From: Diego Novillo <dnovillo@redhat.com>
To: Jan Hubicka <jh@suse.cz>
Cc: "gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
Subject: Re: aliasing question
Date: Tue, 07 Sep 2004 14:22:00 -0000 [thread overview]
Message-ID: <1094566924.21733.36.camel@localhost.localdomain> (raw)
In-Reply-To: <20040907105142.GD28472@kam.mff.cuni.cz>
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.
Diego.
next prev parent reply other threads:[~2004-09-07 14:22 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-09-07 10:51 Jan Hubicka
2004-09-07 14:22 ` Diego Novillo [this message]
2004-09-08 20:05 ` Jan Hubicka
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