8188046: java.lang.Math.mutliplyHigh does not run in constant time

Reviewed-by: rriggs, darcy

Author: Brian Burkhalter

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

@@ -1161,31 +1161,20 @@ public static long multiplyFull(int x, int y) {
      */
     @IntrinsicCandidate
     public static long multiplyHigh(long x, long y) {
-        if (x < 0 || y < 0) {
-            // Use technique from section 8-2 of Henry S. Warren, Jr.,
-            // Hacker's Delight (2nd ed.) (Addison Wesley, 2013), 173-174.
-            long x1 = x >> 32;
-            long x2 = x & 0xFFFFFFFFL;
-            long y1 = y >> 32;
-            long y2 = y & 0xFFFFFFFFL;
-            long z2 = x2 * y2;
-            long t = x1 * y2 + (z2 >>> 32);
-            long z1 = t & 0xFFFFFFFFL;
-            long z0 = t >> 32;
-            z1 += x2 * y1;
-            return x1 * y1 + z0 + (z1 >> 32);
-        } else {
-            // Use Karatsuba technique with two base 2^32 digits.
-            long x1 = x >>> 32;
-            long y1 = y >>> 32;
-            long x2 = x & 0xFFFFFFFFL;
-            long y2 = y & 0xFFFFFFFFL;
-            long A = x1 * y1;
-            long B = x2 * y2;
-            long C = (x1 + x2) * (y1 + y2);
-            long K = C - A - B;
-            return (((B >>> 32) + K) >>> 32) + A;
-        }
+        // Use technique from section 8-2 of Henry S. Warren, Jr.,
+        // Hacker's Delight (2nd ed.) (Addison Wesley, 2013), 173-174.
+        long x1 = x >> 32;
+        long x2 = x & 0xFFFFFFFFL;
+        long y1 = y >> 32;
+        long y2 = y & 0xFFFFFFFFL;
+
+        long z2 = x2 * y2;
+        long t = x1 * y2 + (z2 >>> 32);
+        long z1 = t & 0xFFFFFFFFL;
+        long z0 = t >> 32;
+        z1 += x2 * y1;
+
+        return x1 * y1 + z0 + (z1 >> 32);
     }
 
     /**