Optimize vector frequency estimation

lib/src/phy/utils/vector_simd.c

@@ -1671,7 +1671,7 @@ void srslte_vec_apply_cfo_simd(const cf_t* x, float cfo, cf_t* z, int len)
 
 float srslte_vec_estimate_frequency_simd(const cf_t* x, int len)
 {
-  float sum_sin = 0.0f;
+  cf_t sum = 0.0f;
 
   /* Asssumes x[n] = cexp(j·2·pi·n·O) = cos(j·2·pi·n·O) + j · sin(j·2·pi·n·O)
    * where O = f / f_s */
@@ -1679,44 +1679,34 @@ float srslte_vec_estimate_frequency_simd(const cf_t* x, int len)
   int i = 1;
 
 #if SRSLTE_SIMD_CF_SIZE
-  simd_f_t _sum_sin = srslte_simd_f_zero();
+  if (len > SRSLTE_SIMD_CF_SIZE) {
+    simd_cf_t _sum = srslte_simd_cf_zero();
 
-  for (; i < len - SRSLTE_SIMD_CF_SIZE + 1; i += SRSLTE_SIMD_CF_SIZE) {
-    simd_cf_t a1  = srslte_simd_cfi_loadu(&x[i]);
-    simd_f_t  re1 = srslte_simd_cf_re(a1);
-    simd_f_t  im1 = srslte_simd_cf_im(a1);
-
-    simd_cf_t a2  = srslte_simd_cfi_loadu(&x[i - 1]);
-    simd_f_t  re2 = srslte_simd_cf_re(a2);
-    simd_f_t  im2 = srslte_simd_cf_im(a2);
+    for (; i < len - SRSLTE_SIMD_CF_SIZE + 1; i += SRSLTE_SIMD_CF_SIZE) {
+      simd_cf_t a1 = srslte_simd_cfi_loadu(&x[i]);
+      simd_cf_t a2 = srslte_simd_cfi_loadu(&x[i - 1]);
 
-    simd_f_t _pow = srslte_simd_f_sqrt(
-        srslte_simd_f_mul(srslte_simd_f_add(srslte_simd_f_mul(re1, re1), srslte_simd_f_mul(im1, im1)),
-                          srslte_simd_f_add(srslte_simd_f_mul(re2, re2), srslte_simd_f_mul(im2, im2))));
+      // Multiply by conjugate and accumulate
+      _sum = srslte_simd_cf_add(srslte_simd_cf_conjprod(a1, a2), _sum);
+    }
 
-    simd_f_t _sin = srslte_simd_f_mul(srslte_simd_f_sub(srslte_simd_f_mul(re1, im2), srslte_simd_f_mul(re2, im1)),
-                                      srslte_simd_f_rcp(_pow));
-    _sum_sin      = srslte_simd_f_add(_sum_sin, _sin);
-  }
+    // Accumulate using horizontal addition
+    for (int k = 2; k < SRSLTE_SIMD_CF_SIZE; k *= 2) {
+      _sum.re = srslte_simd_f_hadd(_sum.re, _sum.re);
+      _sum.im = srslte_simd_f_hadd(_sum.im, _sum.im);
+    }
 
-  float _sum_sin_v[SRSLTE_SIMD_CF_SIZE];
-  srslte_simd_f_storeu(_sum_sin_v, _sum_sin);
-  for (int k = 0; k < SRSLTE_SIMD_CF_SIZE; k++) {
-    sum_sin += _sum_sin_v[k];
+    // Get accumulator
+    srslte_simd_aligned cf_t _sum_v[SRSLTE_SIMD_CF_SIZE];
+    srslte_simd_cfi_store(_sum_v, _sum);
+    sum = _sum_v[0];
   }
 #endif /* SRSLTE_SIMD_CF_SIZE */
 
   for (; i < len; i++) {
-    /* Load current Sample */
-    float re1 = crealf(x[i]);
-    float im1 = cimagf(x[i]);
-
-    /* Load previous sample */
-    float re2 = crealf(x[i - 1]);
-    float im2 = cimagf(x[i - 1]);
-
-    float pow = sqrtf((re1 * re1 + im1 * im1) * (re2 * re2 + im2 * im2));
-    sum_sin += (re1 * im2 - re2 * im1) / pow;
+    sum += x[i] * conjf(x[i - 1]);
   }
-  return asinf(sum_sin / (float)(len - 1)) / (2.0f * (float)M_PI);
+
+  // Extract argument and divide by (-2·PI)
+  return -cargf(sum) * M_1_PI * 0.5f;
 }