[PATCH 8/8] Document the explictcc calling convention

docs/LangRef.rst

     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.
+
+"``explicitcc``" - A calling convention which allows function argument
+   register assignments and callee save registers to be explicitly
+   specified using the hwreg() parameter- and noclobber()
+   function-attributes.
+
 "``cc <n>``" - Numbered convention
     Any calling convention may be specified by number, allowing
     target-specific calling conventions to be used. Target specific

     array), however ``dereferenceable(<n>)`` does imply ``nonnull`` in
     ``addrspace(0)`` (which is the default address space).
 
+``hwreg(<n>)``
+    This indicates that the parameter is located in a particular
+    physical register. This attribute may only be applied to
+    parameters of a function using the ``explicitcc`` calling
+    convention or to argments to llvm.experimental.retwr.
+
 .. _gc:
 
 Garbage Collector Strategy Names

     show that no exceptions passes by it. This is normally the case for
     the ELF x86-64 abi, but it can be disabled for some compilation
     units.
+``noclobber(<n>,...)``
+    This attribute is valid for functions using the ``explicitcc``
+    calling convention. It specifies a list of callee save registers.
 
 .. _moduleasm:
 

 LLVM provides experimental intrinsics to support runtime patching
 mechanisms commonly desired in dynamic language JITs. These intrinsics
 are described in :doc:`StackMaps`.
+
+Explicitcc Intrinsics
+---------------------
+
+'``llvm.experimental.retwr``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+      declare void @llvm.experimental.retwr(...) noreturn
+
+Overview:
+"""""""""
+
+The ``llvm.`experimental.retwr` allows a function declared with the
+explicitcc calling convention to return to the caller while
+simultaneously loading values into registers.
+
+Arguments:
+""""""""""
+
+The values to return. The destination register is given by a hwreg()
+parameter attribute.
+
+Semantics:
+""""""""""
+
+The current function is terminated and execution is transferred to the
+caller. Before control is transferred, the values given in the
+argument list are loaded into the physical registers as specified by
+the hwreg() attributes. It is an error to provide an argument without
+a hwreg() attribute or use ``@llvm.experimental.retwr`` in a function
+which is not using the explicitcc calling convention.