The C/C++ standards allow compilers to do whatever they want as long as it does not cause observable output changes. So LLVM could do runtime profiling if it w

The C/C++ standards allow compilers to do whatever they want as long as it does not cause observable output changes. So LLVM could do runtime profiling if it wanted. But I suspect that in the majority of the cases the cost of the profiling would be worse than the potential gains.