__fastcall jitting

Hi,

We're using LLVM MCJIT on X86 platform.
When calling
  Function::setCallingConv(CallingConv::X86_StdCall)
the jitted code indeed conforms __stdcall convention (instead of default __cdecl).
However applying
  Function::setCallingConv(CallingConv::X86_FastCall)
seem does not have proper effect; jitted code is still __stdcall.
Is it a bug, or am I missing something?

Regards,
Mikhail