Replace `isnan()` with fp equality test in neon code (#925)

* Replace serialized isnan(x) calls with cmp_eq(x,x) (changing sense of associated tests accordingly).

arbor/include/arbor/simd/neon.hpp

@@ -395,12 +395,7 @@ struct neon_double2 : implbase<neon_double2> {
 
         auto is_large = cmp_gt(x, broadcast(exp_maxarg));
         auto is_small = cmp_lt(x, broadcast(exp_minarg));
-
-        bool a[2];
-        a[0] = isnan(vgetq_lane_f64(x, 0)) == 0 ? 0 : 1;
-        a[1] = isnan(vgetq_lane_f64(x, 1)) == 0 ? 0 : 1;
-
-        auto is_nan = mask_copy_from(a);
+        auto is_not_nan = cmp_eq(x, x);
 
         // Compute n and g.
 
@@ -430,7 +425,7 @@ struct neon_double2 : implbase<neon_double2> {
 
         return ifelse(is_large, broadcast(HUGE_VAL),
                       ifelse(is_small, broadcast(0),
-                             ifelse(is_nan, broadcast(NAN), result)));
+                             ifelse(is_not_nan, result, broadcast(NAN))));
     }
 
     // Use same rational polynomial expansion as for exp(x), without
@@ -443,12 +438,7 @@ struct neon_double2 : implbase<neon_double2> {
     static float64x2_t expm1(const float64x2_t& x) {
         auto is_large = cmp_gt(x, broadcast(exp_maxarg));
         auto is_small = cmp_lt(x, broadcast(expm1_minarg));
-
-        bool a[2];
-        a[0] = isnan(vgetq_lane_f64(x, 0)) == 0 ? 0 : 1;
-        a[1] = isnan(vgetq_lane_f64(x, 1)) == 0 ? 0 : 1;
-
-        auto is_nan = mask_copy_from(a);
+        auto is_not_nan = cmp_eq(x, x);
 
         auto half = broadcast(0.5);
         auto one = broadcast(1.);
@@ -484,9 +474,7 @@ struct neon_double2 : implbase<neon_double2> {
 
         return ifelse(is_large, broadcast(HUGE_VAL),
                       ifelse(is_small, broadcast(-1),
-                             ifelse(is_nan, broadcast(NAN),
-                                    ifelse(nzero, expgm1, scaled))));
-    }
+                             ifelse(is_not_nan, ifelse(nzero, expgm1, scaled), broadcast(NAN))));
 
     // Natural logarithm:
     //
@@ -514,11 +502,7 @@ struct neon_double2 : implbase<neon_double2> {
         auto is_small = cmp_lt(x, broadcast(log_minarg));
         auto is_domainerr = cmp_lt(x, broadcast(0));
 
-        bool a[2];
-        a[0] = isnan(vgetq_lane_f64(x, 0)) == 0 ? 0 : 1;
-        a[1] = isnan(vgetq_lane_f64(x, 0)) == 0 ? 0 : 1;
-
-        auto is_nan = mask_copy_from(a);
+        auto is_nan = logical_not(cmp_eq(x, x));
         is_domainerr = logical_or(is_nan, is_domainerr);
 
         float64x2_t g = vcvt_f64_f32(vcvt_f32_s32(logb_normal(x)));