llvm dev - Jun 2016 - New to LLVM, need some help with JIT

Hi,

I'm a grad student working on a C++11 project and I have no compiler
background. I apologize for asking basic questions here.

Currently I'm looking for ways to optimize my code and as a starting point
I wanted to experiment with loop unrolling and further I might consider
function in-lining . I have been introduced to JIT compiler of LLVM and the
possible application of it in my project.

Currently I know the basics about creating a module and getting a JIT
compiler. I followed this:
https://llvm.org/svn/llvm-project/llvm/trunk/examples/HowToUseJIT/HowToUseJIT.cpp

I have two questions:

1) What's the right way to add a class or an instance of it to a module? I
could only find ways to add structs to modules and I'm not sure how to deal
with classes. Any good references to check? Or any good
article/projects/code samples to look into?

2) Is there a way to declare that certain parts of a structure won’t change
during execution? For instance, let’s say that I have a class 'foo' and
foo.x is 7 and doesn’t change over the life of the code, but foo.y does.
There is a function in class foo and the function uses foo.x as the
condition in the loop, thus I’d like to tell LLVM that foo.x is always 7,
so that it can unroll the loop "for(int i = 0;i < foo.x;i++)". But
I’m a
little confused about how LLVM uses constants – they appear to be tied to
global variables only, and only an entire global variable, so I don’t see
how to make one component of it a constant.

Thank you,

-P.S.P

--
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Hi Pooya,

1) What's the right way to add a class or an instance of it to a module?
I
> could only find ways to add structs to modules and I'm not sure how to
deal
> with classes. Any good references to check? Or any good
> article/projects/code samples to look into?

LLVM IR is a low level representation - it doesn't have any built-in
representation of classes beyond the structs that you've seen. Your
frontend has to map those concepts onto LLVM IR.

2) Is there a way to declare that certain parts of a structure won’t
change
> during execution?

This is also a problem for the frontend.

You could take a look at how clang maps these concepts from C++ to LLVM IR
by writing some simple C++ examples and then running:

clang++ -emit-llvm -S -o foo.ll foo.cpp

If your input language is C/C++ you may want to use clang to build the IR
that you want to JIT. There is an example of that in the
clang/examples/clang-interpreter directory.

Finally, if your ultimate goal is to experiment with optimizations in the
JIT you may find the new tutorial series that I'm working on helpful:
"Building A JIT in LLVM" at http://llvm.org/docs/tutorial/ . These
tutorials are very new, and only the first chapter has a complete write-up,
but all of the code is complete, and the code for Chapter 2, "Adding
Optimizations", may be relevant to you.

Cheers,
Lang.


On Thu, Jun 2, 2016 at 4:36 PM, Pooya SaadatPanah via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> I'm a grad student working on a C++11 project and I have no compiler
> background. I apologize for asking basic questions here.
>
> Currently I'm looking for ways to optimize my code and as a starting
point
> I wanted to experiment with loop unrolling and further I might consider
> function in-lining . I have been introduced to JIT compiler of LLVM and the
> possible application of it in my project.
>
> Currently I know the basics about creating a module and getting a JIT
> compiler. I followed this:
>
>
https://llvm.org/svn/llvm-project/llvm/trunk/examples/HowToUseJIT/HowToUseJIT.cpp
>
> I have two questions:
>
> 1) What's the right way to add a class or an instance of it to a
module? I
> could only find ways to add structs to modules and I'm not sure how to
deal
> with classes. Any good references to check? Or any good
> article/projects/code samples to look into?
>
> 2) Is there a way to declare that certain parts of a structure won’t
> change during execution? For instance, let’s say that I have a class
'foo'
> and foo.x is 7 and doesn’t change over the life of the code, but foo.y
> does. There is a function in class foo and the function uses foo.x as the
> condition in the loop, thus I’d like to tell LLVM that foo.x is always 7,
> so that it can unroll the loop "for(int i = 0;i < foo.x;i++)".
But I’m a
> little confused about how LLVM uses constants – they appear to be tied to
> global variables only, and only an entire global variable, so I don’t see
> how to make one component of it a constant.
>
> Thank you,
>
> -P.S.P
>
> --
>
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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FWIW, invariant groups/invariant intrinsics may help with #2 (though, as
Lang noted, it's the job of your frontend to pass these on to LLVM):

 - http://llvm.org/docs/LangRef.html#invariant-group-metadata
 - http://llvm.org/docs/LangRef.html#llvm-invariant-start-intrinsic

On Fri, Jun 3, 2016 at 10:30 AM, Lang Hames via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi Pooya,
>
> 1) What's the right way to add a class or an instance of it to a
module? I
>> could only find ways to add structs to modules and I'm not sure how
to deal
>> with classes. Any good references to check? Or any good
>> article/projects/code samples to look into?
>
>
> LLVM IR is a low level representation - it doesn't have any built-in
> representation of classes beyond the structs that you've seen. Your
> frontend has to map those concepts onto LLVM IR.
>
> 2) Is there a way to declare that certain parts of a structure won’t
>> change during execution?
>
>
> This is also a problem for the frontend.
>
> You could take a look at how clang maps these concepts from C++ to LLVM IR
> by writing some simple C++ examples and then running:
>
> clang++ -emit-llvm -S -o foo.ll foo.cpp
>
> If your input language is C/C++ you may want to use clang to build the IR
> that you want to JIT. There is an example of that in the
> clang/examples/clang-interpreter directory.
>
> Finally, if your ultimate goal is to experiment with optimizations in the
> JIT you may find the new tutorial series that I'm working on helpful:
> "Building A JIT in LLVM" at http://llvm.org/docs/tutorial/ .
These
> tutorials are very new, and only the first chapter has a complete write-up,
> but all of the code is complete, and the code for Chapter 2, "Adding
> Optimizations", may be relevant to you.
>
> Cheers,
> Lang.
>
>
> On Thu, Jun 2, 2016 at 4:36 PM, Pooya SaadatPanah via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>> I'm a grad student working on a C++11 project and I have no
compiler
>> background. I apologize for asking basic questions here.
>>
>> Currently I'm looking for ways to optimize my code and as a
starting
>> point I wanted to experiment with loop unrolling and further I might
>> consider function in-lining . I have been introduced to JIT compiler of
>> LLVM and the possible application of it in my project.
>>
>> Currently I know the basics about creating a module and getting a JIT
>> compiler. I followed this:
>>
>>
https://llvm.org/svn/llvm-project/llvm/trunk/examples/HowToUseJIT/HowToUseJIT.cpp
>>
>> I have two questions:
>>
>> 1) What's the right way to add a class or an instance of it to a
module?
>> I could only find ways to add structs to modules and I'm not sure
how to
>> deal with classes. Any good references to check? Or any good
>> article/projects/code samples to look into?
>>
>> 2) Is there a way to declare that certain parts of a structure won’t
>> change during execution? For instance, let’s say that I have a class
'foo'
>> and foo.x is 7 and doesn’t change over the life of the code, but foo.y
>> does. There is a function in class foo and the function uses foo.x as
the
>> condition in the loop, thus I’d like to tell LLVM that foo.x is always
7,
>> so that it can unroll the loop "for(int i = 0;i <
foo.x;i++)". But I’m a
>> little confused about how LLVM uses constants – they appear to be tied
to
>> global variables only, and only an entire global variable, so I don’t
see
>> how to make one component of it a constant.
>>
>> Thank you,
>>
>> -P.S.P
>>
>> --
>>
>>
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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