[llvm-dev] Compiling the code generated by LLVM C++ backend

[Roel Jordans via llvm-dev]

Hi Dipanjan,

The llc utility is complete interface to the LLVM back-ends, it helps 
you translate LLVM IR into a target's native code.  As such there are 
more "target architectures" that you can select when using llc, you can 
get a list of them with "llc --version" (at least, that's how it works 
with the current versions of LLVM, 2.9 is rather ancient).

The C and C++ back-ends are rather special cases.  The C back-end (which 
no-longer exists in the current LLVM version as it wasn't maintained 
anymore) aims at translating the IR back to compilable C code, as you 
already observed.  The intent of this back-end was that it allows a user 
to utilize the optimizations and front-end of LLVM even if LLVM doesn't 
support the user's target architecture by generating an "optimized" C 
code which can then be used with another compiler.

The C++ back-end has a different purpose, it serves to show how IR code 
can be generated from LLVM's C++ library interface.  This is more useful 
if you are writing a program that should generate IR code input for LLVM 
and want to see how a particular IR construct can be created.  There are 
probably other uses for the C++ back-end but that's the main one I've 
seen so far.

So, in general, if you want to generate code that is compilable into a 
native binary, use the C back-end (although it may be broken in many 
ways due to lack of maintenance), or try to see if there is a real 
back-end for your target architecture so that you can generate code more 
directly.

Cheers,
  Roel

On 09/09/15 11:43, Dipanjan Das via llvm-dev wrote:
>
> Hi,
>
> I am working with LLVM 2.9. So far I have used "llc -march=c" and "llc
> -march=cpp" respectively to generate C and C++ source code from LLVM
> IR/BitCode. My questions follow:
>
>  1. I can see that there is a significant difference between the code
>     generated by C and C++ backends. Former one generates code to be
>     compiled by gcc to generate a native executable, while the later one
>     is targeted to generated LLVM IR itself. Surprisingly, both the
>     diametrically opposite functionalities have been put together in the
>     same utility called ''llc'. I wonder if there is any official doc
>     available online to point out the striking difference.
>  2. This question is simpler even: How can I make use of the C++ code
>     generated by LLVM C++ backend? In other words, can it be compiled to
>     a standalone, native binary that can be used to write out
>     corresponding LLVM IR? If so, what will the compiler and linker
>     switches be?
>
> Thanks in advance.
>
> --
>
> Thanks & Regards,
>
> Dipanjan
>
>
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