Summary

Add a new method to compute the upper 64 bits of the unsigned 128-bit product of two unsigned 64-bit integers.

Problem

There is no interface to compute the high half of the product of two unsigned 64-bit integers. Such a function would be useful to cryptographic and other algorithms especially once intrinsified.

Solution

Add a method unsignedMultiplyHigh(long x, long y) to java.lang.Math and java.lang.StrictMath which computes the upper 64 bits of the 128-bit product of two unsigned 64-bit integers. The algorithm used in jdk.internal.math.FDBigInteger#mult(int[] src, int srcLen, int v0, int v1, int[] dst) is effectively unrolled into a loop-free solution.

Specification

--- a/src/java.base/share/classes/java/lang/Math.java
+++ b/src/java.base/share/classes/java/lang/Math.java
@@ -1156,6 +1156,7 @@ public final class Math {
      * @param x the first value
      * @param y the second value
      * @return the result
+     * @see #unsignedMultiplyHigh
      * @since 9
      */
     @IntrinsicCandidate
@@ -1187,6 +1188,29 @@ public final class Math {
         }
     }

+    /**
+     * 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) {}


--- a/src/java.base/share/classes/java/lang/StrictMath.java
+++ b/src/java.base/share/classes/java/lang/StrictMath.java
@@ -982,6 +982,7 @@ public final class StrictMath {
      * @param x the first value
      * @param y the second value
      * @return the result
+     * @see #unsignedMultiplyHigh
      * @see Math#multiplyHigh(long,long)
      * @since 9
      */
@@ -989,6 +990,21 @@ public final class StrictMath {
         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) {}