Re: Urgent: bug fixes needed for GCC 3.0.2

[Jakub Jelinek <jakub at redhat dot com>]

On Thu, Oct 04, 2001 at 08:57:13AM -0700, Mark Mitchell wrote:
> 
> >   > I'm not sure.  This is a MEM on the stack, and one that is not
> >   > necessarily going to be around for the entire function -- witness
> >   > the fact that we are going to overwrite it.  Perhaps I misunderstand
> >   > a subtlety of RTX_UNCHANGING_P (quite likely, in fact!) but to me
> >   > it doesn't make sense to set it for this MEM; the value there is
> >   > likely to change.
> >   >
> >   > Am I speaking nonsense?
> > Well, if it wasn't for the sibcall optimization the stack slot would be
> > unchanging.  That's the crux of the issue.

This looks to me like ressurected
http://gcc.gnu.org/ml/gcc-patches/2001-03/msg00806.html
thread which was exactly about this.

I'm using following patch for this in gcc-2.96-RH (which Richard Kenner did
not like but it certainly is less invasive change then ignoring
RTX_UNCHANGING_P with sibling call optimization). IMHO clearing
RTX_UNCHANGING_P is a safe thing to do even if it cleared some innocent
variable's (which would be sharing the same stack slot) RTX_UNCHANGING_P
flag as opposed to setting it.

2001-03-16  Jakub Jelinek  <jakub@redhat.com>

        * expr.c (expand_expr): If TARGET_EXPR is putting a non-constant
        variable into place of constant variable, clear the /u flag if
        possible.

--- gcc/expr.c.jj       Mon Mar 12 13:41:38 2001
+++ gcc/expr.c  Fri Mar 16 19:46:26 2001
@@ -8167,6 +8167,17 @@ expand_expr (exp, target, tmode, modifie
            else
              {
                DECL_RTL (slot) = target;
+
+               /* If target is unchanging, but slot is not, we could end up
+                  initializing the unchanging target through non-unchanging
+                  references.  */
+               if (RTX_UNCHANGING_P (target) && ! TREE_READONLY (slot)
+                   && (GET_CODE (target) == REG
+                       || (GET_CODE (target) == MEM
+                           && GET_CODE (XEXP (target, 0)) == ADDRESSOF
+                           && GET_CODE (XEXP (XEXP (target, 0), 0)) == REG)))
+                 RTX_UNCHANGING_P (target) = 0;
+
                /* If we must have an addressable slot, then make sure that
                   the RTL that we just stored in slot is OK.  */
                if (TREE_ADDRESSABLE (slot))


	Jakub