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8241900: Loop unswitching may cause dependence on null check to be lost

Reviewed-by: thartmann, kvn
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rwestrel committed Apr 2, 2020
1 parent 6b7a466 commit 6405d75070301465824784cfdcee017095525a27
@@ -221,11 +221,11 @@ void PhaseIdealLoop::do_unswitching(IdealLoopTree *loop, Node_List &old_new) {

// Hardwire the control paths in the loops into if(true) and if(false)
_igvn.rehash_node_delayed(unswitch_iff);
short_circuit_if(unswitch_iff, proj_true);
dominated_by(proj_true, unswitch_iff, false, false);

IfNode* unswitch_iff_clone = old_new[unswitch_iff->_idx]->as_If();
_igvn.rehash_node_delayed(unswitch_iff_clone);
short_circuit_if(unswitch_iff_clone, proj_false);
dominated_by(proj_false, unswitch_iff_clone, false, false);

// Reoptimize loops
loop->record_for_igvn();
@@ -0,0 +1,83 @@
/*
* Copyright (c) 2020, Red Hat, Inc. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/

/*
* @test
* @bug 8241900
* @summary Loop unswitching may cause dependence on null check to be lost
*
* @run main/othervm -XX:-TieredCompilation -XX:-BackgroundCompilation -XX:+StressGCM -XX:+StressLCM TestLoopUnswitchingLostCastDependency
*/

import java.util.Arrays;

public class TestLoopUnswitchingLostCastDependency {
private static Object objectField;

public static void main(String[] args) {
Object[] array = new Object[100];
Arrays.fill(array, new Object());
for (int i = 0; i < 20_000; i++) {
array[1] = null;
test(array);
array[1] = new Object();
objectField = null;
test(array);
array[1] = new Object();
objectField = new Object();
test(array);
}
}

private static void test(Object[] array) {
Object o = objectField;
Object o3 = array[1];
int j = 0;
for (int i = 1; i < 100; i *= 2) {
Object o2 = array[i];
// Both branches taken: loop is unswitched.
if (o3 != null) {
if (o2 == null) {
}
// Both branches taken: loop is unswitched next.
if (o != null) {
// CastPP here becomes control dependent on o2 ==
// null check above.
if (o.getClass() == Object.class) {
}
// This causes partial peeling. When that happens,
// the o2 == null check becomes redundant with the
// o3 != null check in the peeled iteration. The
// CastPP with o as input that was control
// dependent on the o2 == null check becomes
// control dependent on the o3 != null check,
// above the o != null check.
if (j > 7) {

}
j++;
}
}
}
}
}

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