[llvm-dev] GSoC Opportunity

Tue Mar 17 07:09:29 PDT 2020

Hi Nick,

What you said makes sense, but it's not called a call graph. :)
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.

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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