Add CodeGen support for functions that always return arguments via a new parameter attribute 'returned', which is taken advantage of in target-independent tail call opportunity detection and in ARM call lowering (when placed on an integral first parameter).

llvm-svn: 179925

1 files changed, 11 insertions, 1 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 905053fef0b..7dafbb98779 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -719,7 +719,17 @@ Currently, only the following parameter attributes are defined:
 ``nest``
     This indicates that the pointer parameter can be excised using the
     :ref:`trampoline intrinsics <int_trampoline>`. This is not a valid
-    attribute for return values.
+    attribute for return values and can only be applied to one parameter.
+
+``returned``
+    This indicates that the value of the function always returns the value
+    of the parameter as its return value. This is an optimization hint to
+    the code generator when generating the caller, allowing tail call
+    optimization and omission of register saves and restores in some cases;
+    it is not checked or enforced when generating the callee. The parameter
+    and the function return type must be valid operands for the
+    :ref:`bitcast instruction <i_bitcast>`. This is not a valid attribute for
+    return values and can only be applied to one parameter.
 
 .. _gc: