Fixed log-add-exp per review feedback

Author: oleksandr-pavlyk

dpctl/tensor/libtensor/include/kernels/elementwise_functions/logaddexp.hpp

@@ -69,11 +69,17 @@ template <typename argT1, typename argT2, typename resT> struct LogAddExpFunctor
                                        const sycl::vec<argT2, vec_sz> &in2)
     {
         sycl::vec<resT, vec_sz> res;
-        auto diff = in1 - in2;
+        auto diff = in1 - in2; // take advantange of faster vec arithmetic
 
 #pragma unroll
         for (int i = 0; i < vec_sz; ++i) {
-            res[i] = impl<resT>(in1[i], in2[i]);
+            if (std::isfinite(diff[i])) {
+                res[i] = std::max<resT>(in1[i], in2[i]) +
+                         impl_finite<resT>(-std::abs(diff[i]));
+            }
+            else {
+                res[i] = impl<resT>(in1[i], in2[i]);
+            }
         }
 
         return res;
@@ -82,19 +88,28 @@ template <typename argT1, typename argT2, typename resT> struct LogAddExpFunctor
 private:
     template <typename T> T impl(T const &in1, T const &in2)
     {
-        T max = std::max<T>(in1, in2);
-        if (std::isnan(max)) {
-            return std::numeric_limits<T>::quiet_NaN();
+        if (in1 == in2) { // handle signed infinities
+            const T log2 = std::log(T(2));
+            return in1 + log2;
         }
         else {
-            if (std::isinf(max)) {
-                // if both args are -inf, and hence max is -inf
-                // the result is -inf as well
-                return max;
+            const T tmp = in1 - in2;
+            if (tmp > 0) {
+                return in1 + std::log1p(std::exp(-tmp));
+            }
+            else if (tmp <= 0) {
+                return in2 + std::log1p(std::exp(tmp));
+            }
+            else {
+                return std::numeric_limits<T>::quiet_NaN();
             }
         }
-        T min = std::min<T>(in1, in2);
-        return max + std::log1p(std::exp(min - max));
+    }
+
+    template <typename T> T impl_finite(T const &in)
+    {
+        return (in > 0) ? (in + std::log1p(std::exp(-in)))
+                        : std::log1p(std::exp(in));
     }
 };