8371558: C2: Missing optimization opportunity in AbsNode::Ideal

Reviewed-by: thartmann, rcastanedalo, chagedorn

Author: benoitmaillard

src/hotspot/share/opto/phaseX.cpp

@@ -2596,6 +2596,16 @@ void PhaseIterGVN::add_users_of_use_to_worklist(Node* n, Node* use, Unique_Node_
       }
     }
   }
+  // Check for "abs(0-x)" into "abs(x)" conversion
+  if (use->is_Sub()) {
+    for (DUIterator_Fast i2max, i2 = use->fast_outs(i2max); i2 < i2max; i2++) {
+      Node* u = use->fast_out(i2);
+      if (u->Opcode() == Op_AbsD || u->Opcode() == Op_AbsF ||
+          u->Opcode() == Op_AbsL || u->Opcode() == Op_AbsI) {
+        worklist.push(u);
+      }
+    }
+  }
   auto enqueue_init_mem_projs = [&](ProjNode* proj) {
     add_users_to_worklist0(proj, worklist);
   };

test/hotspot/jtreg/compiler/c2/TestMissingOptAbsZeroMinusX.java

@@ -0,0 +1,85 @@
+/*
+ * 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 8371558
+ * @summary An expression of the form "abs(0-x)" should be transformed to "abs(x)".
+ *          This test ensures that updates to the Sub node’s inputs propagate as
+ *          expected and that the optimization is not missed.
+ * @run main/othervm -XX:+IgnoreUnrecognizedVMOptions -XX:-TieredCompilation -Xcomp
+ *      -XX:CompileCommand=compileonly,compiler.c2.TestMissingOptAbsZeroMinusX::test*
+ *      -XX:VerifyIterativeGVN=1110 compiler.c2.TestMissingOptAbsZeroMinusX
+ * @run main compiler.c2.TestMissingOptAbsZeroMinusX
+ *
+ */
+
+package compiler.c2;
+
+public class TestMissingOptAbsZeroMinusX {
+    static int a;
+    static boolean b;
+
+    public static void main(String[] strArr) {
+        // no known reproducer for AbsL
+        testAbsI();
+        testAbsF();
+        testAbsD();
+    }
+
+    static void testAbsI() {
+        int d = 4;
+        for (int i = 8; i < 133; i += 3) {
+            d -= a;
+            b = (d != Math.abs(d));
+            for (long f = 3; f < 127; f++) {
+                for (int e = 1; e < 3; e++) {}
+            }
+            d = 0;
+        }
+    }
+
+    static void testAbsF() {
+        float d = 12.3f;
+        for (int i = 8; i < 133; i += 3) {
+            d -= a;
+            b = (d != Math.abs(d));
+            for (long f = 3; f < 127; f++) {
+                for (int e = 1; e < 3; e++) {}
+            }
+            d = 0.0f;
+        }
+    }
+
+    static void testAbsD() {
+        double d = 12.3;
+        for (int i = 8; i < 133; i += 3) {
+            d -= a;
+            b = (d != Math.abs(d));
+            for (long f = 3; f < 127; f++) {
+                for (int e = 1; e < 3; e++) {}
+            }
+            d = 0.0;
+        }
+    }
+}