[llvm-dev] Query about JIT

[Sachkov, Alexey via llvm-dev]

Hi Rajesh,

If I understand correctly, libclang is a C interface to Clang features, not LLVM. That means that you cannot get LLVM IR via libclang: The C Interface to Clang provides a relatively small API that exposes facilities for parsing source code into an abstract syntax tree (AST), loading already-parsed ASTs, traversing the AST, associating physical source locations with elements within the AST, and other facilities that support Clang-based development tools.

To get LLVM IR and perform JIT compilation you need to use C++ API of Clang and LLVM.

Here is some links which might be helpful:
* [cfe-dev] How to use clang and llvm for JIT, preferably via the C API  http://lists.llvm.org/pipermail/cfe-dev/2015-August/044869.html
* [cfe-dev] Help on Generating LLVM Module from C++ file using libClang  http://lists.llvm.org/pipermail/cfe-dev/2017-November/056033.html
* [LLVMdev] libclang JIT frontend  http://lists.llvm.org/pipermail/llvm-dev/2013-October/066088.html
* Generate assembly from C code in memory using libclang  https://stackoverflow.com/questions/34828480/generate-assembly-from-c-code-in-memory-using-libclang

BTW, there is a library for translating OpenCL C source code into a LLVM IR which features in-memory translation:
* https://github.com/intel/opencl-clang
It is not suitable for performing the whole JIT compilation, but it is used by https://github.com/intel/intel-graphics-compiler as a front-end. Also I’m not sure that you will be able to use it as-is for your purposes, but at least it can be used as an example

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Rajesh Jayaprakash via llvm-dev
Sent: Friday, March 8, 2019 2:14 PM
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Query about JIT

Dear llvm-dev list,

Apologies if this list is not the right venue for this query - suitable redirection would be appreciated in that case.

I have a JIT use case that I'd like to know the best way to implement using LLVM.

I am looking to migrate from the existing native compilation option (Tiny C Compiler - TCC) for pLisp, a Lisp dialect and IDE.

At present, the native compilation is done by converting the Lisp code to C, storing the C code in a char buffer, and passing it to TCC programmatically via the API provided. I get a function pointer in return, which I store and invoke as needed.

Delving into the LLVM documentation, I found that one possible way to achieve the same functionality in LLVM is to use clag/libclang to convert the C source to LLVM IR, load this IR into the the JIT context and (skipping some steps I'm yet to figure out) get the desired function pointer.

Is this approach the right one? One issue I foresee is that libclang's clang_parseTranslateUnit() function expects the C code to be from a file (although the file can be in-memory), whereas in my case the C code needs to be picked up from a char buffer - necessitating fmemopen(), etc.

Thanks for your assistance.

Regards,
Rajesh Jayaprakash
(https://github.com/shikantaza/plisp)

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