This commit adds a new target-independent calling convention for C++ TLS
access functions. It aims to minimize overhead in the caller by perserving as
many registers as possible.
The target-specific implementation for X86-64 is defined as following:
Arguments are passed as for the default C calling convention
The same applies for the return value(s)
The callee preserves all GPRs - except RAX and RDI
The access function makes C-style TLS function calls in the entry and exit
block, C-style TLS functions save a lot more registers than normal calls.
The added calling convention ties into the existing implementation of the
C-style TLS functions, so we can't simply use existing calling conventions
such as preserve_mostcc.
rdar://9001553
llvm-svn: 254737
Manman Ren