In the current implementation this ABI only works for functions that take only integer and pointer arguments.

Word description
This symbol is passed as the abi argument to alien-indirect, alien-callback, alien-assembly, and add-library to indicate that the "fast call" calling convention should be used, where the first two integer or pointer arguments are passed in registers and the function cleans up its own stack frame before returning to the caller. This symbol only has meaning on 32-bit x86 platforms.

See also
cdecl, stdcall, thiscall