8373731: C2: Missed optimization opportunity for AddI

Reviewed-by: bmaillard, epeter, dlong

src/hotspot/share/opto/phaseX.cpp

@@ -2007,19 +2007,6 @@ void PhaseIterGVN::verify_Identity_for(Node* n) {
     case Op_ConvI2L:
       return;
 
-    // AddINode::Identity
-    // Converts (x-y)+y to x
-    // Could be issue with notification
-    //
-    // Turns out AddL does the same.
-    //
-    // Found with:
-    //  compiler/c2/Test6792161.java
-    //  -ea -esa -XX:CompileThreshold=100 -XX:+UnlockExperimentalVMOptions -server -XX:-TieredCompilation -XX:+IgnoreUnrecognizedVMOptions -XX:VerifyIterativeGVN=1110
-    case Op_AddI:
-    case Op_AddL:
-      return;
-
     // AbsINode::Identity
     // Not investigated yet.
     case Op_AbsI:
@@ -2778,6 +2765,18 @@ void PhaseIterGVN::add_users_of_use_to_worklist(Node* n, Node* use, Unique_Node_
     };
     use->visit_uses(push_and_to_worklist, is_boundary);
   }
+
+  // If changed Sub inputs, check Add for identity.
+  // e.g., (x - y) + y -> x; x + (y - x) -> y.
+  if (use_op == Op_SubI || use_op == Op_SubL) {
+    const int add_op = (use_op == Op_SubI) ? Op_AddI : Op_AddL;
+    for (DUIterator_Fast i2max, i2 = use->fast_outs(i2max); i2 < i2max; i2++) {
+      Node* u = use->fast_out(i2);
+      if (u->Opcode() == add_op) {
+        worklist.push(u);
+      }
+    }
+  }
 }
 
 /**

test/hotspot/jtreg/compiler/c2/igvn/TestMissingAddSubElimination.java

@@ -0,0 +1,117 @@
+/*
+ * 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.
+ *
+ * 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.c2.igvn;
+
+import compiler.lib.ir_framework.*;
+import java.util.Random;
+import jdk.test.lib.Asserts;
+import jdk.test.lib.Utils;
+
+/*
+ * @test
+ * @bug 8373731
+ * @summary C2 IGVN should eliminate a + (b - c) after c = a * i folds to a,
+ *          i.e. a + (b - a) -> b
+ * @library /test/lib /
+ * @run driver ${test.main.class}
+ */
+
+public class TestMissingAddSubElimination {
+    private static final Random R = Utils.getRandomInstance();
+
+    public static void main(String[] args) {
+        TestFramework.runWithFlags("-XX:+IgnoreUnrecognizedVMOptions",
+                                   "-XX:VerifyIterativeGVN=1000",
+                                   "-XX:CompileCommand=compileonly,compiler.c2.igvn.TestMissingAddSubElimination::*");
+    }
+
+    @Run(test = {"testAddI1", "testAddI2", "testAddL1", "testAddL2"})
+    public void runTestAdd(){
+        int x = R.nextInt();
+        int y = R.nextInt();
+        Asserts.assertEQ(testAddI1(x, y), y);
+        Asserts.assertEQ(testAddI2(x, y), x);
+
+        long xl = R.nextLong();
+        long yl = R.nextLong();
+        Asserts.assertEQ(testAddL1(xl, yl), yl);
+        Asserts.assertEQ(testAddL2(xl, yl), xl);
+    }
+
+    // int: x + (y - x) -> y
+    @Test
+    @IR(counts = {
+            IRNode.ADD_I, "0",
+            IRNode.SUB_I, "0",
+            IRNode.MUL_I, "0"
+    })
+    int testAddI1(int x, int y) {
+        int i;
+        for (i = -10; i < 1; i++) { }
+        int c = x * i;
+        return x + (y - c);
+    }
+
+    // int: (x - y) + y -> x
+    @Test
+    @IR(counts = {
+            IRNode.ADD_I, "0",
+            IRNode.SUB_I, "0",
+            IRNode.MUL_I, "0"
+    })
+    int testAddI2(int x, int y) {
+        int i;
+        for (i = -10; i < 1; i++) { }
+        int c =  y * i;
+        return (x - c) + y;
+    }
+
+    // long: x + (y - x) -> y
+    @Test
+    @IR(counts = {
+            IRNode.ADD_L, "0",
+            IRNode.SUB_L, "0",
+            IRNode.MUL_L, "0"
+    })
+    long testAddL1(long x, long y) {
+        int i;
+        for (i = -10; i < 1; i++) { }
+        long c = x * i;
+        return x + (y - c);
+    }
+
+    // long: (x - y) + y -> x
+    @Test
+    @IR(counts = {
+            IRNode.ADD_L, "0",
+            IRNode.SUB_L, "0",
+            IRNode.MUL_L, "0"
+    })
+    long testAddL2(long x, long y) {
+        int i;
+        for (i = -10; i < 1; i++) { }
+        long c =  y * i;
+        return (x - c) + y;
+    }
+}