CompileBlocks isn't thread safe
https://github.com/FEX-Emu/FEX/blob/master/External/FEXCore/Source/Interface/Core/Core.cpp#L475
This function is called from multiple guest threads, which is fine except that the FrontendDecoder object is shared and not thread safe.
Best way to work around this issue would be to give each thread object its own FrontendDecoder object.
This way multiple threads can be compiling code as it pleases