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8249605: C2: assert(no_dead_loop) failed: dead loop detected
Fixed dead loop detection in PhiNode::Ideal() to additionally account for dead MergeMemNodes

Reviewed-by: kvn, thartmann
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chhagedorn committed Aug 3, 2020
1 parent 5a8c995 commit 0a4ddac11778076a8ff2585b4078e604fa29f74c
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Showing 2 changed files with 80 additions and 7 deletions.
@@ -2229,23 +2229,30 @@ Node *PhiNode::Ideal(PhaseGVN *phase, bool can_reshape) {
} else {
// We know that at least one MergeMem->base_memory() == this
// (saw_self == true). If all other inputs also references this phi
// (directly or through data nodes) - it is dead loop.
// (directly or through data nodes) - it is a dead loop.
bool saw_safe_input = false;
for (uint j = 1; j < req(); ++j) {
Node *n = in(j);
if (n->is_MergeMem() && n->as_MergeMem()->base_memory() == this)
continue; // skip known cases
Node* n = in(j);
if (n->is_MergeMem()) {
MergeMemNode* mm = n->as_MergeMem();
if (mm->base_memory() == this || mm->base_memory() == mm->empty_memory()) {
// Skip this input if it references back to this phi or if the memory path is dead
continue;
}
}
if (!is_unsafe_data_reference(n)) {
saw_safe_input = true; // found safe input
break;
}
}
if (!saw_safe_input)
return top; // all inputs reference back to this phi - dead loop
if (!saw_safe_input) {
// There is a dead loop: All inputs are either dead or reference back to this phi
return top;
}

// Phi(...MergeMem(m0, m1:AT1, m2:AT2)...) into
// MergeMem(Phi(...m0...), Phi:AT1(...m1...), Phi:AT2(...m2...))
PhaseIterGVN *igvn = phase->is_IterGVN();
PhaseIterGVN* igvn = phase->is_IterGVN();
Node* hook = new Node(1);
PhiNode* new_base = (PhiNode*) clone();
// Must eagerly register phis, since they participate in loops.
@@ -0,0 +1,66 @@
/*
* Copyright (c) 2020, Oracle and/or its affiliates. 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 8249605
* @summary A dead memory loop is detected when replacing an actually dead memory phi node in PhiNode::Ideal()
* by a dead MergeMemNode which builds a cycle with one of its slices.
*
* @run main/othervm -Xcomp -XX:-TieredCompilation
* -XX:CompileCommand=compileonly,compiler.c2.TestDeadPhiMergeMemLoop::main
* -XX:CompileCommand=dontinline,compiler.c2.TestDeadPhiMergeMemLoop::dontInline
* compiler.c2.TestDeadPhiMergeMemLoop
*/

package compiler.c2;

public class TestDeadPhiMergeMemLoop {

public static boolean bFld = false;
public static double dArrFld[] = new double[400];

public static void main(String[] strArr) {
int x = 1;
int i = 0;
int iArr[] = new int[400];
dontInline();

if (bFld) {
x += x;
} else if (bFld) {
float f = 1;
while (++f < 132) {
if (bFld) {
dArrFld[5] = 3;
for (i = (int)(f); i < 12; i++) {
}
}
}
}
}

// Not inlined
public static void dontInline() {
}
}

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