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Re: [gimplefe] [gsoc16] Gimple Front End Project


On 8 March 2016 at 16:47, David Malcolm <dmalcolm@redhat.com> wrote:
>> > Isn't this what -fopt-info does?
>> > https://gcc.gnu.org/onlinedocs/gcc/Developer-Options.html
>>
>> Yes.
>
> One difference is that in this proposal, the output is emitted as a
> diagnostic, rather than to a file.

-fopt-info prints to stderr by default. I haven't used the current
version. The original version of -fopt-info was very similar to what
you described above.

> FWIW, I find those existing options to be very fiddly to use (and
> they're categorized as "GCC Developer Options").

Not sure what you mean exactly by fiddly. I think they are categorized
as "GCC Developer Options" because the main users are developers
working on GCC optimizations, and there has been little interest in
making the output consistent and user-friendly. I seem to remember
that the original proposal by Google devs was to make -fopt-info a way
for users to understand how their code was or not optimized. That
would not match your description either, because you want to use it
for unit-testing, thus it will still be a "developer" option.

>> > Of course, ideally, the pass manager would be all modular, based on
>> > dependencies and re-configurable like
>> > http://llvm.org/docs/CommandGuide/opt.html I wonder how far away is
>> > GCC's
>> > middle-end from being able to do that.
>>
>> For any kind of unit testing you need to develop some pass manager
>> support.
>
> (nods)

Which seems orthogonal to having a gimple-fe. That is, a gcc version
of the llvm opt tool would be useful even if it needed to go through
the existing FEs. It would be very useful to unit-testing specific
passes or sequences of passes. It would also be pretty
uncontroversial. And not something that a GSoC student could even
attempt, given the need to spent significant time/effort in coming up
with an overall design for the pass manager.

> This is possibly a silly question, but why would a custom C-like FE for
> a language that isn't quite C rule out torturing unit tests?  Can't we
> inject the gimple at the stage it was generated, and then supply
> various -O options to run various combinations of passes?

Indeed.

> (Though hacking up the C frontend might be a suitable way to prototype
> a dedicated gimple frontend; runs into issues of having them diverge.
>  I don't know how much we'd want in common vs how much has to be
> different)
>
> (potentially we could define a subset of e.g. C99 that would be
> acceptable as gimple.  I don't know if that's helpful)

A first step would be to dump something that is parseable as a subset
of C. I don't think we are nowhere near that. And that would still not
be enough, since I don't think we dump all details of gimple. The C FE
also would need to be able to parse as gimple things like look like C
but are significantly different (like Fortran explicit parentheses).

Adding stuff to the C FE might seem the fast path forward, but we will
end up with a gimple-fe as complex as the whole C FE plus the extra
bits for parsing all the non-C stuff. That seems very undesirable in
the long run. To be clear, what I would propose if a C-like
representation is desired is "cp -a c cgimple", and start deleting all
the things that are not needed by cgimple and adding all the extra
stuff needed.

Since the goal seems to be to be able to dump/reload some kind of IR
rather than a textual representation of GIMPLE tuples, why not
dump/load LLVM IR? The GIMPLE=>LLVM is already implemented as a GPL
plugin in dragonegg.
http://llvm.org/devmtg/2009-10/Sands_LLVMGCCPlugin.pdf
It is not a suggestion, but I would like to know what you think about
the advantages/disadvantages.

Cheers,

Manuel.


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