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8362394: C2: Repeated stacked string concatenation fails with "Hit MemLimit" and other resourcing errors

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Reviewed-by: chagedorn, rcastanedalo
This commit is contained in:
Daniel Skantz 2025-10-07 09:04:39 +00:00
parent 07549f3e15
commit c06d6805aa
2 changed files with 116 additions and 1 deletions
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22
src/hotspot/share/opto/stringopts.cpp
View File

@ -53,6 +53,11 @@ class StringConcat : public ResourceObj {
Node_List _uncommon_traps; // Uncommon traps that needs to be rewritten
// to restart at the initial JVMState.
static constexpr uint STACKED_CONCAT_UPPER_BOUND = 256; // argument limit for a merged concat.
// The value 256 was derived by measuring
// compilation time on variable length sequences
// of stackable concatenations and chosen to keep
// a safe margin to any critical point.
public:
// Mode for converting arguments to Strings
enum {
@ -295,6 +300,8 @@ StringConcat* StringConcat::merge(StringConcat* other, Node* arg) {
}
assert(result->_control.contains(other->_end), "what?");
assert(result->_control.contains(_begin), "what?");
uint arguments_appended = 0;
for (int x = 0; x < num_arguments(); x++) {
Node* argx = argument_uncast(x);
if (argx == arg) {
@ -303,8 +310,21 @@ StringConcat* StringConcat::merge(StringConcat* other, Node* arg) {
for (int y = 0; y < other->num_arguments(); y++) {
result->append(other->argument(y), other->mode(y));
}
arguments_appended += other->num_arguments();
} else {
result->append(argx, mode(x));
arguments_appended++;
}
// Check if this concatenation would result in an excessive number of arguments
// -- leading to high memory use, compilation time, and later, a large number of IR nodes
// -- and bail out in that case.
if (arguments_appended > STACKED_CONCAT_UPPER_BOUND) {
#ifndef PRODUCT
if (PrintOptimizeStringConcat) {
tty->print_cr("Merge candidate of length %d exceeds argument limit", arguments_appended);
}
#endif
return nullptr;
}
}
result->set_allocation(other->_begin);
@ -680,7 +700,7 @@ PhaseStringOpts::PhaseStringOpts(PhaseGVN* gvn):
#endif
StringConcat* merged = sc->merge(other, arg);
if (merged->validate_control_flow() && merged->validate_mem_flow()) {
if (merged != nullptr && merged->validate_control_flow() && merged->validate_mem_flow()) {
#ifndef PRODUCT
AtomicAccess::inc(&_stropts_merged);
if (PrintOptimizeStringConcat) {

95
test/hotspot/jtreg/compiler/stringopts/TestStackedConcatsMany.java Normal file
View File

@ -0,0 +1,95 @@
/*
* Copyright (c) 2025, 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 8362394
* @summary Test that repeated stacked string concatenations do not
* consume too many compilation resources.
* @requires vm.compiler2.enabled
* @library /test/lib /
* @run main/othervm -XX:-OptoScheduling compiler.stringopts.TestStackedConcatsMany
* @run main/othervm -XX:-TieredCompilation -Xcomp -XX:-OptoScheduling
* -XX:CompileOnly=compiler.stringopts.TestStackedConcatsMany::f
* compiler.stringopts.TestStackedConcatsMany
*/
// The test uses -XX:-OptoScheduling to avoid the assert "too many D-U pinch points" on aarch64 (JDK-8328078).
package compiler.stringopts;
import jdk.test.lib.Asserts;
public class TestStackedConcatsMany {
public static void main (String... args) {
new StringBuilder(); // Trigger loading of the StringBuilder class.
String s = f();
String z = "xy";
for (int i = 0; i < 24; i++) {
z = z + z;
}
Asserts.assertEQ(s, z);
}
static String f() {
String s = "xy";
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
s = new StringBuilder().append(s).append(s).toString();
return s;
}
}