# Why You Can't Use Prebuilt LLVM 10.0 with C++17

C++17 introduced an alignment argument to ::operator new(). It's important to note that if you allocate something using aligned new, you absolutely must deallocate it using aligned delete, or the behavior is undefined. LLVM 10.x takes advantage of this alignment parameter, if the compiler supports it. That means if you are compiling on Windows with MSVC set to C++14, __cpp_aligned_new is not defined and the extra argument isn't passed. Otherwise, if it's compiled with MSVC set to C++17, __cpp_aligned_new is defined and the extra argument is passed.

There's just one teeny tiny little problem - the check was in a header file.

Now, if this header file were completely internal and LLVM itself only ever called ::operator new() from inside it's libraries, this wouldn't be a problem. As you might have guessed, this was not the case. LLVM was calling allocate_buffer() from inside header files. The corresponding deallocate_buffer call was inside the same header file, but sadly, it was called from a destructor, and that destructor had been called from inside one of LLVM's libraries, which meant it didn't know about aligned allocations... Oops!

This means, if your program uses C++17, but LLVM was compiled with C++14, your program will happily pass LLVM aligned memory, which LLVM will then pass into the wrong delete operator, because it's calling the unaligned delete function from C++14 instead of the aligned delete function from C++17. This results in strange and bizarre heap corruption errors because of the mismatched ::operator new and ::operator delete. Arguably, the real bug here is LLVM calling any allocation function from a header file in the first place, as this is just begging for ABI problems.

Of course, one might ask why a C++17 application would be linking against LLVM compiled with C++14. Well, you see, the prebuilt LLVM binaries were compiled against C++14... OOPS! Suddenly if you wanted to avoid compiling LLVM yourself, you couldn't use C++17 anymore!

Luckily this has now been fixed after a bunch of people complained about the extremely unintuitive errors resulting from this mismatch. Unfortunately, as of this writing, LLVM still hasn't provided a new release, which means you will still encounter this problem using the latest precompiled binaries of LLVM. Personally, I recompile LLVM using C++17 for my own projects, just to be safe, since LLVM does not guarantee ABI compatibility in these situations.

Still, this is a particularly nasty case of an unintentional ABI break between C++ versions, which is easy to miss because most people assume C++ versions are backwards compatible. Be careful, and stop allocating memory in header files that might be deallocated inside your library!