1 /* K=7 r=1/2 Viterbi decoder for SSE2
2 * Feb 2004, Phil Karn, KA9Q
3 */
4 #include <stdio.h>
5 #include <stdlib.h>
6 #include <memory.h>
7 #include <xmmintrin.h>
8 #include "fec.h"
9
10 typedef union { unsigned char c[64]; __m128i v[4]; } metric_t;
11 typedef union { unsigned long w[2]; unsigned char c[8]; unsigned short s[4]; __m64 v[1];} decision_t;
12 union branchtab27 { unsigned char c[32]; __m128i v[2];} Branchtab27_sse2[2];
13 static int Init = 0;
14
15 /* State info for instance of Viterbi decoder
16 * Don't change this without also changing references in sse2bfly27.s!
17 */
18 struct v27 {
19 metric_t metrics1; /* path metric buffer 1 */
20 metric_t metrics2; /* path metric buffer 2 */
21 decision_t *dp; /* Pointer to current decision */
22 metric_t *old_metrics,*new_metrics; /* Pointers to path metrics, swapped on every bit */
23 decision_t *decisions; /* Beginning of decisions for block */
24 };
25
26 /* Initialize Viterbi decoder for start of new frame */
init_viterbi27_sse2(void * p,int starting_state)27 int init_viterbi27_sse2(void *p,int starting_state){
28 struct v27 *vp = p;
29 int i;
30
31 if(p == NULL)
32 return -1;
33 for(i=0;i<64;i++)
34 vp->metrics1.c[i] = 63;
35
36 vp->old_metrics = &vp->metrics1;
37 vp->new_metrics = &vp->metrics2;
38 vp->dp = vp->decisions;
39 vp->old_metrics->c[starting_state & 63] = 0; /* Bias known start state */
40 return 0;
41 }
42
set_viterbi27_polynomial_sse2(int polys[2])43 void set_viterbi27_polynomial_sse2(int polys[2]){
44 int state;
45
46 for(state=0;state < 32;state++){
47 Branchtab27_sse2[0].c[state] = (polys[0] < 0) ^ parity((2*state) & abs(polys[0])) ? 255 : 0;
48 Branchtab27_sse2[1].c[state] = (polys[1] < 0) ^ parity((2*state) & abs(polys[1])) ? 255 : 0;
49 }
50 Init++;
51 }
52
53
54 /* Create a new instance of a Viterbi decoder */
create_viterbi27_sse2(int len)55 void *create_viterbi27_sse2(int len){
56 void *p;
57 struct v27 *vp;
58
59 if(!Init){
60 int polys[2] = { V27POLYA, V27POLYB };
61 set_viterbi27_polynomial_sse2(polys);
62 }
63 /* Ordinary malloc() only returns 8-byte alignment, we need 16 */
64 if(posix_memalign(&p, sizeof(__m128i),sizeof(struct v27)))
65 return NULL;
66 vp = (struct v27 *)p;
67
68 if((p = malloc((len+6)*sizeof(decision_t))) == NULL){
69 free(vp);
70 return NULL;
71 }
72 vp->decisions = (decision_t *)p;
73 init_viterbi27_sse2(vp,0);
74
75 return vp;
76 }
77
78 /* Viterbi chainback */
chainback_viterbi27_sse2(void * p,unsigned char * data,unsigned int nbits,unsigned int endstate)79 int chainback_viterbi27_sse2(
80 void *p,
81 unsigned char *data, /* Decoded output data */
82 unsigned int nbits, /* Number of data bits */
83 unsigned int endstate){ /* Terminal encoder state */
84 struct v27 *vp = p;
85 decision_t *d;
86
87 if(p == NULL)
88 return -1;
89 d = vp->decisions;
90 /* Make room beyond the end of the encoder register so we can
91 * accumulate a full byte of decoded data
92 */
93 endstate %= 64;
94 endstate <<= 2;
95
96 /* The store into data[] only needs to be done every 8 bits.
97 * But this avoids a conditional branch, and the writes will
98 * combine in the cache anyway
99 */
100 d += 6; /* Look past tail */
101 while(nbits-- != 0){
102 int k;
103
104 k = (d[nbits].c[(endstate>>2)/8] >> ((endstate>>2)%8)) & 1;
105 data[nbits>>3] = endstate = (endstate >> 1) | (k << 7);
106 }
107 return 0;
108 }
109
110 /* Delete instance of a Viterbi decoder */
delete_viterbi27_sse2(void * p)111 void delete_viterbi27_sse2(void *p){
112 struct v27 *vp = p;
113
114 if(vp != NULL){
115 free(vp->decisions);
116 free(vp);
117 }
118 }
119
120
121 #if 0
122 /* This code is turned off because it's slower than my hand-crafted assembler in sse2bfly27.s. But it does work. */
123 void update_viterbi27_blk_sse2(void *p,unsigned char *syms,int nbits){
124 struct v27 *vp = p;
125 decision_t *d;
126
127 if(p == NULL)
128 return;
129 d = (decision_t *)vp->dp;
130 while(nbits--){
131 __m128i sym0v,sym1v;
132 void *tmp;
133 int i;
134
135 /* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */
136 sym0v = _mm_set1_epi8(syms[0]);
137 sym1v = _mm_set1_epi8(syms[1]);
138 syms += 2;
139
140 for(i=0;i<2;i++){
141 __m128i decision0,decision1,metric,m_metric,m0,m1,m2,m3,survivor0,survivor1;
142
143 /* Form branch metrics */
144 metric = _mm_avg_epu8(_mm_xor_si128(Branchtab27_sse2[0].v[i],sym0v),_mm_xor_si128(Branchtab27_sse2[1].v[i],sym1v));
145 /* There's no packed bytes right shift in SSE2, so we use the word version and mask
146 * (I'm *really* starting to like Altivec...)
147 */
148 metric = _mm_srli_epi16(metric,3);
149 metric = _mm_and_si128(metric,_mm_set1_epi8(31));
150 m_metric = _mm_sub_epi8(_mm_set1_epi8(31),metric);
151
152 /* Add branch metrics to path metrics */
153 m0 = _mm_add_epi8(vp->old_metrics->v[i],metric);
154 m3 = _mm_add_epi8(vp->old_metrics->v[2+i],metric);
155 m1 = _mm_add_epi8(vp->old_metrics->v[2+i],m_metric);
156 m2 = _mm_add_epi8(vp->old_metrics->v[i],m_metric);
157
158 /* Compare and select, using modulo arithmetic */
159 decision0 = _mm_cmpgt_epi8(_mm_sub_epi8(m0,m1),_mm_setzero_si128());
160 decision1 = _mm_cmpgt_epi8(_mm_sub_epi8(m2,m3),_mm_setzero_si128());
161 survivor0 = _mm_or_si128(_mm_and_si128(decision0,m1),_mm_andnot_si128(decision0,m0));
162 survivor1 = _mm_or_si128(_mm_and_si128(decision1,m3),_mm_andnot_si128(decision1,m2));
163
164 /* Pack each set of decisions into 16 bits */
165 d->s[2*i] = _mm_movemask_epi8(_mm_unpacklo_epi8(decision0,decision1));
166 d->s[2*i+1] = _mm_movemask_epi8(_mm_unpackhi_epi8(decision0,decision1));
167
168 /* Store surviving metrics */
169 vp->new_metrics->v[2*i] = _mm_unpacklo_epi8(survivor0,survivor1);
170 vp->new_metrics->v[2*i+1] = _mm_unpackhi_epi8(survivor0,survivor1);
171 }
172 d++;
173 /* Swap pointers to old and new metrics */
174 tmp = vp->old_metrics;
175 vp->old_metrics = vp->new_metrics;
176 vp->new_metrics = tmp;
177 }
178 vp->dp = d;
179 }
180 #endif
181