[cfe-users] using Clang for runtime compilation of a class

Sun Oct 4 07:46:10 PDT 2015

Hi Georg (and others interested in Clang+JIT),

We just implemented this (Clang+JIT) in our open-source project CasADi (
http://casadi.org). Feel free to have a look and/or reuse components.

We used a plugin-design for using Clang, which is a bit different from the
examples we could find for using LLVM/Clang. We added a generic "Compiler"
class and made Clang/JIT a plugin for this Compiler class. Here is the
implementation of the plugin:
https://github.com/casadi/casadi/blob/develop/casadi/interfaces/clang/clang_compiler.cpp.
We also made a second plugin (only for Linux/Mac) which simply calls the
system compiler using shell commands. Mainly for benchmarking and when
Clang/JIT isn't available:
https://github.com/casadi/casadi/blob/develop/casadi/solvers/shell_compiler.cpp
.

The trickiest was to set up the build system (we use CMake) and in
particular redistribute the C and C++ standard libraries needed to compile
any useful C/C++ code. And do it in a way that makes the compiled code
portable and not relying on e.g. setting environmental variables.

This all works now, also on Windows, although it is not yet completely
mature (especially when trying to JIT compile C++ code). The solution used
in the LLVM examples was to use the arguments passed to "main", e.g. to
locate the "resource", but this didn't work for us since or tool is itself
a library and we don't have a "main". So the design we chose instead was to
place the compilation resources (including C/C++ standard library headers)
in a relative location to the plugin (which is a shared library) since the
path of the plugin can be retrieved at runtime by "dladdr" in Linux/Mac and
GetModuleHandleEx+GetModuleFileName on Windows.

Best regards,
Joel

2015-10-03 20:37 GMT+02:00 David Blaikie via cfe-users <
cfe-users at lists.llvm.org>:

>
>
> On Sat, Oct 3, 2015 at 12:30 AM, Georg Bartels via cfe-users <
> cfe-users at lists.llvm.org> wrote:
>
>> Dear Clang Users,
>>
>> I have a particular programming problem, and several discussion forums
>> suggested Clang as a possible solution. Being totally new to Clang and its
>> community, is this the right place to ask questions about how to use Clang
>> for a particular programming task? If not, where should I turn to?
>>
>> In case this is the right forum for such questions, I continue describing
>> my problem. Basically, I want to compile a class which inherits from a
>> virtual interface class at runtime of my program. The program should then
>> create an instance of this new class and use it through its interface.
>>
>> Let me quickly sketch an example. The static version of my program would
>> look like this:
>>
>> class Base
>> {
>>   public:
>>     virtual int calc() const;
>> };
>>
>> class Derived: public Base
>> {
>>   public:
>>     virtual int calc() const
>>     {
>>       return 42;
>>     }
>> };
>>
>> int main()
>> {
>>     std::cout << Derived().calc() << std::endl;
>>     return 0;
>> }
>>
>>
>> The dynamic version (with two fuzzy lines of code) shall look like this:
>>
>> class Base
>> {
>>   public:
>>     virtual void calc() const;
>> };
>>
>> int main()
>> {
>>   std::string code = read_derived_code();
>>
>>   // two fuzzy lines for which I am looking for a solution
>>   SomeCompiledObjectClass o = compile(code);
>>   Base* d = SomeFactoryClass(o).createInstance();
>>
>>   std::cout << d->calc() << std::endl;
>>   return 0;
>> }
>>
>> Is Clang the right tool for solving my two fuzzy lines of code? If yes,
>> which class should have a look at? If not, do you maybe know about another
>> library/compile which can achieve this task?
>>
>
> This migth be the sort of thing that would escalate to cfe-dev and/or
> llvm-dev.
>
> What you're looking to do is JIT (Just In Time) compile some code. That
> involves using clang to turn the source code into LLVM IR and then LLVM to
> make machine code from that, put it in memory, make it executable, and call
> some part of it.
>
> Your two fuzzy lines of code would look slightly different (chances are
> what you'd do is generate source code that included a private function that
> return an instance, then you would JIT compile that function, indirectly
> code generating anything in the class that was needed, etc).
>
> I'm not sure where the best examples of the Clang half of this problem
> are. But for the second have, LLVM has the Kaleidoscope tutorials
> (especially the recently added ORC examples) that show how to use LLVM as a
> JIT compiler. You shuold be able to wrap up all these APIs into a fairly
> simple API for your one specific use case.
>
> I think /maybe/ out there is an existing C++ rapid development environment
> using Clang (essentially a C++ interpreter/interactive command prompt like
> Python) that you might be able to grab the basics of "feed code to
> compiler, get IR" from.
>
> - David
>
>
>>
>> Thanks a lot for your help!
>>
>> Cheers,
>> Georg.
>>
>> _______________________________________________
>> cfe-users mailing list
>> cfe-users at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
>>
>
>
> _______________________________________________
> cfe-users mailing list
> cfe-users at lists.llvm.org
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>
>

-- 
-- 
Joel Andersson, PhD
Ptge. Busquets 11-13, atico 3
E-08940 Cornella de Llobregat (Barcelona), Spain
Home: +34-93-6034011
Mobile: +34-63-4408800 (in Sweden also +46-707-360512)
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