Generate optimized bytecode for 'for' in progression

Instead of checking for overflow at the end of each iteration, calculate the actual final loop parameter value before the loop, save it to the local variable and at the end of the iteration check if the loop parameter is exactly equal to this final value. ProgressionUtil.getProgressionFinalElement() credits go to @geevee
2013-06-20 18:06:55 +04:00
parent 8a14b62a23
commit d85ca47184
2 changed files with 252 additions and 181 deletions
@@ -0,0 +1,83 @@
+/*
+ * Copyright 2010-2013 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package jet.runtime;
+
+public class ProgressionUtil {
+    private ProgressionUtil() {
+    }
+
+    // a mod b (in arithmetical sense)
+    private static int mod(int a, int b) {
+        return a >= 0 ? a % b : a % b + b;
+    }
+
+    private static long mod(long a, long b) {
+        return a >= 0 ? a % b : a % b + b;
+    }
+
+    // (a - b) mod c
+    private static int differenceModulo(int a, int b, int c) {
+        return mod(mod(a, c) - mod(b, c), c);
+    }
+
+    private static long differenceModulo(long a, long b, long c) {
+        return mod(mod(a, c) - mod(b, c), c);
+    }
+
+
+    /**
+     * Calculates the final element of a bounded arithmetic progression, i.e. the last element of the progression which is in the range
+     * from {@code start} to {@code end} in case of a positive {@code increment}, or from {@code end} to {@code start} in case of a negative
+     * increment.
+     *
+     * <p>No validation on passed parameters is performed. The given parameters should satisfy the condition: either
+     * {@code increment&nbsp;&gt; 0} and {@code start&nbsp;&lt;= end}, or {@code increment&nbsp;&lt; 0} and {@code start&nbsp;&gt;= end}.
+     * @param start first element of the progression
+     * @param end ending bound for the progression
+     * @param increment increment, or difference of successive elements in the progression
+     * @return the final element of the progression
+     */
+    public static int getProgressionFinalElement(int start, int end, int increment) {
+        if (increment > 0) {
+            return end - differenceModulo(end, start, increment);
+        }
+        else {
+            return end + differenceModulo(start, end, -increment);
+        }
+    }
+
+    /**
+     * Calculates the final element of a bounded arithmetic progression, i.e. the last element of the progression which is in the range
+     * from {@code start} to {@code end} in case of a positive {@code increment}, or from {@code end} to {@code start} in case of a negative
+     * increment.
+     *
+     * <p>No validation on passed parameters is performed. The given parameters should satisfy the condition: either
+     * {@code increment&nbsp;&gt; 0} and {@code start&nbsp;&lt;= end}, or {@code increment&nbsp;&lt; 0} and {@code start&nbsp;&gt;= end}.
+     * @param start first element of the progression
+     * @param end ending bound for the progression
+     * @param increment increment, or difference of successive elements in the progression
+     * @return the final element of the progression
+     */
+    public static long getProgressionFinalElement(long start, long end, long increment) {
+        if (increment > 0) {
+            return end - differenceModulo(end, start, increment);
+        }
+        else {
+            return end + differenceModulo(start, end, -increment);
+        }
+    }
+}