[Bug other/61300] powerpc64le miscompile with K&R-style function definition at -O0

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61300

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Alan Modra from comment #1)
> So, the "writes way past this" is writing into the parameter save area.  
> 
> compare_kr is assuming that it was called with a parameter save area because
> it isn't prototyped, but that is quite wrong because when compiling the
> function body we don't know how the function was called.  A call may well
> have a prototype in scope, and thus not set up a parameter save area.
> 
> It's a bug in rs6000_function_parms_need_stack.  This function can't blindly
> test !prototype_p (fun).

So you can simply drop that check instead as it seems to be an optimization
only?  Thus

Index: gcc/config/rs6000/rs6000.c
===================================================================
--- gcc/config/rs6000/rs6000.c  (revision 211011)
+++ gcc/config/rs6000/rs6000.c  (working copy)
@@ -10492,7 +10492,7 @@ rs6000_function_parms_need_stack (tree f
     fun = TREE_TYPE (fun);

   /* Varargs functions need the parameter save area.  */
-  if (!prototype_p (fun) || stdarg_p (fun))
+  if (stdarg_p (fun))
     return true;

   INIT_CUMULATIVE_INCOMING_ARGS (args_so_far_v, fun, NULL_RTX);

?