I'm not sure what that means, but I can discuss the issue with LTO. LTO means that the normal compile step turns the source into an intermediate form. The final compilation doesn't happen until link time. The problem is this intermediate form which is a compiler (even compiler version) -specific internal format. GCC and LLVM use different formats so you cannot mix the two compilers on the same LTO program. I'm not certain but it might be possible to use ld -r to obtain a real compiled object from each toolchain and then link those together at the end, but of course that loses some benefits of LTO, especially cross-module inlining.