8188044: We need Math.unsignedMultiplyHigh

Reviewed-by: rriggs, aph, darcy

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 +1188,24 @@ 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) {
+        // 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.