[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, 10 insertions, 0 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index cf1ceab1f1c..82b33557c12 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -406,6 +406,16 @@ added in the future:
     This calling convention, like the `PreserveMost` calling convention, will be
     used by a future version of the ObjectiveC runtime and should be considered
     experimental at this time.
+"``cxx_fast_tlscc``" - The `CXX_FAST_TLS` calling convention for access functions
+    This calling convention aims to minimize overhead in the caller by
+    preserving as many registers as possible. This calling convention behaves
+    identical to the `C` calling convention on how arguments and return values
+    are passed, but it uses a different set of caller/callee-saved registers.
+    Given that C-style TLS on Darwin has its own special CSRs, we can't use the
+    existing `PreserveMost`.
+
+    - On X86-64 the callee preserves all general purpose registers, except for
+      RDI and RAX.
 "``cc <n>``" - Numbered convention
     Any calling convention may be specified by number, allowing
     target-specific calling conventions to be used. Target specific