1/* 2 * 3 * Optimized version of the standard strlen() function 4 * 5 * 6 * Inputs: 7 * in0 address of string 8 * 9 * Outputs: 10 * ret0 the number of characters in the string (0 if empty string) 11 * does not count the \0 12 * 13 * Copyright (C) 1999, 2001 Hewlett-Packard Co 14 * Stephane Eranian <eranian@hpl.hp.com> 15 * 16 * 09/24/99 S.Eranian add speculation recovery code 17 */ 18 19#include <asm/asmmacro.h> 20 21// 22// 23// This is an enhanced version of the basic strlen. it includes a combination 24// of compute zero index (czx), parallel comparisons, speculative loads and 25// loop unroll using rotating registers. 26// 27// General Ideas about the algorithm: 28// The goal is to look at the string in chunks of 8 bytes. 29// so we need to do a few extra checks at the beginning because the 30// string may not be 8-byte aligned. In this case we load the 8byte 31// quantity which includes the start of the string and mask the unused 32// bytes with 0xff to avoid confusing czx. 33// We use speculative loads and software pipelining to hide memory 34// latency and do read ahead safely. This way we defer any exception. 35// 36// Because we don't want the kernel to be relying on particular 37// settings of the DCR register, we provide recovery code in case 38// speculation fails. The recovery code is going to "redo" the work using 39// only normal loads. If we still get a fault then we generate a 40// kernel panic. Otherwise we return the strlen as usual. 41// 42// The fact that speculation may fail can be caused, for instance, by 43// the DCR.dm bit being set. In this case TLB misses are deferred, i.e., 44// a NaT bit will be set if the translation is not present. The normal 45// load, on the other hand, will cause the translation to be inserted 46// if the mapping exists. 47// 48// It should be noted that we execute recovery code only when we need 49// to use the data that has been speculatively loaded: we don't execute 50// recovery code on pure read ahead data. 51// 52// Remarks: 53// - the cmp r0,r0 is used as a fast way to initialize a predicate 54// register to 1. This is required to make sure that we get the parallel 55// compare correct. 56// 57// - we don't use the epilogue counter to exit the loop but we need to set 58// it to zero beforehand. 59// 60// - after the loop we must test for Nat values because neither the 61// czx nor cmp instruction raise a NaT consumption fault. We must be 62// careful not to look too far for a Nat for which we don't care. 63// For instance we don't need to look at a NaT in val2 if the zero byte 64// was in val1. 65// 66// - Clearly performance tuning is required. 67// 68// 69// 70#define saved_pfs r11 71#define tmp r10 72#define base r16 73#define orig r17 74#define saved_pr r18 75#define src r19 76#define mask r20 77#define val r21 78#define val1 r22 79#define val2 r23 80 81GLOBAL_ENTRY(strlen) 82 .prologue 83 .save ar.pfs, saved_pfs 84 alloc saved_pfs=ar.pfs,11,0,0,8 // rotating must be multiple of 8 85 86 .rotr v[2], w[2] // declares our 4 aliases 87 88 extr.u tmp=in0,0,3 // tmp=least significant 3 bits 89 mov orig=in0 // keep trackof initial byte address 90 dep src=0,in0,0,3 // src=8byte-aligned in0 address 91 .save pr, saved_pr 92 mov saved_pr=pr // preserve predicates (rotation) 93 ;; 94 95 .body 96 97 ld8 v[1]=[src],8 // must not speculate: can fail here 98 shl tmp=tmp,3 // multiply by 8bits/byte 99 mov mask=-1 // our mask 100 ;; 101 ld8.s w[1]=[src],8 // speculatively load next 102 cmp.eq p6,p0=r0,r0 // sets p6 to true for cmp.and 103 sub tmp=64,tmp // how many bits to shift our mask on the right 104 ;; 105 shr.u mask=mask,tmp // zero enough bits to hold v[1] valuable part 106 mov ar.ec=r0 // clear epilogue counter (saved in ar.pfs) 107 ;; 108 add base=-16,src // keep track of aligned base 109 or v[1]=v[1],mask // now we have a safe initial byte pattern 110 ;; 1111: 112 ld8.s v[0]=[src],8 // speculatively load next 113 czx1.r val1=v[1] // search 0 byte from right 114 czx1.r val2=w[1] // search 0 byte from right following 8bytes 115 ;; 116 ld8.s w[0]=[src],8 // speculatively load next to next 117 cmp.eq.and p6,p0=8,val1 // p6 = p6 and val1==8 118 cmp.eq.and p6,p0=8,val2 // p6 = p6 and mask==8 119(p6) br.wtop.dptk 1b // loop until p6 == 0 120 ;; 121 // 122 // We must return try the recovery code iff 123 // val1_is_nat || (val1==8 && val2_is_nat) 124 // 125 // XXX Fixme 126 // - there must be a better way of doing the test 127 // 128 cmp.eq p8,p9=8,val1 // p6 = val1 had zero (disambiguate) 129 tnat.nz p6,p7=val1 // test NaT on val1 130(p6) br.cond.spnt .recover // jump to recovery if val1 is NaT 131 ;; 132 // 133 // if we come here p7 is true, i.e., initialized for // cmp 134 // 135 cmp.eq.and p7,p0=8,val1// val1==8? 136 tnat.nz.and p7,p0=val2 // test NaT if val2 137(p7) br.cond.spnt .recover // jump to recovery if val2 is NaT 138 ;; 139(p8) mov val1=val2 // the other test got us out of the loop 140(p8) adds src=-16,src // correct position when 3 ahead 141(p9) adds src=-24,src // correct position when 4 ahead 142 ;; 143 sub ret0=src,orig // distance from base 144 sub tmp=8,val1 // which byte in word 145 mov pr=saved_pr,0xffffffffffff0000 146 ;; 147 sub ret0=ret0,tmp // adjust 148 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 149 br.ret.sptk.many rp // end of normal execution 150 151 // 152 // Outlined recovery code when speculation failed 153 // 154 // This time we don't use speculation and rely on the normal exception 155 // mechanism. that's why the loop is not as good as the previous one 156 // because read ahead is not possible 157 // 158 // IMPORTANT: 159 // Please note that in the case of strlen() as opposed to strlen_user() 160 // we don't use the exception mechanism, as this function is not 161 // supposed to fail. If that happens it means we have a bug and the 162 // code will cause of kernel fault. 163 // 164 // XXX Fixme 165 // - today we restart from the beginning of the string instead 166 // of trying to continue where we left off. 167 // 168.recover: 169 ld8 val=[base],8 // will fail if unrecoverable fault 170 ;; 171 or val=val,mask // remask first bytes 172 cmp.eq p0,p6=r0,r0 // nullify first ld8 in loop 173 ;; 174 // 175 // ar.ec is still zero here 176 // 1772: 178(p6) ld8 val=[base],8 // will fail if unrecoverable fault 179 ;; 180 czx1.r val1=val // search 0 byte from right 181 ;; 182 cmp.eq p6,p0=8,val1 // val1==8 ? 183(p6) br.wtop.dptk 2b // loop until p6 == 0 184 ;; // (avoid WAW on p63) 185 sub ret0=base,orig // distance from base 186 sub tmp=8,val1 187 mov pr=saved_pr,0xffffffffffff0000 188 ;; 189 sub ret0=ret0,tmp // length=now - back -1 190 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 191 br.ret.sptk.many rp // end of successful recovery code 192END(strlen) 193