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8261730: C2 compilation fails with assert(store->find_edge(load) != -…
…1) failed: missing precedence edge

Relax assertion in PhaseCFG::verify() to accept the case where a store is used
to implement an implicit null check and a load is placed in the null block.

Reviewed-by: thartmann, kvn
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robcasloz committed Mar 4, 2021
1 parent 7915a1f commit 4cfecceb044da6ac76f79ac95e3ad26fe32b64f8
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Showing 4 changed files with 82 additions and 2 deletions.
@@ -200,6 +200,7 @@ macro(Lock)
macro(Loop)
macro(LoopLimit)
macro(Mach)
macro(MachNullCheck)
macro(MachProj)
macro(MulAddS2I)
macro(MaxI)
@@ -770,7 +770,24 @@ Block* PhaseCFG::insert_anti_dependences(Block* LCA, Node* load, bool verify) {
// Add an anti-dep edge, and squeeze 'load' into the highest block.
assert(store != load->find_exact_control(load->in(0)), "dependence cycle found");
if (verify) {
assert(store->find_edge(load) != -1, "missing precedence edge");
#ifdef ASSERT
// We expect an anti-dependence edge from 'load' to 'store', except when
// implicit_null_check() has hoisted 'store' above its early block to
// perform an implicit null check, and 'load' is placed in the null
// block. In this case it is safe to ignore the anti-dependence, as the
// null block is only reached if 'store' tries to write to null.
Block* store_null_block = NULL;
Node* store_null_check = store->find_out_with(Op_MachNullCheck);
if (store_null_check != NULL) {
Node* if_true = store_null_check->find_out_with(Op_IfTrue);
assert(if_true != NULL, "null check without null projection");
Node* null_block_region = if_true->find_out_with(Op_Region);
assert(null_block_region != NULL, "null check without null region");
store_null_block = get_block_for_node(null_block_region);
}
#endif
assert(LCA == store_null_block || store->find_edge(load) != -1,
"missing precedence edge");
} else {
store->add_prec(load);
}
@@ -1,5 +1,5 @@
/*
* Copyright (c) 1997, 2020, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 1997, 2021, 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
@@ -699,6 +699,7 @@ class MachNullCheckNode : public MachBranchNode {
add_req(ctrl);
add_req(memop);
}
virtual int Opcode() const;
virtual uint size_of() const { return sizeof(*this); }

virtual void emit(CodeBuffer &cbuf, PhaseRegAlloc *ra_) const;
@@ -0,0 +1,61 @@
/*
* Copyright (c) 2021, 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.
*/

package compiler.uncommontrap;

/**
* @test
* @bug 8261730
* @summary Test that no anti-dependence violation is reported between a store
* used as an implicit null check and a load placed in the null block.
* @run main/othervm -XX:-BackgroundCompilation
* compiler.uncommontrap.TestNullCheckAntiDependence
*/

public class TestNullCheckAntiDependence {

private static class MyInteger {
int val;
}

private static MyInteger foo = new MyInteger();
private static MyInteger bar = new MyInteger();

static void setFooToZero() {
for (int i = 0; i < 1; i++) {
// This load is placed in the null block.
foo.val = -bar.val;
for (int k = 0; k < 10; k++) {
// This store is hoisted and used as an implicit null check.
foo.val = 0;
}
}
}

public static void main(String[] args) {
for (int i = 0; i < 10_000; i++) {
setFooToZero();
}
}

}

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