Re: Expansion of __builtin_frame_address

[Mark Shinwell <shinwell at codesourcery dot com>]

Sorry, I just mistakenly sent this to gcc@ (where it was being
discussed) instead of gcc-patches@.

Mark

--

Mark Mitchell wrote:
> Mark Shinwell wrote:
> > As for the remaining problem, I suggest that we could:
> >
> > (i) always return the hard frame pointer, and disable FP elimination in
> > the current function; or
> >
> > (iii) ...the same as option (i), but allow targets to define another macro
> > that will cause the default code to use the soft frame pointer rather than
> > the hard frame pointer, and hence allow FP elimination.  (If such a macro
> > were set by a particular target, am I right in thinking that it would be
> > safe to use the soft frame pointer even in the count >= 1 cases?)
>
> > I tend to think that option (iii) might be best, although perhaps it
> > is overkill and option (i) would do.  But I'm not entirely sure;
> > still being a gcc novice I have to admit to not being quite thoroughly
> > clear on this myself at this stage.  So any advice or comments would be
> > appreciated!
>
> I agree that option (iii) is best, as it provides the ability to
> optimize on platforms where that is feasible, and still provides a
> working default elsewhere.  I will review and approve a suitable patch
> to implement (iii), assuming that there are no objections from Jim or
> others.

This having been discussed some more, and my understanding improved,
I now believe that option (i) is in fact the correct thing to do.  The
attach patch implements this, which basically amounts to the same logic
that is currently in the compiler save for the removal of the special
case when count == 0.

OK for mainline?

Mark

--

2006-06-01  Mark Shinwell  <shinwell@codesourcery.com>

	* gcc/builtins.c (expand_builtin_return_addr): Always use
	hard_frame_pointer_rtx and prevent frame pointer elimination
	if INITIAL_FRAME_ADDRESS_RTX isn't set.

Index: gcc/builtins.c
===================================================================
--- gcc/builtins.c	(revision 114277)
+++ gcc/builtins.c	(working copy)
@@ -496,20 +496,14 @@ expand_builtin_return_addr (enum built_i
 #else
   rtx tem;
 
-  /* For a zero count, we don't care what frame address we return, so frame
-     pointer elimination is OK, and using the soft frame pointer is OK.
-     For a non-zero count, we require a stable offset from the current frame
-     pointer to the previous one, so we must use the hard frame pointer, and
-     we must disable frame pointer elimination.  */
-  if (count == 0)
-    tem = frame_pointer_rtx;
-  else 
-    {
-      tem = hard_frame_pointer_rtx;
+  /* We require a stable offset from the current frame pointer to the
+     previous one, so we must use the hard frame pointer, and we must
+     disable frame pointer elimination.  */
 
-      /* Tell reload not to eliminate the frame pointer.  */
-      current_function_accesses_prior_frames = 1;
-    }
+  tem = hard_frame_pointer_rtx;
+
+  /* Tell reload not to eliminate the frame pointer.  */
+  current_function_accesses_prior_frames = 1;
 #endif
 
   /* Some machines need special handling before we can access