[llvm-dev] GSoC Opportunity

Mon Mar 16 20:41:01 PDT 2020

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

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
>>>
>>
> _______________________________________________
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