1/* 2 * Optimized version of the strlen_user() function 3 * 4 * Inputs: 5 * in0 address of buffer 6 * 7 * Outputs: 8 * ret0 0 in case of fault, strlen(buffer)+1 otherwise 9 * 10 * Copyright (C) 1998, 1999, 2001 Hewlett-Packard Co 11 * David Mosberger-Tang <davidm@hpl.hp.com> 12 * Stephane Eranian <eranian@hpl.hp.com> 13 * 14 * 01/19/99 S.Eranian heavily enhanced version (see details below) 15 * 09/24/99 S.Eranian added speculation recovery code 16 */ 17 18#include <asm/asmmacro.h> 19#include <asm/export.h> 20 21// 22// int strlen_user(char *) 23// ------------------------ 24// Returns: 25// - length of string + 1 26// - 0 in case an exception is raised 27// 28// This is an enhanced version of the basic strlen_user. it includes a 29// combination of compute zero index (czx), parallel comparisons, speculative 30// loads and loop unroll using rotating registers. 31// 32// General Ideas about the algorithm: 33// The goal is to look at the string in chunks of 8 bytes. 34// so we need to do a few extra checks at the beginning because the 35// string may not be 8-byte aligned. In this case we load the 8byte 36// quantity which includes the start of the string and mask the unused 37// bytes with 0xff to avoid confusing czx. 38// We use speculative loads and software pipelining to hide memory 39// latency and do read ahead safely. This way we defer any exception. 40// 41// Because we don't want the kernel to be relying on particular 42// settings of the DCR register, we provide recovery code in case 43// speculation fails. The recovery code is going to "redo" the work using 44// only normal loads. If we still get a fault then we return an 45// error (ret0=0). Otherwise we return the strlen+1 as usual. 46// The fact that speculation may fail can be caused, for instance, by 47// the DCR.dm bit being set. In this case TLB misses are deferred, i.e., 48// a NaT bit will be set if the translation is not present. The normal 49// load, on the other hand, will cause the translation to be inserted 50// if the mapping exists. 51// 52// It should be noted that we execute recovery code only when we need 53// to use the data that has been speculatively loaded: we don't execute 54// recovery code on pure read ahead data. 55// 56// Remarks: 57// - the cmp r0,r0 is used as a fast way to initialize a predicate 58// register to 1. This is required to make sure that we get the parallel 59// compare correct. 60// 61// - we don't use the epilogue counter to exit the loop but we need to set 62// it to zero beforehand. 63// 64// - after the loop we must test for Nat values because neither the 65// czx nor cmp instruction raise a NaT consumption fault. We must be 66// careful not to look too far for a Nat for which we don't care. 67// For instance we don't need to look at a NaT in val2 if the zero byte 68// was in val1. 69// 70// - Clearly performance tuning is required. 71// 72 73#define saved_pfs r11 74#define tmp r10 75#define base r16 76#define orig r17 77#define saved_pr r18 78#define src r19 79#define mask r20 80#define val r21 81#define val1 r22 82#define val2 r23 83 84GLOBAL_ENTRY(__strlen_user) 85 .prologue 86 .save ar.pfs, saved_pfs 87 alloc saved_pfs=ar.pfs,11,0,0,8 88 89 .rotr v[2], w[2] // declares our 4 aliases 90 91 extr.u tmp=in0,0,3 // tmp=least significant 3 bits 92 mov orig=in0 // keep trackof initial byte address 93 dep src=0,in0,0,3 // src=8byte-aligned in0 address 94 .save pr, saved_pr 95 mov saved_pr=pr // preserve predicates (rotation) 96 ;; 97 98 .body 99 100 ld8.s v[1]=[src],8 // load the initial 8bytes (must speculate) 101 shl tmp=tmp,3 // multiply by 8bits/byte 102 mov mask=-1 // our mask 103 ;; 104 ld8.s w[1]=[src],8 // load next 8 bytes in 2nd pipeline 105 cmp.eq p6,p0=r0,r0 // sets p6 (required because of // cmp.and) 106 sub tmp=64,tmp // how many bits to shift our mask on the right 107 ;; 108 shr.u mask=mask,tmp // zero enough bits to hold v[1] valuable part 109 mov ar.ec=r0 // clear epilogue counter (saved in ar.pfs) 110 ;; 111 add base=-16,src // keep track of aligned base 112 chk.s v[1], .recover // if already NaT, then directly skip to recover 113 or v[1]=v[1],mask // now we have a safe initial byte pattern 114 ;; 1151: 116 ld8.s v[0]=[src],8 // speculatively load next 117 czx1.r val1=v[1] // search 0 byte from right 118 czx1.r val2=w[1] // search 0 byte from right following 8bytes 119 ;; 120 ld8.s w[0]=[src],8 // speculatively load next to next 121 cmp.eq.and p6,p0=8,val1 // p6 = p6 and val1==8 122 cmp.eq.and p6,p0=8,val2 // p6 = p6 and mask==8 123(p6) br.wtop.dptk.few 1b // loop until p6 == 0 124 ;; 125 // 126 // We must return try the recovery code iff 127 // val1_is_nat || (val1==8 && val2_is_nat) 128 // 129 // XXX Fixme 130 // - there must be a better way of doing the test 131 // 132 cmp.eq p8,p9=8,val1 // p6 = val1 had zero (disambiguate) 133 tnat.nz p6,p7=val1 // test NaT on val1 134(p6) br.cond.spnt .recover // jump to recovery if val1 is NaT 135 ;; 136 // 137 // if we come here p7 is true, i.e., initialized for // cmp 138 // 139 cmp.eq.and p7,p0=8,val1// val1==8? 140 tnat.nz.and p7,p0=val2 // test NaT if val2 141(p7) br.cond.spnt .recover // jump to recovery if val2 is NaT 142 ;; 143(p8) mov val1=val2 // val2 contains the value 144(p8) adds src=-16,src // correct position when 3 ahead 145(p9) adds src=-24,src // correct position when 4 ahead 146 ;; 147 sub ret0=src,orig // distance from origin 148 sub tmp=7,val1 // 7=8-1 because this strlen returns strlen+1 149 mov pr=saved_pr,0xffffffffffff0000 150 ;; 151 sub ret0=ret0,tmp // length=now - back -1 152 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 153 br.ret.sptk.many rp // end of normal execution 154 155 // 156 // Outlined recovery code when speculation failed 157 // 158 // This time we don't use speculation and rely on the normal exception 159 // mechanism. that's why the loop is not as good as the previous one 160 // because read ahead is not possible 161 // 162 // XXX Fixme 163 // - today we restart from the beginning of the string instead 164 // of trying to continue where we left off. 165 // 166.recover: 167 EX(.Lexit1, ld8 val=[base],8) // load the initial bytes 168 ;; 169 or val=val,mask // remask first bytes 170 cmp.eq p0,p6=r0,r0 // nullify first ld8 in loop 171 ;; 172 // 173 // ar.ec is still zero here 174 // 1752: 176 EX(.Lexit1, (p6) ld8 val=[base],8) 177 ;; 178 czx1.r val1=val // search 0 byte from right 179 ;; 180 cmp.eq p6,p0=8,val1 // val1==8 ? 181(p6) br.wtop.dptk.few 2b // loop until p6 == 0 182 ;; 183 sub ret0=base,orig // distance from base 184 sub tmp=7,val1 // 7=8-1 because this strlen returns strlen+1 185 mov pr=saved_pr,0xffffffffffff0000 186 ;; 187 sub ret0=ret0,tmp // length=now - back -1 188 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 189 br.ret.sptk.many rp // end of successful recovery code 190 191 // 192 // We failed even on the normal load (called from exception handler) 193 // 194.Lexit1: 195 mov ret0=0 196 mov pr=saved_pr,0xffffffffffff0000 197 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 198 br.ret.sptk.many rp 199END(__strlen_user) 200EXPORT_SYMBOL(__strlen_user) 201