llvm dev - May 2016 - llvm IR to C/C++ conversion

Hello
I am a beginner in llvm. In my one of the projects I need to convert llvm
IR back into C/C++. I read on stackoverflow that llvm has discontinued this
support. Can someone please explain me what are some drawbacks of such a
conversion and can I use the old backend code safely with current versions
of llvm to convert IR into C or if there is some other safe way to achieve
so?

Thanks
Abhishek Kumar
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160513/a84e529d/attachment.html>

I'm not aware of theoretical drawbacks; it was removed due to lack of a
maintainer (no one cared enough to keep it working).  I suspect you'd have a
lot of work to do to get the old one working again.
> On 2016-May-12, at 11:33, Abhishek Kumar via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hello
> I am a beginner in llvm. In my one of the projects I need to convert llvm
IR back into C/C++. I read on stackoverflow that llvm has discontinued this
support. Can someone please explain me what are some drawbacks of such a
conversion and can I use the old backend code safely with current versions of
llvm to convert IR into C or if there is some other safe way to achieve so?
> 
> Thanks
> Abhishek Kumar
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Skimming through the kaleidoscope tutorial should give you enough
clues on the "right" way to build IR using the LLVM C++ API's.

The cpp back end was removed because it had been broken for some time,
and didn't generate nice code anyway.

On Fri, May 13, 2016 at 4:03 AM, Abhishek Kumar via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hello
> I am a beginner in llvm. In my one of the projects I need to convert llvm
IR
> back into C/C++. I read on stackoverflow that llvm has discontinued this
> support. Can someone please explain me what are some drawbacks of such a
> conversion and can I use the old backend code safely with current versions
> of llvm to convert IR into C or if there is some other safe way to achieve
> so?
>
> Thanks
> Abhishek Kumar
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

On 12 May 2016, at 19:33, Abhishek Kumar via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> I am a beginner in llvm. In my one of the projects I need to convert llvm
IR back into C/C++. I read on stackoverflow that llvm has discontinued this
support. Can someone please explain me what are some drawbacks of such a
conversion and can I use the old backend code safely with current versions of
llvm to convert IR into C or if there is some other safe way to achieve so?
There are two discontinued back ends in LLVM that are often confused.  The C++
back end generated calls to LLVM APIs for constructing the IR that was fed into
it.  It was removed largely because it never generated good API calls (e.g. it
created instructions directly rather than via IRBuilder) and it bit-rotted to
the extent that the code that it generated didn’t compile with new APIs.

The other back end was the C back end.  This was largely discontinued because of
lack of interest, but it always had problems because LLVM IR is strictly more
expressive than C.  There are many LLVM intrinsics that can’t be represented by
ISO C, though a lot have GNU builtins that can be used instead.  C can’t
represent exception-handling constructs at all.  C++ can, but implies the use of
the C++ exception personality function and a specific format for the type info,
whereas LLVM IR can include any personality function.  There are also some
subtle issues with regard to calling conventions: LLVM IR has some leaky
abstractions in this regard, for example on some i386 targets a struct {int;
int;} in C is returned as i64 and so transforming this back into C is very
difficult, because there is no way to differentiate this from a function that
returns a long long (or a union).

If the source code that generated the LLVM IR is C/C++ and you know about the
target-specific lowering then you can probably reconstruct some C/C++ from it
that will generate a compatible binary when compiled.  Whether that is a
sensible thing to do is debatable.

David

-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 3719 bytes
Desc: not available
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160513/cb716cde/attachment-0001.bin>