Square root evaluation stubs
PiperOrigin-RevId: 318936593
diff --git a/src/math/sqrt-neon-nr1rsqrts.c b/src/math/sqrt-neon-nr1rsqrts.c
new file mode 100644
index 0000000..f4435f5
--- /dev/null
+++ b/src/math/sqrt-neon-nr1rsqrts.c
@@ -0,0 +1,37 @@
+// Copyright 2020 Google LLC
+//
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree.
+
+#include <assert.h>
+#include <stddef.h>
+
+#include <arm_neon.h>
+
+#include <xnnpack/math.h>
+#include <xnnpack/math-stubs.h>
+
+
+void xnn_math_f32_sqrt__neon_nr1rsqrts(
+ size_t n,
+ const float* input,
+ float* output)
+{
+ assert(n % (4 * sizeof(float)) == 0);
+
+ for (; n != 0; n -= 4 * sizeof(float)) {
+ const float32x4_t vx = vld1q_f32(input); input += 4;
+
+ // Initial approximation
+ float32x4_t vrsqrtx = vrsqrteq_f32(vx);
+
+ // Netwon-Raphson iteration: rsqrt_x <- rsqrt_x * ((3 - x * (rsqrt_x * rsqrt_x)) / 2)
+ // Note: vrsqrtsq_f32(x, y) := (3 - x * y) / 2
+ vrsqrtx = vmulq_f32(vrsqrtx, vrsqrtsq_f32(vx, vmulq_f32(vrsqrtx, vrsqrtx)));
+
+ // Reconstruct sqrt(x) = rsqrt(x) * x
+ const float32x4_t vy = vmulq_f32(vrsqrtx, vx);
+
+ vst1q_f32(output, vy); output += 4;
+ }
+}