[CXX TLS calling convention] Add CXX TLS calling convention.

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

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/docs/BitCodeFormat.rst b/llvm/docs/BitCodeFormat.rst
index 62d66f8..d6e3099 100644
--- a/llvm/docs/BitCodeFormat.rst
+++ b/llvm/docs/BitCodeFormat.rst
@@ -756,6 +756,7 @@ function. The operand fields are:
   * ``anyregcc``: code 13
   * ``preserve_mostcc``: code 14
   * ``preserve_allcc``: code 15
+  * ``cxx_fast_tlscc``: code 17
   * ``x86_stdcallcc``: code 64
   * ``x86_fastcallcc``: code 65
   * ``arm_apcscc``: code 66