8373502: C2 SuperWord: speculative check uses VPointer variable was pinned after speculative check, leading to bad graph

Reviewed-by: mhaessig, chagedorn
Backport-of: 00050f84d44f3ec23e9c6da52bffd68770010749

src/hotspot/share/opto/vectorization.cpp

@@ -1060,6 +1060,29 @@ bool VPointer::can_make_speculative_aliasing_check_with(const VPointer& other) c
     return false;
   }
 
+  // The speculative check also needs to create the pointer expressions for both
+  // VPointers. We must check that we can do that, i.e. that all variables of the
+  // VPointers are available at the speculative check (and not just pre-loop invariant).
+  if (!this->can_make_pointer_expression_at_speculative_check()) {
+#ifdef ASSERT
+    if (_vloop.is_trace_speculative_aliasing_analysis()) {
+      tty->print_cr("VPointer::can_make_speculative_aliasing_check_with: not all variables of VPointer are avaialbe at speculative check!");
+      this->print_on(tty);
+    }
+#endif
+    return false;
+  }
+
+  if (!other.can_make_pointer_expression_at_speculative_check()) {
+#ifdef ASSERT
+    if (_vloop.is_trace_speculative_aliasing_analysis()) {
+      tty->print_cr("VPointer::can_make_speculative_aliasing_check_with: not all variables of VPointer are avaialbe at speculative check!");
+      other.print_on(tty);
+    }
+#endif
+    return false;
+  }
+
   return true;
 }
 
@@ -1147,6 +1170,8 @@ BoolNode* VPointer::make_speculative_aliasing_check_with(const VPointer& other,
   Node* main_init = new ConvL2INode(main_initL);
   phase->register_new_node_with_ctrl_of(main_init, pre_init);
 
+  assert(vp1.can_make_pointer_expression_at_speculative_check(), "variables must be available early enough to avoid cycles");
+  assert(vp2.can_make_pointer_expression_at_speculative_check(), "variables must be available early enough to avoid cycles");
   Node* p1_init = vp1.make_pointer_expression(main_init, ctrl);
   Node* p2_init = vp2.make_pointer_expression(main_init, ctrl);
   Node* size1 = igvn.longcon(vp1.size());

src/hotspot/share/opto/vectorization.hpp

@@ -1188,6 +1188,22 @@ private:
     return true;
   }
 
+  // We already know that all non-iv summands are pre loop invariant.
+  // See init_are_non_iv_summands_pre_loop_invariant
+  // That is good enough for alignment computations in the pre-loop limit. But it is not
+  // sufficient if we want to use the variables of the VPointer at the speculative check,
+  // which is further up before the pre-loop.
+  bool can_make_pointer_expression_at_speculative_check() const {
+    bool success = true;
+    mem_pointer().for_each_non_empty_summand([&] (const MemPointerSummand& s) {
+      Node* variable = s.variable();
+      if (variable != _vloop.iv() && !_vloop.is_available_for_speculative_check(variable)) {
+        success = false;
+      }
+    });
+    return success;
+  }
+
   // In the pointer analysis, and especially the AlignVector analysis, we assume that
   // stride and scale are not too large. For example, we multiply "iv_scale * iv_stride",
   // and assume that this does not overflow the int range. We also take "abs(iv_scale)"

test/hotspot/jtreg/compiler/loopopts/superword/TestAliasingCheckVPointerVariablesNotAvailable.java

@@ -0,0 +1,71 @@
+/*
+ * 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 id=with-flags
+ * @bug 8373502
+ * @summary Test where a VPointer variable was pinned at the pre-loop, but not available at the
+ *          Auto_Vectorization_Check, and so it should not be used for the auto vectorization
+ *          aliasing check, to avoid a bad (circular) graph.
+ * @run main/othervm
+ *      -XX:CompileCommand=compileonly,*TestAliasingCheckVPointerVariablesNotAvailable::test
+ *      -XX:-TieredCompilation
+ *      -Xcomp
+ *      ${test.main.class}
+ */
+
+/*
+ * @test id=vanilla
+ * @bug 8373502
+ * @run main ${test.main.class}
+ */
+
+package compiler.loopopts.superword;
+
+public class TestAliasingCheckVPointerVariablesNotAvailable {
+    static int iFld;
+
+    public static void main(String[] strArr) {
+        test();
+    }
+
+    static void test() {
+        int iArr[] = new int[400];
+        boolean flag = false;
+        for (int i = 6; i < 50000; i++) { // Trigger OSR
+            try {
+                int x = 234 / iFld;
+                iFld = iArr[3];
+            } catch (ArithmeticException a_e) {
+            }
+            for (int j = i; j < 2; j++) {
+                if (flag) {
+                    iArr[j] = 117;
+                } else {
+                    iArr[1] = 34;
+                }
+                iArr[1] += i;
+            }
+        }
+    }
+}