Round-to-nearest-even evaluation stubs
PiperOrigin-RevId: 310412029
diff --git a/src/math/roundne-sse2.c b/src/math/roundne-sse2.c
new file mode 100644
index 0000000..1ef9974
--- /dev/null
+++ b/src/math/roundne-sse2.c
@@ -0,0 +1,54 @@
+// 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 <emmintrin.h>
+
+#include <xnnpack/math-stubs.h>
+
+
+void xnn_math_f32_roundne__sse2(
+ size_t n,
+ const float* input,
+ float* output)
+{
+ assert(n % (4 * sizeof(float)) == 0);
+
+ // This magic number with a bit representation 0x80000000 serves two purposes:
+ // 1. Extract the sign of a floating-point number.
+ // 2. Check if the input to CVTPS2DQ (_mm_cvtps_epi32) is out-of-range, which results in 0x80000000 output.
+ const __m128 vmagic = _mm_set1_ps(-0.0f);
+
+ for (; n != 0; n -= 4 * sizeof(float)) {
+ const __m128 vx = _mm_load_ps(input);
+ input += 4;
+
+ // Extract the sign of the input.
+ // We need the sign to preserve negative zero value, which would otherwise get lost in FP->INT->FP conversion.
+ const __m128 vsignx = _mm_and_ps(vx, vmagic);
+ // Convert floating-point value x to integer, with default rounding (to nearest-even).
+ // If x is beyond [-2**31, 2**31-1] range or x is NaN, the result is -2**31 (0x80000000).
+ const __m128i vintx = _mm_cvtps_epi32(vx);
+
+ // Compute bitmask for out-of-range conversion input.
+ // The bitmask is set to all ones when x is out-of-range for CVTPS2DQ, and also when x == -2**31. The latter case
+ // is ok, because this x is already an integer, and can be passed to output as is.
+ const __m128 vrndmask = _mm_castsi128_ps(_mm_cmpeq_epi32(vintx, _mm_castps_si128(vmagic)));
+
+ // Convert integer back to floating-point.
+ // We binary OR the result with the sign of x to restore the sign of negative zero.
+ const __m128 vrndx = _mm_or_ps(_mm_cvtepi32_ps(vintx), vsignx);
+
+ // Combine x rounded via conversion to integer and the initial x value.
+ // For -2**31 < x < 2**31, the result is x rounded via conversion to integer.
+ // Otherwise (including NaN inputs), the result is x itself.
+ const __m128 vy = _mm_or_ps(_mm_and_ps(vx, vrndmask), _mm_andnot_ps(vrndmask, vrndx));
+
+ _mm_store_ps(output, vy);
+ output += 4;
+ }
+}