address another overflow issue in gradientMultiplier in LOR gradient …

…code

mllib/src/main/scala/org/apache/spark/mllib/optimization/Gradient.scala

@@ -62,7 +62,18 @@ abstract class Gradient extends Serializable {
 class LogisticGradient extends Gradient {
   override def compute(data: Vector, label: Double, weights: Vector): (Vector, Double) = {
     val margin = -1.0 * dot(data, weights)
-    val gradientMultiplier = (1.0 / (1.0 + math.exp(margin))) - label
+    /**
+     * gradientMultiplier = (1.0 / (1.0 + math.exp(margin))) - label
+     * However, the first part of gradientMultiplier can be potentially suffered from overflow,
+     * so we use the equivalent formula but more numerically stable.
+     */
+    val gradientMultiplier = if (margin > 0.0) {
+      val temp = math.exp(-margin)
+      temp / (1.0 + temp) - label
+    } else {
+      1.0 / (1.0 + math.exp(margin)) - label
+    }
+
     val gradient = data.copy
     scal(gradientMultiplier, gradient)
     val loss =