[llvm-dev] [cfe-dev] [PATCH/DRAFT] Embed metadata into object file

[Mehdi Amini via llvm-dev]

> On Apr 5, 2016, at 7:51 AM, Christian Dietrich <dietrich at cs.fau.de> wrote:
> 
> Mehdi Amini <mehdi.amini at apple.com> writes:
> 
>> Worded like that, I can see a close analogy with  "Debug Info".
> 
> Indeed, it is very similar, but there are some differences and
> shortcoming, if a developer only wants to smuggle some metadata out in a
> very specific format:
> 
> For the IR->ELF path, the debug information is encoded as Dwarf (or
>  something else) in the binary. The plugin developer has not much
>  control about the binary format of the data. !llvm.extra_sections
>  would give a quite fine-grained control.
> 
> For the AST->IR path, I don't see an easy way to annotate a few pieces
>  of information in the AST without spinning up the whole
>  debug-information generation process.


Yes, I meant conceptually it is the same as debug info, so it makes sense to solve it the same way (i.e. metadata + backend support for codegen).

> 
>>> [1] This is a _seperate_ drawback of clang plugins at the moment: They
>>>   cannot define an LLVM IR transformation that should be applied when
>>>   the plugin is active.
>> 
>> You can write clang plugin that are LLVM pass and insert them in the
>> pipeline.
> 
> I can? How? I have not seen any possibility to inject a LLVM pass into
> the Clang CodeGen PassManager infrastructure from a clang plugin, which
> is also FrontentAction like CodeGen.

The FrontendAction is totally separated from the LLVM-pass, but I don’t see why your plugin can’t contain both.

To load an LLVM pass into the pipeline, see what Polly is doing for instance: http://polly.llvm.org/example_load_Polly_into_clang.html

— 
Mehdi



> 
>> 
>>> I think it would be useful for many analyzes
>>>   to have access both to the AST and to the IR.
>> 
>> This is more fuzzy to me, I don't know enough about clang but I'm not
>> sure the design allow to keep a link from the IR to the clang AST? (If
>> it is the case, I'd be curious to see how it works).
> 
> If you restrict the possible subjects to top-level functions, global
> variables, and the compilation unit, this should be implementable quite
> straight forward.
> 
> chris
> -- 
> Christian Dietrich, M.Sc. (Wissenschaftlicher Mitarbeiter)
> Lehrstuhl für Informatik 4 (Verteilte Systeme und Betriebssysteme)
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