1 /* K=15 r=1/6 Viterbi decoder for x86 SSE2
2 * Copyright Mar 2004, Phil Karn, KA9Q
3 * May be used under the terms of the GNU Lesser General Public License (LGPL)
4 */
5 #include <emmintrin.h>
6 #include <stdio.h>
7 #include <stdlib.h>
8 #include <memory.h>
9 #include <limits.h>
10 #include "fec.h"
11
12 typedef union { unsigned long w[8]; unsigned short s[16];} decision_t;
13 typedef union { signed short s[256]; __m128i v[32];} metric_t;
14
15 static union branchtab39 { unsigned short s[128]; __m128i v[16];} Branchtab39[3];
16 static int Init = 0;
17
18 /* State info for instance of Viterbi decoder */
19 struct v39 {
20 metric_t metrics1; /* path metric buffer 1 */
21 metric_t metrics2; /* path metric buffer 2 */
22 void *dp; /* Pointer to current decision */
23 metric_t *old_metrics,*new_metrics; /* Pointers to path metrics, swapped on every bit */
24 void *decisions; /* Beginning of decisions for block */
25 };
26
27 /* Initialize Viterbi decoder for start of new frame */
init_viterbi39_sse2(void * p,int starting_state)28 int init_viterbi39_sse2(void *p,int starting_state){
29 struct v39 *vp = p;
30 int i;
31
32 for(i=0;i<256;i++)
33 vp->metrics1.s[i] = (SHRT_MIN+1000);
34
35 vp->old_metrics = &vp->metrics1;
36 vp->new_metrics = &vp->metrics2;
37 vp->dp = vp->decisions;
38 vp->old_metrics->s[starting_state & 255] = SHRT_MIN; /* Bias known start state */
39 return 0;
40 }
41
42 /* Create a new instance of a Viterbi decoder */
create_viterbi39_sse2(int len)43 void *create_viterbi39_sse2(int len){
44 void *p;
45 struct v39 *vp;
46
47 if(!Init){
48 int polys[3] = { V39POLYA, V39POLYB, V39POLYC };
49
50 set_viterbi39_polynomial_sse2(polys);
51 }
52 /* Ordinary malloc() only returns 8-byte alignment, we need 16 */
53 if(posix_memalign(&p, sizeof(__m128i),sizeof(struct v39)))
54 return NULL;
55
56 vp = (struct v39 *)p;
57 if((p = malloc((len+8)*sizeof(decision_t))) == NULL){
58 free(vp);
59 return NULL;
60 }
61 vp->decisions = (decision_t *)p;
62 init_viterbi39_sse2(vp,0);
63 return vp;
64 }
65
set_viterbi39_polynomial_sse2(int polys[3])66 void set_viterbi39_polynomial_sse2(int polys[3]){
67 int state;
68
69 for(state=0;state < 128;state++){
70 Branchtab39[0].s[state] = (polys[0] < 0) ^ parity((2*state) & polys[0]) ? 255:0;
71 Branchtab39[1].s[state] = (polys[1] < 0) ^ parity((2*state) & polys[1]) ? 255:0;
72 Branchtab39[2].s[state] = (polys[2] < 0) ^ parity((2*state) & polys[2]) ? 255:0;
73 }
74 Init++;
75 }
76
77 /* Viterbi chainback */
chainback_viterbi39_sse2(void * p,unsigned char * data,unsigned int nbits,unsigned int endstate)78 int chainback_viterbi39_sse2(
79 void *p,
80 unsigned char *data, /* Decoded output data */
81 unsigned int nbits, /* Number of data bits */
82 unsigned int endstate){ /* Terminal encoder state */
83 struct v39 *vp = p;
84 decision_t *d = (decision_t *)vp->decisions;
85 int path_metric;
86
87 endstate %= 256;
88
89 path_metric = vp->old_metrics->s[endstate];
90
91 /* The store into data[] only needs to be done every 8 bits.
92 * But this avoids a conditional branch, and the writes will
93 * combine in the cache anyway
94 */
95 d += 8; /* Look past tail */
96 while(nbits-- != 0){
97 int k;
98
99 k = (d[nbits].w[endstate/32] >> (endstate%32)) & 1;
100 endstate = (k << 7) | (endstate >> 1);
101 data[nbits>>3] = endstate;
102 }
103 return path_metric;
104 }
105
106 /* Delete instance of a Viterbi decoder */
delete_viterbi39_sse2(void * p)107 void delete_viterbi39_sse2(void *p){
108 struct v39 *vp = p;
109
110 if(vp != NULL){
111 free(vp->decisions);
112 free(vp);
113 }
114 }
115
116
update_viterbi39_blk_sse2(void * p,unsigned char * syms,int nbits)117 int update_viterbi39_blk_sse2(void *p,unsigned char *syms,int nbits){
118 struct v39 *vp = p;
119 decision_t *d = (decision_t *)vp->dp;
120 int path_metric = 0;
121
122 while(nbits--){
123 __m128i sym0v,sym1v,sym2v;
124 void *tmp;
125 int i;
126
127 /* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */
128 sym0v = _mm_set1_epi16(syms[0]);
129 sym1v = _mm_set1_epi16(syms[1]);
130 sym2v = _mm_set1_epi16(syms[2]);
131 syms += 3;
132
133 /* SSE2 doesn't support saturated adds on unsigned shorts, so we have to use signed shorts */
134 for(i=0;i<16;i++){
135 __m128i decision0,decision1,metric,m_metric,m0,m1,m2,m3,survivor0,survivor1;
136
137 /* Form branch metrics
138 * Because Branchtab takes on values 0 and 255, and the values of sym?v are offset binary in the range 0-255,
139 * the XOR operations constitute conditional negation.
140 * metric and m_metric (-metric) are in the range 0-765
141 */
142 m0 = _mm_add_epi16(_mm_xor_si128(Branchtab39[0].v[i],sym0v),_mm_xor_si128(Branchtab39[1].v[i],sym1v));
143 metric = _mm_add_epi16(_mm_xor_si128(Branchtab39[2].v[i],sym2v),m0);
144 m_metric = _mm_sub_epi16(_mm_set1_epi16(765),metric);
145
146 /* Add branch metrics to path metrics */
147 m0 = _mm_adds_epi16(vp->old_metrics->v[i],metric);
148 m3 = _mm_adds_epi16(vp->old_metrics->v[16+i],metric);
149 m1 = _mm_adds_epi16(vp->old_metrics->v[16+i],m_metric);
150 m2 = _mm_adds_epi16(vp->old_metrics->v[i],m_metric);
151
152 /* Compare and select */
153 survivor0 = _mm_min_epi16(m0,m1);
154 survivor1 = _mm_min_epi16(m2,m3);
155 decision0 = _mm_cmpeq_epi16(survivor0,m1);
156 decision1 = _mm_cmpeq_epi16(survivor1,m3);
157
158 /* Pack each set of decisions into 8 8-bit bytes, then interleave them and compress into 16 bits */
159 d->s[i] = _mm_movemask_epi8(_mm_unpacklo_epi8(_mm_packs_epi16(decision0,_mm_setzero_si128()),_mm_packs_epi16(decision1,_mm_setzero_si128())));
160
161 /* Store surviving metrics */
162 vp->new_metrics->v[2*i] = _mm_unpacklo_epi16(survivor0,survivor1);
163 vp->new_metrics->v[2*i+1] = _mm_unpackhi_epi16(survivor0,survivor1);
164 }
165 /* See if we need to renormalize */
166 if(vp->new_metrics->s[0] >= SHRT_MAX-5000){
167 int i,adjust;
168 __m128i adjustv;
169 union { __m128i v; signed short w[8]; } t;
170
171 /* Find smallest metric and set adjustv to bring it down to SHRT_MIN */
172 adjustv = vp->new_metrics->v[0];
173 for(i=1;i<32;i++)
174 adjustv = _mm_min_epi16(adjustv,vp->new_metrics->v[i]);
175
176 adjustv = _mm_min_epi16(adjustv,_mm_srli_si128(adjustv,8));
177 adjustv = _mm_min_epi16(adjustv,_mm_srli_si128(adjustv,4));
178 adjustv = _mm_min_epi16(adjustv,_mm_srli_si128(adjustv,2));
179 t.v = adjustv;
180 adjust = t.w[0] - SHRT_MIN;
181 path_metric += adjust;
182 adjustv = _mm_set1_epi16(adjust);
183
184 /* We cannot use a saturated subtract, because we often have to adjust by more than SHRT_MAX
185 * This is okay since it can't overflow anyway
186 */
187 for(i=0;i<32;i++)
188 vp->new_metrics->v[i] = _mm_sub_epi16(vp->new_metrics->v[i],adjustv);
189 }
190 d++;
191 /* Swap pointers to old and new metrics */
192 tmp = vp->old_metrics;
193 vp->old_metrics = vp->new_metrics;
194 vp->new_metrics = tmp;
195 }
196 vp->dp = d;
197 return path_metric;
198 }
199
200
201