[LLVMdev] Load MachineFunctionPass plugin from library in llc?

[Matthias Braun]

> On Jul 2, 2015, at 2:05 PM, Jim Grosbach <grosbach at apple.com> wrote:
> 
>> 
>> On Jul 2, 2015, at 1:17 PM, Ethan J. Johnson <ejohns48 at cs.rochester.edu <mailto:ejohns48 at cs.rochester.edu>> wrote:
>> 
>> Hi all,
>>  
>> I am working on creating a MachineFunctionPass to perform an analysis on X86 code. After a bit of trouble, I was able to get my pass compiling and running correctly in llc. However, since a machine pass is compiled directly into the code generator, rerunning “make” across the LLVM build tree involves re-linking most of the major libraries and executables. This is a rather intense process, both in terms of CPU usage and memory (some of the bigger binaries, especially clang, use as much as 5 GB peak memory during linking). With a powerful multicore machine and plenty of RAM, this is doable, but still very time-consuming, and seems wastefully repetitive.
>>  
>> For a front-end pass, I can load the pass from a dynamic shared library using “opt –l path_to_library”; thus I only need to rebuild the pass itself to test a code change. There does not appear to be such an option in llc for dynamically linking a machine pass as a plugin. Does anyone know of a way to achieve something similar?
> 
> There is unfortunately no way to do that currently.
> 
>>  
>> At the very least, it would be nice to be able to just re-compile the parts of the LLVM tree I need (perhaps just llc). It seems particularly odd that clang, which is a front-end, would need to be relinked for a code change in a back-end pass. (clang, and some of its related components, are responsible for the lion’s share of the time-consuming CPU/memory-intensive linking phase.)
>>  
> 
> clang the project is the front end, but clang the executable program is the full compiler, including all of optimization, codegen, object emission, etc..
> 
>> FYI, I am using the Autotools (./configure, make) process to build LLVM, but have also successfully built with CMake and would be willing to try suggestions involving either.
> 
> This, however, can be done, at least w/ CMake+ninja (probably other permutations, too, but that’s the one I use these days). Just do “ninja llc” and it will build just what’s required for llc. Same for any of the other tools.

Also enabling shared libraries "cmake -DBUILD_SHARED_LIBS=ON" reduces link times dramatically in my experience.

- Matthias
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