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Re: aliasing question
- From: Jan Hubicka <jh at suse dot cz>
- To: Diego Novillo <dnovillo at redhat dot com>
- Cc: Jan Hubicka <jh at suse dot cz>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>,gcc-patches at gcc dot gnu dot org
- Date: Wed, 8 Sep 2004 22:05:53 +0200
- Subject: Re: aliasing question
- References: <20040907105142.GD28472@kam.mff.cuni.cz> <1094566924.21733.36.camel@localhost.localdomain>
> On Tue, 2004-09-07 at 06:51, Jan Hubicka wrote:
>
> > # V_MUST_DEF <value_49>;
> > value = T.4862_48;
> >
> > and alias1 dump:
> >
> > # value_229 = V_MAY_DEF <value_19>;
> > value = T.4862_48;
> >
> > Why the MUST_DEF is getting converted to MAY_DEF in such a obvious case
> > for MUST_DEF?
> >
> Before aliasing, 'value' did not belong to any alias set, but after
> aliasing it is. If we didn't do that, suppose that:
>
> *p, alias tag TMT.1, alias set of TMT.1 = { value }
>
> 1. p = (cond) ? &value : foo ();
> 2. *p = 3;
> 3. value = T.4862;
> 4. return *p;
>
> If you put a V_MUST_DEF in statement #3, you will consider '*p = 3'
> dead. Perhaps, we could put a V_MUST_DEF if there were more than one
> element in TMT.1's alias set. Not sure how much would that buy, but it
> may be an interesting experiment.
Hi,
as discussed on the IRC, this check looks unnecesary after all (ie at
least my understanding is that optimizer should not conclude the *p dead
just from the fact that all object in the alias set has been mustdeffed
because the pointer still can point elsewhere). I've
bootstrapped®tested the attached patch and commited it after Diego's
approval.
Honza
2004-09-08 Jan Hubicka <jh@suse.cz>
* tree-ssa-operands.c (add_stmt_operand): Use V_MUST_DEF even for
variables being alias set.
Index: tree-ssa-operands.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/tree-ssa-operands.c,v
retrieving revision 2.36
diff -c -3 -p -r2.36 tree-ssa-operands.c
*** tree-ssa-operands.c 5 Sep 2004 15:24:15 -0000 2.36
--- tree-ssa-operands.c 7 Sep 2004 15:05:47 -0000
*************** add_stmt_operand (tree *var_p, tree stmt
*** 1487,1502 ****
/* The variable is not aliased or it is an alias tag. */
if (flags & opf_is_def)
{
! if (v_ann->is_alias_tag)
! {
! /* Alias tagged vars get V_MAY_DEF to avoid breaking
! def-def chains with the other variables in their
! alias sets. */
! if (s_ann)
! s_ann->makes_aliased_stores = 1;
! append_v_may_def (var);
! }
! else if (flags & opf_kill_def)
{
#if defined ENABLE_CHECKING
/* Only regular variables may get a V_MUST_DEF
--- 1487,1493 ----
/* The variable is not aliased or it is an alias tag. */
if (flags & opf_is_def)
{
! if (flags & opf_kill_def)
{
#if defined ENABLE_CHECKING
/* Only regular variables may get a V_MUST_DEF