commit be34f5e4e7417d426d2b35228eac5310525cce82
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Fri Feb 5 04:43:15 2016 -0800

    Update TARGET_FUNCTION_INCOMING_ARG documentation
    
    On x86, interrupt handlers are only called by processors which push
    interrupt data onto stack at the address where the normal return address
    is.  Since interrupt handlers must access interrupt data via pointers so
    that they can update interrupt data, the pointer argument is passed as
    "argument pointer - word".
    
    TARGET_FUNCTION_INCOMING_ARG defines how callee sees its argument.
    Normally it returns REG, NULL, or CONST_INT.  This patch adds arbitrary
    address computation based on hard register, which can be forced into a
    register, to the list.
    
    When copying an incoming argument onto stack, assign_parm_setup_stack
    has:
    
    if (argument in memory)
      copy argument in memory to stack
    else
      move argument to stack
    
    Since an arbitrary address computation may be passed as an argument, we
    change it to:
    
    if (argument in memory)
      copy argument in memory to stack
    else
      {
        if (argument isn't in register)
          force argument into a register
        move argument to stack
      }
    
    	* function.c (assign_parm_setup_stack): Force source into a
    	register if needed.
    	* target.def (function_incoming_arg): Update documentation to
    	allow arbitrary address computation based on hard register.
    	* doc/tm.texi: Regenerated.

diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 745910f..826f8fb 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -3949,6 +3949,10 @@ which the caller passes the value, and
 fashion to tell the function being called where the arguments will
 arrive.
 
+@code{TARGET_FUNCTION_INCOMING_ARG} can also return arbitrary address
+computation using hard register, which can be forced into a register,
+so that it can be used to pass special arguments.
+
 If @code{TARGET_FUNCTION_INCOMING_ARG} is not defined,
 @code{TARGET_FUNCTION_ARG} serves both purposes.
 @end deftypefn
diff --git a/gcc/function.c b/gcc/function.c
index 1ac8e26..c42e865 100644
--- a/gcc/function.c
+++ b/gcc/function.c
@@ -3466,7 +3466,11 @@ assign_parm_setup_stack (struct assign_parm_data_all *all, tree parm,
 			   BLOCK_OP_NORMAL);
 	}
       else
-	emit_move_insn (dest, src);
+	{
+	  if (!REG_P (src))
+	    src = force_reg (GET_MODE (src), src);
+	  emit_move_insn (dest, src);
+	}
     }
 
   if (to_conversion)
diff --git a/gcc/target.def b/gcc/target.def
index 20f2b32..d3d9963 100644
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -4468,8 +4468,8 @@ a register.",
        bool named),
  default_function_arg)
 
-/* Likewise, but for machines with register windows.  Return the
-   location where the argument will appear to the callee.  */
+/* Likewise, but for machines with register windows or special arguments.
+   Return the location where the argument will appear to the callee.  */
 DEFHOOK
 (function_incoming_arg,
  "Define this hook if the target machine has ``register windows'', so\n\
@@ -4483,6 +4483,10 @@ which the caller passes the value, and\n\
 fashion to tell the function being called where the arguments will\n\
 arrive.\n\
 \n\
+@code{TARGET_FUNCTION_INCOMING_ARG} can also return arbitrary address\n\
+computation using hard register, which can be forced into a register,\n\
+so that it can be used to pass special arguments.\n\
+\n\
 If @code{TARGET_FUNCTION_INCOMING_ARG} is not defined,\n\
 @code{TARGET_FUNCTION_ARG} serves both purposes.",
  rtx, (cumulative_args_t ca, machine_mode mode, const_tree type,