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#include <asm/export.h> 21 22// 23// 24// This is an enhanced version of the basic strlen. it includes a combination 25// of compute zero index (czx), parallel comparisons, speculative loads and 26// loop unroll using rotating registers. 27// 28// General Ideas about the algorithm: 29// The goal is to look at the string in chunks of 8 bytes. 30// so we need to do a few extra checks at the beginning because the 31// string may not be 8-byte aligned. In this case we load the 8byte 32// quantity which includes the start of the string and mask the unused 33// bytes with 0xff to avoid confusing czx. 34// We use speculative loads and software pipelining to hide memory 35// latency and do read ahead safely. This way we defer any exception. 36// 37// Because we don't want the kernel to be relying on particular 38// settings of the DCR register, we provide recovery code in case 39// speculation fails. The recovery code is going to "redo" the work using 40// only normal loads. If we still get a fault then we generate a 41// kernel panic. Otherwise we return the strlen as usual. 42// 43// The fact that speculation may fail can be caused, for instance, by 44// the DCR.dm bit being set. In this case TLB misses are deferred, i.e., 45// a NaT bit will be set if the translation is not present. The normal 46// load, on the other hand, will cause the translation to be inserted 47// if the mapping exists. 48// 49// It should be noted that we execute recovery code only when we need 50// to use the data that has been speculatively loaded: we don't execute 51// recovery code on pure read ahead data. 52// 53// Remarks: 54// - the cmp r0,r0 is used as a fast way to initialize a predicate 55// register to 1. This is required to make sure that we get the parallel 56// compare correct. 57// 58// - we don't use the epilogue counter to exit the loop but we need to set 59// it to zero beforehand. 60// 61// - after the loop we must test for Nat values because neither the 62// czx nor cmp instruction raise a NaT consumption fault. We must be 63// careful not to look too far for a Nat for which we don't care. 64// For instance we don't need to look at a NaT in val2 if the zero byte 65// was in val1. 66// 67// - Clearly performance tuning is required. 68// 69// 70// 71#define saved_pfs r11 72#define tmp r10 73#define base r16 74#define orig r17 75#define saved_pr r18 76#define src r19 77#define mask r20 78#define val r21 79#define val1 r22 80#define val2 r23 81 82GLOBAL_ENTRY(strlen) 83 .prologue 84 .save ar.pfs, saved_pfs 85 alloc saved_pfs=ar.pfs,11,0,0,8 // rotating must be multiple of 8 86 87 .rotr v[2], w[2] // declares our 4 aliases 88 89 extr.u tmp=in0,0,3 // tmp=least significant 3 bits 90 mov orig=in0 // keep trackof initial byte address 91 dep src=0,in0,0,3 // src=8byte-aligned in0 address 92 .save pr, saved_pr 93 mov saved_pr=pr // preserve predicates (rotation) 94 ;; 95 96 .body 97 98 ld8 v[1]=[src],8 // must not speculate: can fail here 99 shl tmp=tmp,3 // multiply by 8bits/byte 100 mov mask=-1 // our mask 101 ;; 102 ld8.s w[1]=[src],8 // speculatively load next 103 cmp.eq p6,p0=r0,r0 // sets p6 to true for cmp.and 104 sub tmp=64,tmp // how many bits to shift our mask on the right 105 ;; 106 shr.u mask=mask,tmp // zero enough bits to hold v[1] valuable part 107 mov ar.ec=r0 // clear epilogue counter (saved in ar.pfs) 108 ;; 109 add base=-16,src // keep track of aligned base 110 or v[1]=v[1],mask // now we have a safe initial byte pattern 111 ;; 1121: 113 ld8.s v[0]=[src],8 // speculatively load next 114 czx1.r val1=v[1] // search 0 byte from right 115 czx1.r val2=w[1] // search 0 byte from right following 8bytes 116 ;; 117 ld8.s w[0]=[src],8 // speculatively load next to next 118 cmp.eq.and p6,p0=8,val1 // p6 = p6 and val1==8 119 cmp.eq.and p6,p0=8,val2 // p6 = p6 and mask==8 120(p6) br.wtop.dptk 1b // loop until p6 == 0 121 ;; 122 // 123 // We must return try the recovery code iff 124 // val1_is_nat || (val1==8 && val2_is_nat) 125 // 126 // XXX Fixme 127 // - there must be a better way of doing the test 128 // 129 cmp.eq p8,p9=8,val1 // p6 = val1 had zero (disambiguate) 130 tnat.nz p6,p7=val1 // test NaT on val1 131(p6) br.cond.spnt .recover // jump to recovery if val1 is NaT 132 ;; 133 // 134 // if we come here p7 is true, i.e., initialized for // cmp 135 // 136 cmp.eq.and p7,p0=8,val1// val1==8? 137 tnat.nz.and p7,p0=val2 // test NaT if val2 138(p7) br.cond.spnt .recover // jump to recovery if val2 is NaT 139 ;; 140(p8) mov val1=val2 // the other test got us out of the loop 141(p8) adds src=-16,src // correct position when 3 ahead 142(p9) adds src=-24,src // correct position when 4 ahead 143 ;; 144 sub ret0=src,orig // distance from base 145 sub tmp=8,val1 // which byte in word 146 mov pr=saved_pr,0xffffffffffff0000 147 ;; 148 sub ret0=ret0,tmp // adjust 149 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 150 br.ret.sptk.many rp // end of normal execution 151 152 // 153 // Outlined recovery code when speculation failed 154 // 155 // This time we don't use speculation and rely on the normal exception 156 // mechanism. that's why the loop is not as good as the previous one 157 // because read ahead is not possible 158 // 159 // IMPORTANT: 160 // Please note that in the case of strlen() as opposed to strlen_user() 161 // we don't use the exception mechanism, as this function is not 162 // supposed to fail. If that happens it means we have a bug and the 163 // code will cause of kernel fault. 164 // 165 // XXX Fixme 166 // - today we restart from the beginning of the string instead 167 // of trying to continue where we left off. 168 // 169.recover: 170 ld8 val=[base],8 // will fail if unrecoverable fault 171 ;; 172 or val=val,mask // remask first bytes 173 cmp.eq p0,p6=r0,r0 // nullify first ld8 in loop 174 ;; 175 // 176 // ar.ec is still zero here 177 // 1782: 179(p6) ld8 val=[base],8 // will fail if unrecoverable fault 180 ;; 181 czx1.r val1=val // search 0 byte from right 182 ;; 183 cmp.eq p6,p0=8,val1 // val1==8 ? 184(p6) br.wtop.dptk 2b // loop until p6 == 0 185 ;; // (avoid WAW on p63) 186 sub ret0=base,orig // distance from base 187 sub tmp=8,val1 188 mov pr=saved_pr,0xffffffffffff0000 189 ;; 190 sub ret0=ret0,tmp // length=now - back -1 191 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 192 br.ret.sptk.many rp // end of successful recovery code 193END(strlen) 194EXPORT_SYMBOL(strlen) 195