Extended Description

The Win64 and System V x86-64 ABIs have different calling conventions. Currently, LLVM picks the right convention based on the target triple.

GCC does this too, but it also allows code to override the default using two attributes, 'ms_abi' and 'sysv_abi', which pick the Win64 and System V x86-64 conventions, respectively.

Sixty-four-bit Wine (http://www.winehq.org/) uses the ms_abi attribute extensively in order for its DLLs to have the correct calling convention. Without this support, LLVM-based compilers can't build 64-bit Wine.