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* [Bug tree-optimization/19804] New: Missed jump threading opportunity on "else" arm of COND_EXPR
@ 2005-02-07 15:33 kazu at cs dot umass dot edu
2005-02-07 17:25 ` [Bug tree-optimization/19804] " pinskia at gcc dot gnu dot org
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: kazu at cs dot umass dot edu @ 2005-02-07 15:33 UTC (permalink / raw)
To: gcc-bugs
Consider:
int
foo (int a, int b)
{
if (a == 0)
if (b != 2)
return 10;
if (b == 1)
return 10;
return 20;
}
Here is t21.dom1:
foo (a, b)
{
int D.1120;
<bb 0>:
if (a_2 == 0) goto <L0>; else goto <L2>;
<L0>:;
if (b_4 != 2) goto <L1>; else goto <L2>;
<L1>:;
D.1120_7 = 10;
goto <bb 6> (<L6>);
<L2>:;
if (b_4 == 1) goto <L4>; else goto <L5>;
<L4>:;
D.1120_6 = 10;
goto <bb 6> (<L6>);
<L5>:;
D.1120_5 = 20;
# D.1120_1 = PHI <10(2), 10(4), 20(5)>;
<L6>:;
return D.1120_1;
}
Note that edge from <L0> to <L2> can be threaded through L2.
This seems to be due to an artificial restriction in
tree-ssa-dom.c:dom_opt_finalize_block.
/* If we have a simple NAME = VALUE equivalency record it.
Until the jump threading selection code improves, only
do this if both the name and value are SSA_NAMEs with
the same underlying variable to avoid missing threading
opportunities. */
if (lhs
&& TREE_CODE (COND_EXPR_COND (last)) == SSA_NAME)
record_const_or_copy (lhs, rhs);
This "if" should just say "if (lhs)".
But be careful though. Making the above change seems to miss other jump
threading opportunities as far as I can tell from my coverage test.
I have yet to find a test case that would be less optimized with the above
change.
By the way, the comment above the "if" statement is just a cut-n-paste from
the "then" case. It does not reflect what the "if" statement does though.
--
Summary: Missed jump threading opportunity on "else" arm of
COND_EXPR
Product: gcc
Version: unknown
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: enhancement
Priority: P2
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: kazu at cs dot umass dot edu
CC: gcc-bugs at gcc dot gnu dot org,law at redhat dot com
OtherBugsDependingO 19794
nThis:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19804
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/19804] Missed jump threading opportunity on "else" arm of COND_EXPR
2005-02-07 15:33 [Bug tree-optimization/19804] New: Missed jump threading opportunity on "else" arm of COND_EXPR kazu at cs dot umass dot edu
@ 2005-02-07 17:25 ` pinskia at gcc dot gnu dot org
2005-02-07 19:36 ` kazu at cs dot umass dot edu
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2005-02-07 17:25 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From pinskia at gcc dot gnu dot org 2005-02-07 04:50 -------
Confirmed.
--
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Ever Confirmed| |1
Last reconfirmed|0000-00-00 00:00:00 |2005-02-07 04:50:55
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19804
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/19804] Missed jump threading opportunity on "else" arm of COND_EXPR
2005-02-07 15:33 [Bug tree-optimization/19804] New: Missed jump threading opportunity on "else" arm of COND_EXPR kazu at cs dot umass dot edu
2005-02-07 17:25 ` [Bug tree-optimization/19804] " pinskia at gcc dot gnu dot org
@ 2005-02-07 19:36 ` kazu at cs dot umass dot edu
2005-02-07 19:48 ` kazu at cs dot umass dot edu
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: kazu at cs dot umass dot edu @ 2005-02-07 19:36 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From kazu at cs dot umass dot edu 2005-02-07 06:45 -------
Here is a test case where a jump threading opportunity would be missed
if the change suggested in the original post were made:
void
foo (int *p)
{
if (*p != 0)
bar ();
if (*p != 0)
bar ();
}
Here is a tree dump right before the first DOM.
foo (p)
{
int D.1120;
<bb 0>:
D.1120_2 = *p_1;
if (D.1120_2 != 0) goto <L0>; else goto <L1>;
<L0>:;
bar ();
<L1>:;
D.1120_3 = *p_1;
if (D.1120_3 != 0) goto <L2>; else goto <L3>;
<L2>:;
bar ();
<L3>:;
return;
}
The change I suggested would record "D.1120_2 == 2" while following
edge from <L0> to <L2>.
Then th jump threading selection code figures out that
D.1120_3 = *p_1;
is the same as
D.1120_3 = 0;
via equality "D.1120_3 == D.1120_2".
For whatever reason, the jump threading selection code does not want to
see anything but SSA_NAME on the rhs.
At that point, DOM throws away this jump threading opportunity.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19804
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/19804] Missed jump threading opportunity on "else" arm of COND_EXPR
2005-02-07 15:33 [Bug tree-optimization/19804] New: Missed jump threading opportunity on "else" arm of COND_EXPR kazu at cs dot umass dot edu
2005-02-07 17:25 ` [Bug tree-optimization/19804] " pinskia at gcc dot gnu dot org
2005-02-07 19:36 ` kazu at cs dot umass dot edu
@ 2005-02-07 19:48 ` kazu at cs dot umass dot edu
2005-02-15 1:37 ` law at redhat dot com
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: kazu at cs dot umass dot edu @ 2005-02-07 19:48 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From kazu at cs dot umass dot edu 2005-02-07 06:46 -------
Note that PR 19516 is very closely related.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19804
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/19804] Missed jump threading opportunity on "else" arm of COND_EXPR
2005-02-07 15:33 [Bug tree-optimization/19804] New: Missed jump threading opportunity on "else" arm of COND_EXPR kazu at cs dot umass dot edu
` (2 preceding siblings ...)
2005-02-07 19:48 ` kazu at cs dot umass dot edu
@ 2005-02-15 1:37 ` law at redhat dot com
2005-04-23 1:14 ` law at redhat dot com
2005-04-25 5:07 ` law at redhat dot com
5 siblings, 0 replies; 7+ messages in thread
From: law at redhat dot com @ 2005-02-15 1:37 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From law at redhat dot com 2005-02-14 21:02 -------
The updated jump thread selection code will catch this case and optimize it in
the appropriate way -- without causing us to miss other jump threading
opportunities :-)
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19804
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/19804] Missed jump threading opportunity on "else" arm of COND_EXPR
2005-02-07 15:33 [Bug tree-optimization/19804] New: Missed jump threading opportunity on "else" arm of COND_EXPR kazu at cs dot umass dot edu
` (3 preceding siblings ...)
2005-02-15 1:37 ` law at redhat dot com
@ 2005-04-23 1:14 ` law at redhat dot com
2005-04-25 5:07 ` law at redhat dot com
5 siblings, 0 replies; 7+ messages in thread
From: law at redhat dot com @ 2005-04-23 1:14 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From law at redhat dot com 2005-04-23 01:14 -------
Fixed now.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19804
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/19804] Missed jump threading opportunity on "else" arm of COND_EXPR
2005-02-07 15:33 [Bug tree-optimization/19804] New: Missed jump threading opportunity on "else" arm of COND_EXPR kazu at cs dot umass dot edu
` (4 preceding siblings ...)
2005-04-23 1:14 ` law at redhat dot com
@ 2005-04-25 5:07 ` law at redhat dot com
5 siblings, 0 replies; 7+ messages in thread
From: law at redhat dot com @ 2005-04-25 5:07 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From law at redhat dot com 2005-04-25 05:07 -------
Fixed by:
http://gcc.gnu.org/ml/gcc-patches/2005-04/msg02426.html
--
What |Removed |Added
----------------------------------------------------------------------------
URL| |http://gcc.gnu.org/ml/gcc-
| |patches/2005-
| |04/msg02426.html
Status|NEW |RESOLVED
Resolution| |FIXED
Target Milestone|--- |4.1.0
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19804
^ permalink raw reply [flat|nested] 7+ messages in thread
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2005-02-07 15:33 [Bug tree-optimization/19804] New: Missed jump threading opportunity on "else" arm of COND_EXPR kazu at cs dot umass dot edu
2005-02-07 17:25 ` [Bug tree-optimization/19804] " pinskia at gcc dot gnu dot org
2005-02-07 19:36 ` kazu at cs dot umass dot edu
2005-02-07 19:48 ` kazu at cs dot umass dot edu
2005-02-15 1:37 ` law at redhat dot com
2005-04-23 1:14 ` law at redhat dot com
2005-04-25 5:07 ` law at redhat dot com
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