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Re: GCC Status Report (2004-03-09)
- From: Eric Botcazou <ebotcazou at libertysurf dot fr>
- To: Mark Mitchell <mark at codesourcery dot com>
- Cc: Jakub Jelinek <jakub at redhat dot com>,gcc at gcc dot gnu dot org,gcc-patches at gcc dot gnu dot org
- Date: Fri, 19 Mar 2004 19:35:42 +0100
- Subject: Re: GCC Status Report (2004-03-09)
- References: <200403091809.i29I9P04020607@sirius.codesourcery.com> <200403191413.10043.ebotcazou@libertysurf.fr> <405B302B.40006@codesourcery.com>
> Well, so, let's remove that chunk of code; it should no longer be needed.
No, not the whole chunk of code. The pessimization was introduced by:
2003-04-07 Glen Nakamura <glen@imodulo.com>
PR opt/8634
* explow.c (maybe_set_unchanging): Don't flag non-static const
aggregate type initializers with RTX_UNCHANGING_P.
===================================================================
RCS file: /cvs/gcc/gcc/gcc/explow.c,v
retrieving revision 1.107
retrieving revision 1.108
diff -u -r1.107 -r1.108
--- gcc/gcc/explow.c 2003/03/20 22:51:39 1.107
+++ gcc/gcc/explow.c 2003/04/07 22:57:41 1.108
@@ -657,8 +657,18 @@
/* We can set RTX_UNCHANGING_P from TREE_READONLY for decls whose
initialization is only executed once, or whose initializer always
has the same value. Currently we simplify this to PARM_DECLs in the
- first case, and decls with TREE_CONSTANT initializers in the second.
*/
+ first case, and decls with TREE_CONSTANT initializers in the second.
+
+ We cannot do this for non-static aggregates, because of the double
+ writes that can be generated by store_constructor, depending on the
+ contents of the initializer. Yes, this does eliminate a good fraction
+ of the number of uses of RTX_UNCHANGING_P for a language like Ada.
+ It also eliminates a good quantity of bugs. Let this be incentive to
+ eliminate RTX_UNCHANGING_P entirely in favour of a more reliable
+ solution, perhaps based on alias sets. */
+
if ((TREE_READONLY (t) && DECL_P (t)
+ && (TREE_STATIC (t) || ! AGGREGATE_TYPE_P (TREE_TYPE (t)))
&& (TREE_CODE (t) == PARM_DECL
|| (DECL_INITIAL (t) && TREE_CONSTANT (DECL_INITIAL (t)))))
|| TREE_CODE_CLASS (TREE_CODE (t)) == 'c')
Do you want me to revert that patch?
> Eric, I can tell you're unhappy with this approach, and so you're
> casting about for something better. That's good, but I think we've come
> as far as we can for 3.4.0.
Yes, I'm pretty frustrated because we're (again) chasing down a far-reaching
bug just days before a release.
> If all this is sufficiently annoyhing to you, maybe you can work on ripping
> out RTX_UNCHANGING_P for 3.5 and replacing it with something better!
Before I start thinking about a replacement, I'd like to understand what I'll
be trying the replace. It appears that no-bo-dy can tell what is the
purpose of RTX_UNCHANGING_P.
--
Eric Botcazou