVM?
>>> Yes, totally. I'm not familiar with GCC internals but
>>> running the LLVM suite is super easy (so easy that you
>>> don't really learn anything by doing it :P )
>>> So, the LLVM project has moved to a common repository:
>>> https://github.com/llvm/llvm-project
>>> You can clone the project and then use CMake to build
>>> it. The cmake configuration for LLVM has a bunch of
>>> flags: https://llvm.org/docs/CMake.html
>>> and you may get lost. So, I'll say start simple:
>>> Go to the llvm-project dir (the one you cloned) and do:
>>> cmake ./llvm -DLLVM_ENABLE_PROJECTS="clang"
>>> -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON
>>> -DLLVM_TARGETS_TO_BUILD="X86"
>>>
>>> In the link above you can read what the flags do. llvm
>>> middle / back-end (i.e. opt / llc, ask if you don't know
>>> what these mean) is always built. But to build clang
>>> we have to enable it explicitly. We set build type to
>>> release because doing a debug build will take a lot of
>>> time and a lot of space. Also, when starting out,
>>> you probably don't need it. We enable assertions mostly
>>> because you can use the -debug option say in opt and see
>>> debug prints.
>>> Finally, we only build for x86 arch because that's
>>> probably what you have and you don't need any other for now.
>>>
>>> Hit enter and once the configuration is complete you can do:
>>> make
>>> or
>>> make -j<number of threads> <-- this is faster but
>>> limit it depending on your systemS
>>>
>>> When that's finished, the llvm-project/bin/ dir will
>>> have executables like clang, clang++, opt, llc etc.
>>> Which you can run (also ask if you don't know what to do
>>> with them. With clang you probably will know, it's like
>>> invoking
>>> most compilers like gcc to compile .c / .cpp files).
>>>
>>> To run the test suite, you can go to
>>> llvm-project/llvm/test and do:
>>> <dir of llvm-project>/bin/llvm-lit .
>>> That will run only llvm's test suite but you'll get an idea.
>>>
>>> Also, you can watch these videos:
>>> https://www.youtube.com/watch?v=J5xExRGaIIY
>>> https://www.youtube.com/watch?v=5kkMpJpIGYU
>>>
>>> Hope this helped!
>>>
>>> Kind regards,
>>> Stefanos Baziotis
>>>
>>> Στις Σάβ, 14 Μαρ 2020 στις 2:04 π.μ., ο/η Benson Bin Bin
>>> Li via llvm-dev <llvm-dev at lists.llvm.org
>>> <mailto:llvm-dev at lists.llvm.org>> έγραψε:
>>>
>>> Dear LLVM Team,
>>>
>>>
>>> I would like to contribute to/participate in LLVM’s
>>> GSOC, because I would very much like to combine my
>>> knowledge of graph theory/algorithms and my interest
>>> in C++ together. Contributing to the LLVM code seems
>>> like a fantastic challenge and learning experience
>>> for these two interests of mine, as well as computer
>>> science in general (For example, the use of a new
>>> syntactic category to disambiguate a grammar
>>> demonstrates 1) indirection 2) the power of naming
>>> things).
>>>
>>>
>>> But to be up front about this, I have not done any
>>> full scale C++ project(Although we had to modify the
>>> Linux kernel in my OS class, that was in C).
>>> However, I do believe my C++ skills are at an
>>> intermediate level, as C++, like Python, is a
>>> language in which I will spend my free time learning
>>> more about. Like vim, there is always more to learn
>>> in C++, and to that end I will watch CppCon Videos
>>> or peruse blogs such as Fluent C++(which is a
>>> treasure trove of material to nerd out on) in my
>>> free time. I also have a layman’s knowledge of
>>> CMake, from using it to configure ccls to lint C++
>>> code with specific flags, and am aware of Google’s
>>> Test framework. Finally, I am currently taking
>>> Professor Stroustrap’s C++ class, and the compilers
>>> course here at Columbia.
>>>
>>>
>>> Regarding the logistics:
>>>
>>> 1. Do I need to submit a resume/screening/patches?
>>>
>>> 2. Although I do have interests in certain projects
>>> posted on you website(Implement missing tab
>>> completion, createLoopPass, and PostDominatorTree),
>>> I am uncertain if I have enough expertise to decide
>>> what would be an appropriate project to contribute
>>> to given my current knowledge and experience.
>>>
>>> 3. The GCC GSoC website suggested checking out their
>>> source code, compiling and running their test suite.
>>> Can I do something similar for LLVM?
>>>
>>>
>>> Anyways, thank you for taking the time to read this
>>> email, and I hope to hear back!
>>>
>>>
>>> Best regards,
>>>
>>> Benson Li
>>>
>>> _______________________________________________
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>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
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>>
>
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