From c06d6805aae3af2e6175f3f43deea46c9ce08bc6 Mon Sep 17 00:00:00 2001 From: Daniel Skantz Date: Tue, 7 Oct 2025 09:04:39 +0000 Subject: [PATCH] 8362394: C2: Repeated stacked string concatenation fails with "Hit MemLimit" and other resourcing errors Reviewed-by: chagedorn, rcastanedalo --- src/hotspot/share/opto/stringopts.cpp | 22 ++++- .../stringopts/TestStackedConcatsMany.java | 95 +++++++++++++++++++ 2 files changed, 116 insertions(+), 1 deletion(-) create mode 100644 test/hotspot/jtreg/compiler/stringopts/TestStackedConcatsMany.java diff --git a/src/hotspot/share/opto/stringopts.cpp b/src/hotspot/share/opto/stringopts.cpp index 25aa82870c3..420423dd246 100644 --- a/src/hotspot/share/opto/stringopts.cpp +++ b/src/hotspot/share/opto/stringopts.cpp @@ -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) { diff --git a/test/hotspot/jtreg/compiler/stringopts/TestStackedConcatsMany.java b/test/hotspot/jtreg/compiler/stringopts/TestStackedConcatsMany.java new file mode 100644 index 00000000000..dbc6e495594 --- /dev/null +++ b/test/hotspot/jtreg/compiler/stringopts/TestStackedConcatsMany.java @@ -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; + } +}