1 // Copyright 2020 Google LLC
2 //
3 // This source code is licensed under the BSD-style license found in the
4 // LICENSE file in the root directory of this source tree.
5
6 #include <assert.h>
7 #include <stddef.h>
8 #include <stdint.h>
9
10 #include <arm_neon.h>
11
12 #include <xnnpack/math-stubs.h>
13
14
xnn_math_f32_roundz__neon_cvt(size_t n,const float * input,float * output)15 void xnn_math_f32_roundz__neon_cvt(
16 size_t n,
17 const float* input,
18 float* output)
19 {
20 assert(n % (4 * sizeof(float)) == 0);
21
22 // Threshold of non-integral values in single-precision floating-point representation.
23 // All inputs above this threshold (by absolute value) are integer numbers.
24 const float32x4_t vintegral_threshold = vmovq_n_f32(0x1.000000p+23f);
25 // Mask for the sign of a single-precision floating-point number.
26 const uint32x4_t vsign_mask = vmovq_n_u32(UINT32_C(0x80000000));
27
28 for (; n != 0; n -= 4 * sizeof(float)) {
29 const float32x4_t vx = vld1q_f32(input); input += 4;
30
31 // Convert floating-point value x to integer, with rounding towards zero, and then back to floating-point.
32 // Note: the result is valid only for abs(x) < 2**31, but we further restrict its use to 2**23.
33 const float32x4_t vrndx = vcvtq_f32_s32(vcvtq_s32_f32(vx));
34
35 // Compute bitmask for the bits we want to copy from the rounded x. Other bits will be copied from x.
36 // If abs(x) is below the integral threshold, use all but the sign bit from the rounded x and the sign bit from x.
37 // If x is guaranteed integral or NaN, use all bits from x.
38 const uint32x4_t vrndmask = vbicq_u32(vcaltq_f32(vx, vintegral_threshold), vsign_mask);
39
40 // Combine x rounded towardz zero via FP->INT->FP conversion and the input x value.
41 // For 0.0 <= x < 2**23, the result is x rounded via FP->INT->FP conversion.
42 // For -2**23 < x <= -0.0, the result is abs(x) rounded via FP->INT->FP conversion with the sign of x.
43 // For abs(x) >= 2**23 or NaN inputs, the result is x itself.
44 const float32x4_t vy = vbslq_f32(vrndmask, vrndx, vx);
45
46 vst1q_f32(output, vy); output += 4;
47 }
48 }
49