8360023: Add an insertion sort implementation to Hotspot

src/hotspot/share/utilities/sort.hpp

@@ -0,0 +1,72 @@
+/*
+ * 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.
+ *
+ */
+#ifndef SHARE_UTILITIES_SORT_HPP
+#define SHARE_UTILITIES_SORT_HPP
+
+#include "memory/allStatic.hpp"
+
+// An insertion sort that is stable and inplace.
+// This algorithm should be the ideal solution to sort a sequence with few elements. Arrays::sort
+// uses insertion sort for arrays up to around 50 elements.
+// comp should return a value > 0 iff the first argument is larger than the second argument. A full
+// comparison function satisfies this requirement but a simple a > b ? 1 : 0 also satisfies it.
+class InsertionSort : AllStatic {
+public:
+  template <class T, class Compare>
+  static void sort(T* data, int size, Compare comp) {
+    if (size == 0) {
+      return;
+    }
+
+    T* begin = data;
+    T* end = data + size;
+    for (T* current = begin + 1; current < end; current++) {
+      T current_elem = *current;
+
+      // Elements in [begin, current) has already been sorted, we search backward to find a
+      // location to insert the element at current. In the meantime, shift all elements on the way
+      // up by 1.
+      T* pos = current;
+      while (pos > begin) {
+        // Because the sort is stable, we must insert the current element at the first location at
+        // which the element is not greater than the current element (note that we are traversing
+        // backward)
+        T* prev = pos - 1;
+        if (comp(*prev, current_elem) <= 0) {
+          break;
+        }
+
+        *pos = *prev;
+        pos = prev;
+      }
+
+      // Move current_elem to pos since all elements in [pos, current) have been shifted up by 1
+      if (pos < current) {
+        *pos = current_elem;
+      }
+    }
+  }
+};
+
+#endif // SHARE_UTILITIES_SORT_HPP

test/hotspot/gtest/utilities/test_sort.cpp

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+/*
+ * 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.
+ *
+ */
+
+#include "runtime/os.hpp"
+#include "unittest.hpp"
+#include "utilities/powerOfTwo.hpp"
+#include "utilities/sort.hpp"
+
+constexpr int TEST_ARRAY_SIZE = 128;
+
+class TwoInt {
+public:
+  int val;
+  int idx;
+
+  TwoInt() : val(0), idx(0) {}
+  TwoInt(int val, int idx) : val(val), idx(idx) {}
+};
+
+int ARRAY0[TEST_ARRAY_SIZE];
+TwoInt ARRAY1[TEST_ARRAY_SIZE];
+
+// Verify that the sort is correct, i.e. a[i] <= a[i + 1]
+void test_insertion_sort(int size) {
+  assert(size <= TEST_ARRAY_SIZE, "invalid parameter");
+  for (int i = 0; i < size; i++) {
+    ARRAY0[i] = os::random();
+  }
+  InsertionSort::sort(ARRAY0, size, [](int a, int b) {
+    return a > b ? 1 : 0;
+  });
+  for (int i = 0; i < size - 1; i++) {
+    ASSERT_TRUE(ARRAY0[i] <= ARRAY0[i + 1]);
+  }
+}
+
+// Verify that the sort is stable. Since there are 128 elements but the keys can only take 16
+// values, there will inevitably be a lot of elements with the same key. We then verify that if the
+// keys of 2 elements are the same, then the element that has the smaller idx will be ordered
+// before the one with the larger idx.
+void test_insertion_sort_stable(int size, int key_bound) {
+  assert(size <= TEST_ARRAY_SIZE, "invalid parameter");
+  assert(is_power_of_2(key_bound), "invalid parameter");
+  for (int i = 0; i < size; i++) {
+    ARRAY1[i] = TwoInt(os::random() & (key_bound - 1), i);
+  }
+  InsertionSort::sort(ARRAY1, size, [](TwoInt a, TwoInt b) {
+    return a.val > b.val ? 1 : 0;
+  });
+  for (int i = 0; i < size - 1; i++) {
+    TwoInt a = ARRAY1[i];
+    TwoInt b = ARRAY1[i + 1];
+    ASSERT_TRUE(a.val <= b.val);
+    if (a.val == b.val) {
+      ASSERT_TRUE(a.idx < b.idx);
+    }
+  }
+}
+
+TEST(utilities, insertion_sort) {
+  for (int i = 0; i < 100; i++) {
+    test_insertion_sort(0);
+    test_insertion_sort(1);
+    test_insertion_sort(2);
+    test_insertion_sort(10);
+    test_insertion_sort(TEST_ARRAY_SIZE);
+    test_insertion_sort_stable(1, 1);
+    test_insertion_sort_stable(2, 1);
+    test_insertion_sort_stable(3, 2);
+    test_insertion_sort_stable(10, 4);
+    test_insertion_sort_stable(TEST_ARRAY_SIZE, 16);
+  }
+}