From: Andrew MacLeod <amacleod@redhat.com>
To: Andrew MacLeod <amacleod@redhat.com>
Cc: gcc mailing list <gcc@gcc.gnu.org>
Subject: Re: [tree-ssa] Out of SSA status and issues
Date: Mon, 12 May 2003 18:57:00 -0000 [thread overview]
Message-ID: <1052765152.3372.28.camel@p4> (raw)
In-Reply-To: <1052750563.2730.317.camel@p4>
On Mon, 2003-05-12 at 10:42, Andrew MacLeod wrote:
>
> The second case occurs when a derefernce is copy propagated into a PHI
> node:
>
> if (T.1_5 != 0B)
> {
>
> # VUSE <(*map)_3>;
> # VUSE <map_4>;
> T.2_8 = map->compact_to_partition;
> i.3_9 = (unsigned int)i_6;
> T.4_10 = i.3_9 * 4;
> T.5_11 = (int *)T.4_10;
>
> # (*T.6)_13 = VDEF <(*T.6)_7>;
> T.6_12 = T.2_8 + T.5_11;
>
> # VUSE <T.6_12>;
> i_14 = (*T.6)_13
> };
> # i_1 = PHI <i_6(0), (*T.6)_13(1)>;
> # (*T.6)_2 = PHI <(*T.6)_7(0), (*T.6)_13(1)>;
>
> # (*T.7)_17 = VDEF <(*T.7)_16>;
> # VUSE <(*map)_3>;
> # VUSE <map_4>;
>
> The value of i_14 has been propagated into the PHI node. DCE the deletes
> the stmt
> i_14 = (*T.6)_13
>
> When we go to rewrite this, all we know is that its a derefernce of T.6.
> There is no VUSE now to look at to figure out what the correct pointer
> it. The original def has it as T.6_12. The information could be found
> by looking for the def of (*T.6)_13 (which is virtual), and looking at
> the real def, which is T.6_12. I am about to try that in my hack and see
> if it works.
>
Actually, is propagating this copy a safe thing to do?
Copy propagation simply looks at the def, and if the stmt is a copy,
copies it... So if there was a store after the definition of i_14 which
killed the memory location that T.6_12 points to, then the PHI is going
to get the wrong result... isn't it?
ie it would look something like this hacked up example:
# (*T.6)_13 = VDEF <(*T.6)_7>;
T.6_12 = T.2_8 + T.5_11;
# VUSE <T.6_12>;
i_14 = (*T.6)_13
# (*T.6)_22 = VDEF <(*T.6)_13>
# VUSE <T.6_12>
*T.6 = 30;
};
# i_1 = PHI <i_6(0), (*T.6)_13(1)>;
Or is there a reason that copyprop would never happen?
Should we never copyprop an indirect reference?
Andrew
next prev parent reply other threads:[~2003-05-12 18:57 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-05-12 14:42 Andrew MacLeod
2003-05-12 15:38 ` Diego Novillo
2003-05-12 15:57 ` Andrew MacLeod
2003-05-12 16:05 ` Michael Matz
2003-05-12 16:10 ` Andrew MacLeod
2003-05-12 16:16 ` law
2003-05-12 17:08 ` law
2003-05-12 17:12 ` Andrew MacLeod
2003-05-12 17:26 ` law
2003-05-12 18:57 ` Andrew MacLeod [this message]
2003-05-13 9:07 ` Michael Matz
2003-05-13 12:42 ` Diego Novillo
2003-05-13 12:50 ` Andrew MacLeod
2003-05-13 13:05 ` Diego Novillo
2003-05-13 13:29 ` Andrew MacLeod
2003-05-13 13:57 ` Diego Novillo
2003-05-13 12:57 ` Michael Matz
2003-05-13 13:11 ` Diego Novillo
2003-05-13 13:18 ` Andrew MacLeod
2003-05-14 17:19 ` Jan Vroonhof
2003-05-14 18:05 ` Andrew MacLeod
2003-05-14 18:33 ` Diego Novillo
2003-05-14 19:11 ` Daniel Berlin
2003-05-13 15:01 ` Daniel Berlin
2003-05-13 12:33 ` Diego Novillo
2003-05-13 12:49 ` Andrew MacLeod
2003-05-13 12:58 ` Diego Novillo
2003-05-13 13:17 Richard Kenner
2003-05-13 13:27 ` Diego Novillo
2003-05-13 13:40 ` Michael Matz
2003-05-13 15:08 ` Michael S. Zick
2003-05-13 13:42 Richard Kenner
2003-05-13 15:23 Richard Kenner
2003-05-13 18:50 ` Geoff Keating
2003-05-13 23:28 ` Michael S. Zick
2003-05-17 17:19 ` Michael S. Zick
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