Generate LLVM C++ API code as backend - c++

Question

The Online LLVM demo page had an option to generate LLVM C++ API code as backend from a source code. However, that demo page is now disabled. I was wondering how we can do it ourselves using the available LLVM tools.
I tried the following
clang++ -c -emit-llvm input.cpp -o input.ll
llc -march=cpp -o input.ll.cpp input.ll
which gives the following error
llc: error: invalid target 'cpp'.
I am using LLVM/Clang version 3.2.

Answer 1

The LLVM C++ backend has to be enabled during configuration when building LLVM. It's enabled by default in the configure (autotools) build, but not in the CMake build when you build on Windows. You can enable it by setting the appropriate flags while configuring with CMake. See this page for more information.
Quote:
LLVM_TARGETS_TO_BUILD:STRING
Semicolon-separated list of targets to build, or all for building all targets. Case-sensitive. For Visual C++ defaults to X86. On the
other cases defaults to all. Example:
-DLLVM_TARGETS_TO_BUILD="X86;PowerPC".
UPDATE
Since version 3.9 the CppBackend is no more a valid target. They've removed from their code as the generated code were presenting a few issues.
Check this commit
Remove bit-rotten CppBackend.
This backend was supposed to generate C++ code which will re-construct
the LLVM IR passed as input. This seems to me to have very marginal
usefulness in the first place.
However, the code has never been updated to use IRBuilder, which makes
its current value negative -- people who look at the output may be
steered to use the *wrong* C++ APIs to construct IR.
Furthermore, it's generated code that doesn't compile since at least
2013.
Differential Revision: http://reviews.llvm.org/D19942
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk#268631 91177308-0d34-0410-b5e6-96231b3b80d8

Answer 2

Sadly, this appears to no longer be possible in more recent versions of LLVM. The associated commit message explains it pretty well.
As you can see in the following commit,
Remove bit-rotten CppBackend, the generated code would show issues.
commit 257fabb18605265a79397d35dd79a3973760ffaf
Author: ---
Date: Thu May 5 14:35:40 2016 +0000
Remove bit-rotten CppBackend.
This backend was supposed to generate C++ code which will re-construct
the LLVM IR passed as input. This seems to me to have very marginal
usefulness in the first place.
However, the code has never been updated to use IRBuilder, which makes
its current value negative -- people who look at the output may be
steered to use the *wrong* C++ APIs to construct IR.
Furthermore, it's generated code that doesn't compile since at least
2013.
Differential Revision: http://reviews.llvm.org/D19942
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk#268631 91177308-0d34-0410-b5e6-96231b3b80d8