Address negative zero test issues (#45)

Author: ekomarova

mkl_umath/src/mkl_umath_loops.c.src

@@ -1161,7 +1161,11 @@ pairwise_sum_@TYPE@(char *a, npy_intp n, npy_intp stride)
 {
     if (n < 8) {
         npy_intp i;
-        @type@ res = 0.;
+        /*
+         * Start with -0 to preserve -0 values.  The reason is that summing
+         * only -0 should return -0, but `0 + -0 == 0` while `-0 + -0 == -0`.
+         */
+        @type@ res = -0.0;
 
         for (i = 0; i < n; i++) {
             res += @trf@(*((@dtype@*)(a + i * stride)));
@@ -2263,8 +2267,8 @@ pairwise_sum_@TYPE@(@ftype@ *rr, @ftype@ * ri, char * a, npy_intp n,
     if (n < 8) {
         npy_intp i;
 
-        *rr = 0.;
-        *ri = 0.;
+        *rr = -0.0;
+        *ri = -0.0;
         for (i = 0; i < n; i += 2) {
             *rr += *((@ftype@ *)(a + i * stride + 0));
             *ri += *((@ftype@ *)(a + i * stride + sizeof(@ftype@)));