[GR-80584] Add empty-loop removal in low tier.

PullRequest: graal/25701

Author: pecimuth

compiler/src/jdk.graal.compiler.test/src/jdk/graal/compiler/loop/test/LateRemoveEmptyLoopsTest.java

@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2026, 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.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * 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 jdk.graal.compiler.loop.test;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import jdk.graal.compiler.api.directives.GraalDirectives;
+import jdk.graal.compiler.core.test.GraalCompilerTest;
+import jdk.graal.compiler.loop.phases.AggressivePartialUnrollPhase;
+import jdk.graal.compiler.nodes.LoopBeginNode;
+import jdk.graal.compiler.nodes.StructuredGraph;
+import jdk.graal.compiler.options.OptionValues;
+import jdk.graal.compiler.vector.phases.VectorLoopUtility;
+
+/**
+ * Regression test for late empty-loop removal (GR-80584).
+ */
+public class LateRemoveEmptyLoopsTest extends GraalCompilerTest {
+
+    /**
+     * Checks that the mid-tier graph contains a main loop.
+     */
+    @Override
+    protected void checkMidTierGraph(StructuredGraph graph) {
+        super.checkMidTierGraph(graph);
+        boolean hasMainLoop = false;
+        for (LoopBeginNode loop : graph.getNodes(LoopBeginNode.TYPE)) {
+            hasMainLoop |= loop.isMainLoop();
+        }
+        Assert.assertTrue("expected main loop in mid-tier graph", hasMainLoop);
+    }
+
+    /**
+     * Checks that the low-tier graph contains no main loop.
+     */
+    @Override
+    protected void checkLowTierGraph(StructuredGraph graph) {
+        super.checkLowTierGraph(graph);
+        for (LoopBeginNode loop : graph.getNodes(LoopBeginNode.TYPE)) {
+            Assert.assertFalse("unexpected main loop: " + loop, loop.isMainLoop());
+        }
+    }
+
+    /**
+     * Sets {@code state} to 1 on odd iterations.
+     *
+     * @param limit the exclusive loop bound
+     * @param initial the initial state
+     * @return {@code initial} if {@code limit <= 1}, otherwise 1
+     */
+    public static int alternatingState(int limit, int initial) {
+        int state = initial;
+        for (int i = 0; GraalDirectives.injectIterationCount(1_000_000, i < limit); i++) {
+            if (GraalDirectives.injectBranchProbability(0.5, (i & 1) != 0)) {
+                state = 1;
+            }
+        }
+        return state;
+    }
+
+    /**
+     * Checks the result and absence of main loops.
+     */
+    @Test
+    public void testAlternatingState() {
+        OptionValues options = new OptionValues(getInitialOptions(),
+                        AggressivePartialUnrollPhase.Options.HighTierPartialUnrolling, false,
+                        VectorLoopUtility.Options.RespectVectorization, false);
+        test(options, "alternatingState", 20, 23);
+    }
+}

compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/core/phases/LowTier.java

@@ -56,6 +56,7 @@
 import jdk.graal.compiler.phases.schedule.SchedulePhase.SchedulingStrategy;
 import jdk.graal.compiler.phases.tiers.LowTierContext;
 import jdk.graal.compiler.vector.phases.VectorLoweringPhaseSuite;
+import jdk.graal.compiler.vector.phases.RemoveEmptyLoopsPhase;
 import jdk.graal.compiler.vector.replacements.VectorIntrinsics;
 import jdk.graal.compiler.virtual.phases.ea.LowTierReadEliminationPhase;
 
@@ -121,6 +122,8 @@ public LowTier(OptionValues options) {
 
         appendPhase(canonicalizerWithoutGVN);
 
+        appendPhase(new RemoveEmptyLoopsPhase(canonicalizerWithoutGVN));
+
         /*
          * This placeholder should be replaced by an instance of {@link AddressLoweringPhase}
          * specific to the target architecture for this compilation. This should be done by the