1 files changed, 22 insertions, 4 deletions
diff --git a/gcc/config/mmix/mmix.c b/gcc/config/mmix/mmix.c
index 26668e76da8..1ce88001277 100644
--- a/gcc/config/mmix/mmix.c
+++ b/gcc/config/mmix/mmix.c
@@ -392,15 +392,33 @@ mmix_conditional_register_usage (void)
 
 /* INCOMING_REGNO and OUTGOING_REGNO worker function.
    Those two macros must only be applied to function argument
-   registers.  FIXME: for their current use in gcc, it'd be better
-   with an explicit specific additional FUNCTION_INCOMING_ARG_REGNO_P
-   a'la TARGET_FUNCTION_ARG / TARGET_FUNCTION_INCOMING_ARG instead of
+   registers and the function return value register for the opposite
+   use.  FIXME: for their current use in gcc, it'd be better with an
+   explicit specific additional FUNCTION_INCOMING_ARG_REGNO_P a'la
+   TARGET_FUNCTION_ARG / TARGET_FUNCTION_INCOMING_ARG instead of
    forcing the target to commit to a fixed mapping and for any
-   unspecified register use.  */
+   unspecified register use.  Particularly when thinking about the
+   return-value, it is better to imagine INCOMING_REGNO and
+   OUTGOING_REGNO as named CALLEE_TO_CALLER_REGNO and INNER_REGNO as
+   named CALLER_TO_CALLEE_REGNO because the direction.  The "incoming"
+   and "outgoing" is from the perspective of the parameter-registers,
+   but the same macro is (must be, lacking an alternative like
+   suggested above) used to map the return-value-register from the
+   same perspective.  To make directions even more confusing, the macro
+   MMIX_OUTGOING_RETURN_VALUE_REGNUM holds the number of the register
+   in which to return a value, i.e. INCOMING_REGNO for the return-value-
+   register as received from a called function; the return-value on the
+   way out.  */
 
 int
 mmix_opposite_regno (int regno, int incoming)
 {
+  if (incoming && regno == MMIX_OUTGOING_RETURN_VALUE_REGNUM)
+    return MMIX_RETURN_VALUE_REGNUM;
+
+  if (!incoming && regno == MMIX_RETURN_VALUE_REGNUM)
+    return MMIX_OUTGOING_RETURN_VALUE_REGNUM;
+
   if (!mmix_function_arg_regno_p (regno, incoming))
     return regno;