Re: JIT function compile to file

[David Malcolm <dmalcolm@redhat.com>]

On Thu, 2023-08-03 at 15:07 +0000, Li, Yicheng via Jit wrote:
> Hi,
> 
> I’m looking for examples that can let me build a jit function on one
> run, save it to a file, and grab from file again on the second run
> without rebuilding and recompilation.
> I’ve seen the function, gcc_jit_context_compile_to_file, and I’m
> wondering how to use it.

gcc_jit_context_compile_to_file is for ahead-of-time compilation.  You
could compile the function into a shared library, and then load the
shared library when the process restarts.

Emacs does something like this for its Lisp files; see:
  https://akrl.sdf.org/gccemacs.html


> Throughout the documentation, I see the gcc_jit_location type
> variable, I’m unsure how to use it, and if it is related to calling a
> saved-to-file jit function.

gcc_jit_location is for providing "source location" information for use
when stepping through the code in a debugger.  It doesn't affect the
destination where code is compiled to.

> It would be great if there’s any example that can be provided.

I don't think there's an existing example in the documentation or
testsuite, but use gcc_jit_context_compile_to_file with
GCC_JIT_OUTPUT_KIND_DYNAMIC_LIBRARY to make a .so file, then on
rerunning the process use dlopen to load the .so file, and dlsym to
extract the function.

FWIW the testsuite
has:https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/testsuite/jit.dg/test-compile-to-dynamic-library.c
but that testcase merely does the compilation-to-dynamic-library, not
the loading of the built library, and it heavily uses the preprocessor
so probably isn't good as an example.


Hope this is helpful
Dave