14e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi/* K=9 r=1/3 Viterbi decoder for PowerPC G4/G5 Altivec vector instructions 24e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * 8-bit offset-binary soft decision samples 34e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * Copyright Aug 2006, Phil Karn, KA9Q 44e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * May be used under the terms of the GNU Lesser General Public License (LGPL) 54e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi */ 64e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi#include <stdio.h> 74e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi#include <stdlib.h> 84e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi#include <memory.h> 94e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi#include <limits.h> 104e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi#include "fec.h" 114e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 124e213d510f437769f8a28578dd4f786fb7d16c4Bill Yitypedef union { unsigned char c[2][16]; vector unsigned char v[2]; } decision_t; 134e213d510f437769f8a28578dd4f786fb7d16c4Bill Yitypedef union { unsigned short s[256]; vector unsigned short v[32]; } metric_t; 144e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 154e213d510f437769f8a28578dd4f786fb7d16c4Bill Yistatic union branchtab39 { unsigned short s[128]; vector unsigned short v[16];} Branchtab39[3]; 164e213d510f437769f8a28578dd4f786fb7d16c4Bill Yistatic int Init = 0; 174e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 184e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi/* State info for instance of Viterbi decoder */ 194e213d510f437769f8a28578dd4f786fb7d16c4Bill Yistruct v39 { 204e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi metric_t metrics1; /* path metric buffer 1 */ 214e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi metric_t metrics2; /* path metric buffer 2 */ 224e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi void *dp; /* Pointer to current decision */ 234e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi metric_t *old_metrics,*new_metrics; /* Pointers to path metrics, swapped on every bit */ 244e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi void *decisions; /* Beginning of decisions for block */ 254e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi}; 264e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 274e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi/* Initialize Viterbi decoder for start of new frame */ 284e213d510f437769f8a28578dd4f786fb7d16c4Bill Yiint init_viterbi39_av(void *p,int starting_state){ 294e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi struct v39 *vp = p; 304e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi int i; 314e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 324e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi for(i=0;i<32;i++) 334e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->metrics1.v[i] = (vector unsigned short)(1000); 344e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 354e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->old_metrics = &vp->metrics1; 364e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->new_metrics = &vp->metrics2; 374e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->dp = vp->decisions; 384e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->old_metrics->s[starting_state & 255] = 0; /* Bias known start state */ 394e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi return 0; 404e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi} 414e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 424e213d510f437769f8a28578dd4f786fb7d16c4Bill Yivoid set_viterbi39_polynomial_av(int polys[3]){ 434e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi int state; 444e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 454e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi for(state=0;state < 128;state++){ 464e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi Branchtab39[0].s[state] = (polys[0] < 0) ^ parity((2*state) & abs(polys[0])) ? 255 : 0; 474e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi Branchtab39[1].s[state] = (polys[1] < 0) ^ parity((2*state) & abs(polys[1])) ? 255 : 0; 484e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi Branchtab39[2].s[state] = (polys[2] < 0) ^ parity((2*state) & abs(polys[2])) ? 255 : 0; 494e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 504e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi Init++; 514e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi} 524e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 534e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi/* Create a new instance of a Viterbi decoder */ 544e213d510f437769f8a28578dd4f786fb7d16c4Bill Yivoid *create_viterbi39_av(int len){ 554e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi struct v39 *vp; 564e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 574e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi if(!Init){ 584e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi int polys[3] = { V39POLYA, V39POLYB, V39POLYC }; 594e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 604e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi set_viterbi39_polynomial_av(polys); 614e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 624e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp = (struct v39 *)malloc(sizeof(struct v39)); 634e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->decisions = malloc(sizeof(decision_t)*(len+8)); 644e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi init_viterbi39_av(vp,0); 654e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi return vp; 664e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi} 674e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 684e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi/* Viterbi chainback */ 694e213d510f437769f8a28578dd4f786fb7d16c4Bill Yiint chainback_viterbi39_av( 704e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi void *p, 714e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi unsigned char *data, /* Decoded output data */ 724e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi unsigned int nbits, /* Number of data bits */ 734e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi unsigned int endstate){ /* Terminal encoder state */ 744e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi struct v39 *vp = p; 754e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi decision_t *d = (decision_t *)vp->decisions; 764e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi int path_metric; 774e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 784e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Make room beyond the end of the encoder register so we can 794e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * accumulate a full byte of decoded data 804e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi */ 814e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi endstate %= 256; 824e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 834e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi path_metric = vp->old_metrics->s[endstate]; 844e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 854e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* The store into data[] only needs to be done every 8 bits. 864e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * But this avoids a conditional branch, and the writes will 874e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * combine in the cache anyway 884e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi */ 894e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi d += 8; /* Look past tail */ 904e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi while(nbits-- != 0){ 914e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi int k; 924e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 934e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi k = (d[nbits].c[endstate >> 7][endstate & 15] & (0x80 >> ((endstate>>4)&7)) ) ? 1 : 0; 944e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi endstate = (k << 7) | (endstate >> 1); 954e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi data[nbits>>3] = endstate; 964e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 974e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi return path_metric; 984e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi} 994e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1004e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi/* Delete instance of a Viterbi decoder */ 1014e213d510f437769f8a28578dd4f786fb7d16c4Bill Yivoid delete_viterbi39_av(void *p){ 1024e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi struct v39 *vp = p; 1034e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1044e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi if(vp != NULL){ 1054e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi free(vp->decisions); 1064e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi free(vp); 1074e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 1084e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi} 1094e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1104e213d510f437769f8a28578dd4f786fb7d16c4Bill Yiint update_viterbi39_blk_av(void *p,unsigned char *syms,int nbits){ 1114e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi struct v39 *vp = p; 1124e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi decision_t *d = (decision_t *)vp->dp; 1134e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi int path_metric = 0; 1144e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vector unsigned char decisions = (vector unsigned char)(0); 1154e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1164e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi while(nbits--){ 1174e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vector unsigned short symv,sym0v,sym1v,sym2v; 1184e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vector unsigned char s; 1194e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi void *tmp; 1204e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi int i; 1214e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1224e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */ 1234e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi s = (vector unsigned char)vec_perm(vec_ld(0,syms),vec_ld(5,syms),vec_lvsl(0,syms)); 1244e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1254e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi symv = (vector unsigned short)vec_mergeh((vector unsigned char)(0),s); /* Unsigned byte->word unpack */ 1264e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi sym0v = vec_splat(symv,0); 1274e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi sym1v = vec_splat(symv,1); 1284e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi sym2v = vec_splat(symv,2); 1294e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi syms += 3; 1304e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1314e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi for(i=0;i<16;i++){ 1324e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vector bool short decision0,decision1; 1334e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vector unsigned short metric,m_metric,m0,m1,m2,m3,survivor0,survivor1; 1344e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1354e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Form branch metrics 1364e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * Because Branchtab takes on values 0 and 255, and the values of sym?v are offset binary in the range 0-255, 1374e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * the XOR operations constitute conditional negation. 1384e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * the metrics are in the range 0-765 1394e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi */ 1404e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi m0 = vec_add(vec_xor(Branchtab39[0].v[i],sym0v),vec_xor(Branchtab39[1].v[i],sym1v)); 1414e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi m1 = vec_xor(Branchtab39[2].v[i],sym2v); 1424e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi metric = vec_add(m0,m1); 1434e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi m_metric = vec_sub((vector unsigned short)(765),metric); 1444e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1454e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Add branch metrics to path metrics */ 1464e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi m0 = vec_adds(vp->old_metrics->v[i],metric); 1474e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi m3 = vec_adds(vp->old_metrics->v[16+i],metric); 1484e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi m1 = vec_adds(vp->old_metrics->v[16+i],m_metric); 1494e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi m2 = vec_adds(vp->old_metrics->v[i],m_metric); 1504e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1514e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Compare and select */ 1524e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi decision0 = vec_cmpgt(m0,m1); 1534e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi decision1 = vec_cmpgt(m2,m3); 1544e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi survivor0 = vec_min(m0,m1); 1554e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi survivor1 = vec_min(m2,m3); 1564e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1574e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Store decisions and survivors. 1584e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * To save space without SSE2's handy PMOVMSKB instruction, we pack and store them in 1594e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * a funny interleaved fashion that we undo in the chainback function. 1604e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi */ 1614e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi decisions = vec_add(decisions,decisions); /* Shift each byte 1 bit to the left */ 1624e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1634e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Booleans are either 0xff or 0x00. Subtracting 0x00 leaves the lsb zero; subtracting 1644e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * 0xff is equivalent to adding 1, which sets the lsb. 1654e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi */ 1664e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi decisions = vec_sub(decisions,(vector unsigned char)vec_pack(vec_mergeh(decision0,decision1),vec_mergel(decision0,decision1))); 1674e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1684e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->new_metrics->v[2*i] = vec_mergeh(survivor0,survivor1); 1694e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->new_metrics->v[2*i+1] = vec_mergel(survivor0,survivor1); 1704e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1714e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi if((i % 8) == 7){ 1724e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* We've accumulated a total of 128 decisions, stash and start again */ 1734e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi d->v[i>>3] = decisions; /* No need to clear, the new bits will replace the old */ 1744e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 1754e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 1764e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi#if 0 1774e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Experimentally determine metric spread 1784e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * The results are fixed for a given code and input symbol size 1794e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi */ 1804e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi { 1814e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi int i; 1824e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vector unsigned short min_metric; 1834e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vector unsigned short max_metric; 1844e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi union { vector unsigned short v; unsigned short s[8];} t; 1854e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi int minimum,maximum; 1864e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi static int max_spread = 0; 1874e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 1884e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi min_metric = max_metric = vp->new_metrics->v[0]; 1894e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi for(i=1;i<32;i++){ 1904e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi min_metric = vec_min(min_metric,vp->new_metrics->v[i]); 1914e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi max_metric = vec_max(max_metric,vp->new_metrics->v[i]); 1924e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 1934e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,8)); 1944e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,8)); 1954e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,4)); 1964e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,4)); 1974e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,2)); 1984e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,2)); 1994e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 2004e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi t.v = min_metric; 2014e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi minimum = t.s[0]; 2024e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi t.v = max_metric; 2034e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi maximum = t.s[0]; 2044e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi if(maximum-minimum > max_spread){ 2054e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi max_spread = maximum-minimum; 2064e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi printf("metric spread = %d\n",max_spread); 2074e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 2084e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 2094e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi#endif 2104e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 2114e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Renormalize if necessary. This deserves some explanation. 2124e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * The maximum possible spread, found by experiment, for 8 bit symbols is about 3825 2134e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * So by looking at one arbitrary metric we can tell if any of them have possibly saturated. 2144e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * However, this is very conservative. Large spreads occur only at very high Eb/No, where 2154e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * saturating a bad path metric doesn't do much to increase its chances of being erroneously chosen as a survivor. 2164e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 2174e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * At more interesting (low) Eb/No ratios, the spreads are much smaller so our chances of saturating a metric 2184e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * by not not normalizing when we should are extremely low. So either way, the risk to performance is small. 2194e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 2204e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * All this is borne out by experiment. 2214e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi */ 2224e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi if(vp->new_metrics->s[0] >= USHRT_MAX-5000){ 2234e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vector unsigned short scale; 2244e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi union { vector unsigned short v; unsigned short s[8];} t; 2254e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 2264e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Find smallest metric and splat */ 2274e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi scale = vp->new_metrics->v[0]; 2284e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi for(i=1;i<32;i++) 2294e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi scale = vec_min(scale,vp->new_metrics->v[i]); 2304e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 2314e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi scale = vec_min(scale,vec_sld(scale,scale,8)); 2324e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi scale = vec_min(scale,vec_sld(scale,scale,4)); 2334e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi scale = vec_min(scale,vec_sld(scale,scale,2)); 2344e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi 2354e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Subtract it from all metrics 2364e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi * Work backwards to try to improve the cache hit ratio, assuming LRU 2374e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi */ 2384e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi for(i=31;i>=0;i--) 2394e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->new_metrics->v[i] = vec_subs(vp->new_metrics->v[i],scale); 2404e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi t.v = scale; 2414e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi path_metric += t.s[0]; 2424e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 2434e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi d++; 2444e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi /* Swap pointers to old and new metrics */ 2454e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi tmp = vp->old_metrics; 2464e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->old_metrics = vp->new_metrics; 2474e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->new_metrics = tmp; 2484e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi } 2494e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi vp->dp = d; 2504e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi return path_metric; 2514e213d510f437769f8a28578dd4f786fb7d16c4Bill Yi} 252