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.
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Answer
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.
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