llvm dev - Sep 2015 - Compiling the code generated by LLVM C++ backend

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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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
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

Hi Roel,

I understand that C and C++ backends are special cases. My concern is,
since two different functionalities are offered by two deceptively
identical arguments (i.e. march=c and march=cpp), it's easy for a beginner
like me to stumble upon at this inconsistency. I myself had a hard time
grasping the fact, especially when the official documentation
<http://llvm.org/docs/CommandGuide/llc.html> is inadequate.

The llc command compiles LLVM source inputs into assembly language for
a
> specified architecture. The assembly language output can then be passed
> through a native assembler and linker to generate a native executable.
> The choice of architecture for the output assembly code is automatically
> determined from the input file, unless the -march option is used to
> override the default.

 At this point, I know that the support for C backend has been dropped in LLVM
release 3.1 <http://llvm.org/releases/3.1/docs/ReleaseNotes.html> and
above.

The C backend has been removed. It had numerous problems, to the point
of
> not being able to compile any nontrivial program.

At the same time, I'd be interested to know the use-cases of C++ backend
and how to compile the C++ code (to generate IR) that LLVM C++ backend
generates.


On 9 September 2015 at 18:00, Roel Jordans via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> 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
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>


-- 

Thanks & Regards,
Dipanjan
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