Digging a bit deeper into JIT's compile internals

[David Malcolm]

On Sat, 2023-06-10 at 17:26 +0100, Joshua Saxby via Jit wrote:

Hi Joshua

> After following files jit-recording.cc and jit-playback.cc, I think
> I've
> found out where I need to patch JIT to do what I want it to do.

Indeed.  In case you haven't seen it yet, see:
https://gcc.gnu.org/onlinedocs/jit/internals/index.html#overview-of-code-structure

You'll see there (and in jit-playback.cc) that the
gcc_jit_compile[_to_file] functions are basically running the
equivalent of cc1 in-process to generate a .s file in a tempdir, and
then calling out to subprocesses to run the assembler and linker as
needed.  FWIW I've experimented in the past with libraries for the
assembler and linker (keeping it all in-process), and got some
speedups, but it needed nontrivial patches to binutils to turn as and
ld into shared libraries.

> Looks like both compiling to file and memory ultimately rely upon
> playback::context::compile() to do the bulk of their work and then
> override
> a postprocess() method to do additional handling pertaining to their
> specific task.
> 
> If I can find a way to cache or store the intermediate result
> generated by
> compile(), I should then be able to restructure this to have the
> follow-up
> tasks required by both different forms of compilation to be done
> starting
> with this intermediate result.
> 
> Anyone see a problem with my approach? I'm hoping it will be possible
> to
> reüse compilation results produced by compile() in this way without
> it
> causing any conflicts...

I have a higher-level question, which is why do you want to compile to
both/reuse results?

I believe you ought to be able to compile a context multiple times
(provided no errors occur), so if you need both you can currently write
code like this:

  gcc_jit_context *ctxt = populate_ctxt ();
  gcc_jit_context_compile_to_file (ctxt,
                                   GCC_JIT_OUTPUT_KIND_ASSEMBLER,
                                   "foo.s");
  gcc_jit_result *result = gcc_jit_context_compile (ctxt);

albeit with the drawback that it's duplicating work.  Is it the
duplicated work that you're trying to avoid?

Do you have ideas about what the API you'd want would look like?

[...snip...]

Hope this is helpful
Dave

> > 
> > ---------- Forwarded message ----------
> > From: Joshua Saxby <joshua.a.saxby@gmail.com>
> > To: jit@gcc.gnu.org
> > Cc:
> > Bcc:
> > Date: Sat, 10 Jun 2023 00:04:43 +0100
> > Subject: Modifying the jit compiler API?
> > Currently the JIT has two functions allowing you to compile a
> > context
> > either to memory or to file.
> > 
> > But what if you want to compile to both? There doesn't seem to be
> > any way
> > to do this except by calling both functions separately which I
> > believe will
> > effectively be two separate compilations...
> > 
> > Presumably, it should be possible to modify this part of the API to
> > compile
> > to some form of intermediate representation of the work that is
> > common to
> > both kinds of compilation, and then turn that into code in memory
> > and in
> > file, respectively.
> > 
> > Anyone got any pointers for me on where in the code would be the
> > best place
> > for me to modify to dupport this? I did take a look in the
> > implementation
> > code of JIT sone weeks ago and remember seeing lots of complicated
> > stuff
> > regarding recordings and replays that looked relevant...
> > 
> > A slighty simpler alteration I am also interested in making is
> > allowing
> > compile to file to compile to a buffer in memory as well as a file
> > (I
> > suppose it could be termed "compile to binary"). Maybe I should
> > start with
> > that...
> > 
> > 
>