8188044: We need Math.unsignedMultiplyHigh

Reviewed-by: rriggs, aph, darcy

Author: Brian Burkhalter

src/java.base/share/classes/java/lang/Math.java

@@ -1156,6 +1156,7 @@ public static long multiplyFull(int x, int y) {
      * @param x the first value
      * @param y the second value
      * @return the result
+     * @see #unsignedMultiplyHigh
      * @since 9
      */
     @IntrinsicCandidate
@@ -1187,6 +1188,24 @@ public static long multiplyHigh(long x, long y) {
         }
     }
 
+    /**
+     * Returns as a {@code long} the most significant 64 bits of the unsigned
+     * 128-bit product of two unsigned 64-bit factors.
+     *
+     * @param x the first value
+     * @param y the second value
+     * @return the result
+     * @see #multiplyHigh
+     * @since 18
+     */
+    public static long unsignedMultiplyHigh(long x, long y) {
+        // Compute via multiplyHigh() to leverage the intrinsic
+        long result = Math.multiplyHigh(x, y);
+        result += (y & (x >> 63)); // equivalent to `if (x < 0) result += y;`
+        result += (x & (y >> 63)); // equivalent to `if (y < 0) result += x;`
+        return result;
+    }
+
     /**
      * Returns the largest (closest to positive infinity)
      * {@code int} value that is less than or equal to the algebraic quotient.

src/java.base/share/classes/java/lang/StrictMath.java

@@ -982,13 +982,29 @@ public static long multiplyFull(int x, int y) {
      * @param x the first value
      * @param y the second value
      * @return the result
+     * @see #unsignedMultiplyHigh
      * @see Math#multiplyHigh(long,long)
      * @since 9
      */
     public static long multiplyHigh(long x, long y) {
         return Math.multiplyHigh(x, y);
     }
 
+    /**
+     * Returns as a {@code long} the most significant 64 bits of the unsigned
+     * 128-bit product of two unsigned 64-bit factors.
+     *
+     * @param x the first value
+     * @param y the second value
+     * @return the result
+     * @see #multiplyHigh
+     * @see Math#unsignedMultiplyHigh(long,long)
+     * @since 18
+     */
+    public static long unsignedMultiplyHigh(long x, long y) {
+        return Math.unsignedMultiplyHigh(x, y);
+    }
+
     /**
      * Returns the largest (closest to positive infinity)
      * {@code int} value that is less than or equal to the algebraic quotient.

test/jdk/java/lang/Math/MultiplicationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2016, 2021, 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
@@ -26,12 +26,13 @@
  * @library /test/lib
  * @build jdk.test.lib.RandomFactory
  * @run main MultiplicationTests
- * @bug 5100935
+ * @bug 5100935 8188044
  * @summary Tests for multiplication methods (use -Dseed=X to set PRNG seed)
  * @key randomness
  */
 
 import java.math.BigInteger;
+import java.util.function.BiFunction;
 import jdk.test.lib.RandomFactory;
 
 public class MultiplicationTests {
@@ -49,10 +50,25 @@ private static long multiplyHighBigInt(long x, long y) {
             .shiftRight(64).longValue();
     }
 
-    // Check Math.multiplyHigh(x,y) against multiplyHighBigInt(x,y)
-    private static boolean check(long x, long y) {
-        long p1 = multiplyHighBigInt(x, y);
-        long p2 = Math.multiplyHigh(x, y);
+    // Calculate high 64 bits of unsigned 128 product using signed multiply
+    private static long unsignedMultiplyHigh(long x, long y) {
+        long x0 = x & 0xffffffffL;
+        long x1 = x >>> 32;
+        long y0 = y & 0xffffffffL;
+        long y1 = y >>> 32;
+
+        long t = x1 * y0 + ((x0 * y0) >>> 32);
+        long z0 = x0 * y1 + (t & 0xffffffffL);
+        long z1 = t >>> 32;
+
+        return x1 * y1 + z1 + (z0 >>> 32);
+    }
+
+    // Compare results of two functions for a pair of values
+    private static boolean check(BiFunction<Long,Long,Long> reference,
+        BiFunction<Long,Long,Long> multiply, long x, long y) {
+        long p1 = reference.apply(x, y);
+        long p2 = multiply.apply(x, y);
         if (p1 != p2) {
             System.err.printf("Error - x:%d y:%d p1:%d p2:%d\n", x, y, p1, p2);
             return false;
@@ -61,7 +77,19 @@ private static boolean check(long x, long y) {
         }
     }
 
-    private static int testMultiplyHigh() {
+    // Check Math.multiplyHigh(x,y) against multiplyHighBigInt(x,y)
+    private static boolean checkSigned(long x, long y) {
+        return check((a,b) -> multiplyHighBigInt(a,b),
+            (a,b) -> Math.multiplyHigh(a, b), x, y);
+    }
+
+    // Check Math.unsignedMultiplyHigh(x,y) against unsignedMultiplyHigh(x,y)
+    private static boolean checkUnsigned(long x, long y) {
+        return check((a,b) -> unsignedMultiplyHigh(a,b),
+            (a,b) -> Math.unsignedMultiplyHigh(a, b), x, y);
+    }
+
+    private static int test(BiFunction<Long,Long,Boolean> chk) {
         int failures = 0;
 
         // check some boundary cases
@@ -84,23 +112,31 @@ private static int testMultiplyHigh() {
         };
 
         for (long[] xy : v) {
-            if(!check(xy[0], xy[1])) {
+            if(!chk.apply(xy[0], xy[1])) {
                 failures++;
             }
         }
 
         // check some random values
         for (int i = 0; i < COUNT; i++) {
-            if (!check(rnd.nextLong(), rnd.nextLong())) {
+            if (!chk.apply(rnd.nextLong(), rnd.nextLong())) {
                 failures++;
             }
         }
 
         return failures;
     }
 
+    private static int testMultiplyHigh() {
+        return test((x,y) -> checkSigned(x,y));
+    }
+
+    private static int testUnsignedMultiplyHigh() {
+        return test((x,y) -> checkUnsigned(x,y));
+    }
+
     public static void main(String argv[]) {
-        int failures = testMultiplyHigh();
+        int failures = testMultiplyHigh() + testUnsignedMultiplyHigh();
 
         if (failures > 0) {
             System.err.println("Multiplication testing encountered "