1 /* K=15 r=1/6 Viterbi decoder for x86 SSE
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 <xmmintrin.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[512]; unsigned char c[2048];} decision_t;
13 typedef union { signed short s[16384]; __m64 v[4096];} metric_t;
14 
15 static union branchtab615 { unsigned short s[8192]; __m64 v[2048];} Branchtab615[6];
16 static int Init = 0;
17 
18 /* State info for instance of Viterbi decoder */
19 struct v615 {
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_viterbi615_sse(void * p,int starting_state)28 int init_viterbi615_sse(void *p,int starting_state){
29   struct v615 *vp = p;
30   int i;
31 
32   if(p == NULL)
33     return -1;
34   for(i=0;i<16384;i++)
35     vp->metrics1.s[i] = (SHRT_MIN+5000);
36 
37   vp->old_metrics = &vp->metrics1;
38   vp->new_metrics = &vp->metrics2;
39   vp->dp = vp->decisions;
40   vp->old_metrics->s[starting_state & 16383] = SHRT_MIN; /* Bias known start state */
41   return 0;
42 }
43 
44 /* Create a new instance of a Viterbi decoder */
create_viterbi615_sse(int len)45 void *create_viterbi615_sse(int len){
46   struct v615 *vp;
47 
48   if(!Init){
49     int polys[6] = { V615POLYA,V615POLYB,V615POLYC,V615POLYD,V615POLYE,V615POLYF };
50     set_viterbi615_polynomial_sse(polys);
51   }
52 
53   if((vp = (struct v615 *)malloc(sizeof(struct v615))) == NULL){
54     return NULL;
55   }
56   if((vp->decisions = malloc((len+14)*sizeof(decision_t))) == NULL){
57     free(vp);
58     return NULL;
59   }
60   init_viterbi615_sse(vp,0);
61   return vp;
62 }
63 
set_viterbi615_polynomial_sse(int polys[6])64 void set_viterbi615_polynomial_sse(int polys[6]){
65   int state;
66   int i;
67 
68   for(state=0;state < 8192;state++){
69     for(i=0;i<6;i++)
70       Branchtab615[i].s[state] = (polys[i] < 0) ^ parity((2*state) & abs(polys[i])) ? 255 : 0;
71   }
72   Init++;
73 }
74 
75 /* Viterbi chainback */
chainback_viterbi615_sse(void * p,unsigned char * data,unsigned int nbits,unsigned int endstate)76 int chainback_viterbi615_sse(
77       void *p,
78       unsigned char *data, /* Decoded output data */
79       unsigned int nbits, /* Number of data bits */
80       unsigned int endstate){ /* Terminal encoder state */
81   struct v615 *vp = p;
82   decision_t *d;
83 
84   if(p == NULL)
85     return -1;
86   d = (decision_t *)vp->decisions;
87   endstate %= 16384;
88 
89   /* The store into data[] only needs to be done every 8 bits.
90    * But this avoids a conditional branch, and the writes will
91    * combine in the cache anyway
92    */
93   d += 14; /* Look past tail */
94   while(nbits-- != 0){
95     int k;
96 
97     /*    k = (d[nbits].w[endstate/32] >> (endstate%32)) & 1;*/
98     k = (d[nbits].c[endstate/8] >> (endstate%8)) & 1;
99     endstate = (k << 13) | (endstate >> 1);
100     data[nbits>>3] = endstate >> 6;
101   }
102   return 0;
103 }
104 
105 /* Delete instance of a Viterbi decoder */
delete_viterbi615_sse(void * p)106 void delete_viterbi615_sse(void *p){
107   struct v615 *vp = p;
108 
109   if(vp != NULL){
110     free(vp->decisions);
111     free(vp);
112   }
113 }
114 
115 
update_viterbi615_blk_sse(void * p,unsigned char * syms,int nbits)116 int update_viterbi615_blk_sse(void *p,unsigned char *syms,int nbits){
117   struct v615 *vp = p;
118   decision_t *d;
119 
120   if(p == NULL)
121     return -1;
122   d = (decision_t *)vp->dp;
123   while(nbits--){
124     __m64 sym0v,sym1v,sym2v,sym3v,sym4v,sym5v;
125     void *tmp;
126     int i;
127 
128     /* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */
129     sym0v = _mm_set1_pi16(syms[0]);
130     sym1v = _mm_set1_pi16(syms[1]);
131     sym2v = _mm_set1_pi16(syms[2]);
132     sym3v = _mm_set1_pi16(syms[3]);
133     sym4v = _mm_set1_pi16(syms[4]);
134     sym5v = _mm_set1_pi16(syms[5]);
135     syms += 6;
136 
137     for(i=0;i<2048;i++){
138       __m64 decision0,decision1,metric,m_metric,m0,m1,m2,m3,survivor0,survivor1;
139 
140       /* Form branch metrics
141        * Because Branchtab takes on values 0 and 255, and the values of sym?v are offset binary in the range 0-255,
142        * the XOR operations constitute conditional negation.
143        * metric and m_metric (-metric) are in the range 0-1530
144        */
145       m0 = _mm_add_pi16(_mm_xor_si64(Branchtab615[0].v[i],sym0v),_mm_xor_si64(Branchtab615[1].v[i],sym1v));
146       m1 = _mm_add_pi16(_mm_xor_si64(Branchtab615[2].v[i],sym2v),_mm_xor_si64(Branchtab615[3].v[i],sym3v));
147       m2 = _mm_add_pi16(_mm_xor_si64(Branchtab615[4].v[i],sym4v),_mm_xor_si64(Branchtab615[5].v[i],sym5v));
148       metric = _mm_add_pi16(m0,_mm_add_pi16(m1,m2));
149       m_metric = _mm_sub_pi16(_mm_set1_pi16(1530),metric);
150 
151       /* Add branch metrics to path metrics */
152       m0 = _mm_adds_pi16(vp->old_metrics->v[i],metric);
153       m3 = _mm_adds_pi16(vp->old_metrics->v[2048+i],metric);
154       m1 = _mm_adds_pi16(vp->old_metrics->v[2048+i],m_metric);
155       m2 = _mm_adds_pi16(vp->old_metrics->v[i],m_metric);
156 
157       /* Compare and select */
158       survivor0 = _mm_min_pi16(m0,m1);
159       survivor1 = _mm_min_pi16(m2,m3);
160       decision0 = _mm_cmpeq_pi16(survivor0,m1);
161       decision1 = _mm_cmpeq_pi16(survivor1,m3);
162 
163       /* Pack decisions into 8 bits and store */
164       d->c[i] = _mm_movemask_pi8(_mm_unpacklo_pi8(_mm_packs_pi16(decision0,_mm_setzero_si64()),_mm_packs_pi16(decision1,_mm_setzero_si64())));
165 
166       /* Store surviving metrics */
167       vp->new_metrics->v[2*i] = _mm_unpacklo_pi16(survivor0,survivor1);
168       vp->new_metrics->v[2*i+1] = _mm_unpackhi_pi16(survivor0,survivor1);
169     }
170     /* See if we need to renormalize
171      * Max metric spread for this code with 0-255 branch metrics is 12750
172      */
173     if(vp->new_metrics->s[0] >= SHRT_MAX-12750){
174       int i,adjust;
175       __m64 adjustv;
176       union { __m64 v; signed short w[4]; } t;
177 
178       /* Find smallest metric and set adjustv to bring it down to SHRT_MIN */
179       adjustv = vp->new_metrics->v[0];
180       for(i=1;i<4096;i++)
181 	adjustv = _mm_min_pi16(adjustv,vp->new_metrics->v[i]);
182 
183       adjustv = _mm_min_pi16(adjustv,_mm_srli_si64(adjustv,32));
184       adjustv = _mm_min_pi16(adjustv,_mm_srli_si64(adjustv,16));
185       t.v = adjustv;
186       adjust = t.w[0] - SHRT_MIN;
187       adjustv = _mm_set1_pi16(adjust);
188 
189       for(i=0;i<4096;i++)
190 	vp->new_metrics->v[i] = _mm_sub_pi16(vp->new_metrics->v[i],adjustv);
191     }
192     d++;
193     /* Swap pointers to old and new metrics */
194     tmp = vp->old_metrics;
195     vp->old_metrics = vp->new_metrics;
196     vp->new_metrics = tmp;
197   }
198   vp->dp = d;
199   _mm_empty();
200   return 0;
201 }
202