1 // Copyright (c) Facebook, Inc. and its affiliates.
2 // All rights reserved.
3 //
4 // Copyright 2019 Google LLC
5 //
6 // This source code is licensed under the BSD-style license found in the
7 // LICENSE file in the root directory of this source tree.
8
9 #include <assert.h>
10 #include <stdint.h>
11 #include <stddef.h>
12
13 #include <fp16/bitcasts.h>
14
15 #include <xnnpack/math.h>
16 #include <xnnpack/requantization-stubs.h>
17
18
xnn_qs8_requantize_rndnu__scalar(size_t n,const int32_t * input,float scale,int8_t zero_point,int8_t qmin,int8_t qmax,int8_t * output)19 void xnn_qs8_requantize_rndnu__scalar(
20 size_t n,
21 const int32_t* input,
22 float scale,
23 int8_t zero_point,
24 int8_t qmin,
25 int8_t qmax,
26 int8_t* output)
27 {
28 assert(n % 4 == 0);
29 assert(scale < 1.0f);
30 assert(scale >= 0x1.0p-32f);
31
32 const uint32_t scale_bits = fp32_to_bits(scale);
33 const int32_t multiplier = ((int32_t) scale_bits & INT32_C(0x007FFFFF)) | INT32_C(0x00800000);
34 const uint32_t shift = 127 + 23 - (scale_bits >> 23);
35 assert(shift >= 24);
36 assert(shift < 56);
37
38 const int64_t rounding = INT64_C(1) << (shift - 1);
39 const int32_t smin = (int32_t) qmin - (int32_t) zero_point;
40 const int32_t smax = (int32_t) qmax - (int32_t) zero_point;
41 for (; n != 0; n -= 4) {
42 const int32_t x = input[0];
43 const int32_t y = input[1];
44 const int32_t z = input[2];
45 const int32_t w = input[3];
46 input += 4;
47
48 // Compute full 64-bit product of signed 32-bit factors.
49 //
50 // Note: multiplier can be treated as either signed or unsigned.
51 const int64_t x_product = (int64_t) x * (int64_t) multiplier;
52 const int64_t y_product = (int64_t) y * (int64_t) multiplier;
53 const int64_t z_product = (int64_t) z * (int64_t) multiplier;
54 const int64_t w_product = (int64_t) w * (int64_t) multiplier;
55
56 // Arithmetically shift the full 64-bit product right with rounding.
57 // Rounding is performed towards closest integer, with midpoints rounded up.
58 //
59 // Note that although rounding is precomputed, it is dependent on shift value, and on processors with 64-bit
60 // "right shift with rounding" instruction each line below can be represented by just one such instruction
61 // (e.g. VRSHL.S64 on ARM NEON, SRSHL in ARM64 Advanced SIMD).
62 const int32_t x_scaled = (int32_t) asr_s64(x_product + rounding, shift);
63 const int32_t y_scaled = (int32_t) asr_s64(y_product + rounding, shift);
64 const int32_t z_scaled = (int32_t) asr_s64(z_product + rounding, shift);
65 const int32_t w_scaled = (int32_t) asr_s64(w_product + rounding, shift);
66
67 // Clamp scaled value with zero point between (qmin - zero point) and (qmax - zero point).
68 const int32_t x_clamped = math_min_s32(math_max_s32(x_scaled, smin), smax);
69 const int32_t y_clamped = math_min_s32(math_max_s32(y_scaled, smin), smax);
70 const int32_t z_clamped = math_min_s32(math_max_s32(z_scaled, smin), smax);
71 const int32_t w_clamped = math_min_s32(math_max_s32(w_scaled, smin), smax);
72
73 // Add zero point to clamped value.
74 // The result is guaranteed to be in [qmin, qmax] range.
75 //
76 // This addition can not be safely done before clamping, because scaled values are in [-2147483520, 2147483519]
77 // range, so addition of zero point (which can be up to 127) can overflow signed 32-bit integer.
78 const int32_t x_biased = x_clamped + zero_point;
79 const int32_t y_biased = y_clamped + zero_point;
80 const int32_t z_biased = z_clamped + zero_point;
81 const int32_t w_biased = w_clamped + zero_point;
82
83 output[0] = (int8_t) x_biased;
84 output[1] = (int8_t) y_biased;
85 output[2] = (int8_t) z_biased;
86 output[3] = (int8_t) w_biased;
87 output += 4;
88 }
89 }
90