[llvm-dev] GSoC Opportunity

Tue Mar 17 22:45:40 PDT 2020

>
> IMHO, you do. :)

Lol, you have too much faith in me. On a more serious note, how do I know
what issue can be solved in a reasonable amount of time/how do I search for
one?

> Please feel free to email me or Ettore if you encounter any blockers, or
> have further questions.

Hi Whitney, thanks for the video link and info. I was a bit busy today, but
I will work tomorrow and get back to you on this!

Best,
Benson

On Tue, Mar 17, 2020 at 12:54 PM Nicholas Krause <xerofoify at gmail.com>
wrote:

>
>
> On 3/17/20 10:09 AM, Stefanos Baziotis wrote:
>
> Hi Nick,
>
> What you said makes sense, but it's not called a call graph. :)
>
> Sure I assumed so that was just a term I used by mistake :).
>
> You're essentially referring to what LoopInfo does which makes sense, but
> as I mentioned earlier, this is already done
> in the LoopInfo. Now, how much one will be able to use it in a
> LoopNestPass is another issue, which is certainly
> something that mentors could help you with.
>
> I'm not applying for GSoC but that's a hint if other students are applying
> to help them get started.
>
> Nick
>
>
> Best,
> Stefanos
>
> Στις Τρί, 17 Μαρ 2020 στις 3:35 μ.μ., ο/η Nicholas Krause <
> xerofoify at gmail.com> έγραψε:
>
>>
>>
>> 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> 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> 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> 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> 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
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> llvm-dev at lists.llvm.org
>>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>
>>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing listllvm-dev at lists.llvm.orghttps://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>>
>>>
>>
>
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