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 20// 21// int strlen_user(char *) 22// ------------------------ 23// Returns: 24// - length of string + 1 25// - 0 in case an exception is raised 26// 27// This is an enhanced version of the basic strlen_user. it includes a 28// combination of compute zero index (czx), parallel comparisons, speculative 29// loads and loop unroll using rotating registers. 30// 31// General Ideas about the algorithm: 32// The goal is to look at the string in chunks of 8 bytes. 33// so we need to do a few extra checks at the beginning because the 34// string may not be 8-byte aligned. In this case we load the 8byte 35// quantity which includes the start of the string and mask the unused 36// bytes with 0xff to avoid confusing czx. 37// We use speculative loads and software pipelining to hide memory 38// latency and do read ahead safely. This way we defer any exception. 39// 40// Because we don't want the kernel to be relying on particular 41// settings of the DCR register, we provide recovery code in case 42// speculation fails. The recovery code is going to "redo" the work using 43// only normal loads. If we still get a fault then we return an 44// error (ret0=0). Otherwise we return the strlen+1 as usual. 45// The fact that speculation may fail can be caused, for instance, by 46// the DCR.dm bit being set. In this case TLB misses are deferred, i.e., 47// a NaT bit will be set if the translation is not present. The normal 48// load, on the other hand, will cause the translation to be inserted 49// if the mapping exists. 50// 51// It should be noted that we execute recovery code only when we need 52// to use the data that has been speculatively loaded: we don't execute 53// recovery code on pure read ahead data. 54// 55// Remarks: 56// - the cmp r0,r0 is used as a fast way to initialize a predicate 57// register to 1. This is required to make sure that we get the parallel 58// compare correct. 59// 60// - we don't use the epilogue counter to exit the loop but we need to set 61// it to zero beforehand. 62// 63// - after the loop we must test for Nat values because neither the 64// czx nor cmp instruction raise a NaT consumption fault. We must be 65// careful not to look too far for a Nat for which we don't care. 66// For instance we don't need to look at a NaT in val2 if the zero byte 67// was in val1. 68// 69// - Clearly performance tuning is required. 70// 71 72#define saved_pfs r11 73#define tmp r10 74#define base r16 75#define orig r17 76#define saved_pr r18 77#define src r19 78#define mask r20 79#define val r21 80#define val1 r22 81#define val2 r23 82 83GLOBAL_ENTRY(__strlen_user) 84 .prologue 85 .save ar.pfs, saved_pfs 86 alloc saved_pfs=ar.pfs,11,0,0,8 87 88 .rotr v[2], w[2] // declares our 4 aliases 89 90 extr.u tmp=in0,0,3 // tmp=least significant 3 bits 91 mov orig=in0 // keep trackof initial byte address 92 dep src=0,in0,0,3 // src=8byte-aligned in0 address 93 .save pr, saved_pr 94 mov saved_pr=pr // preserve predicates (rotation) 95 ;; 96 97 .body 98 99 ld8.s v[1]=[src],8 // load the initial 8bytes (must speculate) 100 shl tmp=tmp,3 // multiply by 8bits/byte 101 mov mask=-1 // our mask 102 ;; 103 ld8.s w[1]=[src],8 // load next 8 bytes in 2nd pipeline 104 cmp.eq p6,p0=r0,r0 // sets p6 (required because of // cmp.and) 105 sub tmp=64,tmp // how many bits to shift our mask on the right 106 ;; 107 shr.u mask=mask,tmp // zero enough bits to hold v[1] valuable part 108 mov ar.ec=r0 // clear epilogue counter (saved in ar.pfs) 109 ;; 110 add base=-16,src // keep track of aligned base 111 chk.s v[1], .recover // if already NaT, then directly skip to recover 112 or v[1]=v[1],mask // now we have a safe initial byte pattern 113 ;; 1141: 115 ld8.s v[0]=[src],8 // speculatively load next 116 czx1.r val1=v[1] // search 0 byte from right 117 czx1.r val2=w[1] // search 0 byte from right following 8bytes 118 ;; 119 ld8.s w[0]=[src],8 // speculatively load next to next 120 cmp.eq.and p6,p0=8,val1 // p6 = p6 and val1==8 121 cmp.eq.and p6,p0=8,val2 // p6 = p6 and mask==8 122(p6) br.wtop.dptk.few 1b // loop until p6 == 0 123 ;; 124 // 125 // We must return try the recovery code iff 126 // val1_is_nat || (val1==8 && val2_is_nat) 127 // 128 // XXX Fixme 129 // - there must be a better way of doing the test 130 // 131 cmp.eq p8,p9=8,val1 // p6 = val1 had zero (disambiguate) 132 tnat.nz p6,p7=val1 // test NaT on val1 133(p6) br.cond.spnt .recover // jump to recovery if val1 is NaT 134 ;; 135 // 136 // if we come here p7 is true, i.e., initialized for // cmp 137 // 138 cmp.eq.and p7,p0=8,val1// val1==8? 139 tnat.nz.and p7,p0=val2 // test NaT if val2 140(p7) br.cond.spnt .recover // jump to recovery if val2 is NaT 141 ;; 142(p8) mov val1=val2 // val2 contains the value 143(p8) adds src=-16,src // correct position when 3 ahead 144(p9) adds src=-24,src // correct position when 4 ahead 145 ;; 146 sub ret0=src,orig // distance from origin 147 sub tmp=7,val1 // 7=8-1 because this strlen returns strlen+1 148 mov pr=saved_pr,0xffffffffffff0000 149 ;; 150 sub ret0=ret0,tmp // length=now - back -1 151 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 152 br.ret.sptk.many rp // end of normal execution 153 154 // 155 // Outlined recovery code when speculation failed 156 // 157 // This time we don't use speculation and rely on the normal exception 158 // mechanism. that's why the loop is not as good as the previous one 159 // because read ahead is not possible 160 // 161 // XXX Fixme 162 // - today we restart from the beginning of the string instead 163 // of trying to continue where we left off. 164 // 165.recover: 166 EX(.Lexit1, ld8 val=[base],8) // load the initial bytes 167 ;; 168 or val=val,mask // remask first bytes 169 cmp.eq p0,p6=r0,r0 // nullify first ld8 in loop 170 ;; 171 // 172 // ar.ec is still zero here 173 // 1742: 175 EX(.Lexit1, (p6) ld8 val=[base],8) 176 ;; 177 czx1.r val1=val // search 0 byte from right 178 ;; 179 cmp.eq p6,p0=8,val1 // val1==8 ? 180(p6) br.wtop.dptk.few 2b // loop until p6 == 0 181 ;; 182 sub ret0=base,orig // distance from base 183 sub tmp=7,val1 // 7=8-1 because this strlen returns strlen+1 184 mov pr=saved_pr,0xffffffffffff0000 185 ;; 186 sub ret0=ret0,tmp // length=now - back -1 187 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 188 br.ret.sptk.many rp // end of successful recovery code 189 190 // 191 // We failed even on the normal load (called from exception handler) 192 // 193.Lexit1: 194 mov ret0=0 195 mov pr=saved_pr,0xffffffffffff0000 196 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 197 br.ret.sptk.many rp 198END(__strlen_user) 199