1/* SPDX-License-Identifier: GPL-2.0 */ 2/* 3 * 4 * Optimized version of the standard strlen() function 5 * 6 * 7 * Inputs: 8 * in0 address of string 9 * 10 * Outputs: 11 * ret0 the number of characters in the string (0 if empty string) 12 * does not count the \0 13 * 14 * Copyright (C) 1999, 2001 Hewlett-Packard Co 15 * Stephane Eranian <eranian@hpl.hp.com> 16 * 17 * 09/24/99 S.Eranian add speculation recovery code 18 */ 19 20#include <asm/asmmacro.h> 21#include <asm/export.h> 22 23// 24// 25// This is an enhanced version of the basic strlen. it includes a combination 26// of compute zero index (czx), parallel comparisons, speculative loads and 27// loop unroll using rotating registers. 28// 29// General Ideas about the algorithm: 30// The goal is to look at the string in chunks of 8 bytes. 31// so we need to do a few extra checks at the beginning because the 32// string may not be 8-byte aligned. In this case we load the 8byte 33// quantity which includes the start of the string and mask the unused 34// bytes with 0xff to avoid confusing czx. 35// We use speculative loads and software pipelining to hide memory 36// latency and do read ahead safely. This way we defer any exception. 37// 38// Because we don't want the kernel to be relying on particular 39// settings of the DCR register, we provide recovery code in case 40// speculation fails. The recovery code is going to "redo" the work using 41// only normal loads. If we still get a fault then we generate a 42// kernel panic. Otherwise we return the strlen as usual. 43// 44// The fact that speculation may fail can be caused, for instance, by 45// the DCR.dm bit being set. In this case TLB misses are deferred, i.e., 46// a NaT bit will be set if the translation is not present. The normal 47// load, on the other hand, will cause the translation to be inserted 48// if the mapping exists. 49// 50// It should be noted that we execute recovery code only when we need 51// to use the data that has been speculatively loaded: we don't execute 52// recovery code on pure read ahead data. 53// 54// Remarks: 55// - the cmp r0,r0 is used as a fast way to initialize a predicate 56// register to 1. This is required to make sure that we get the parallel 57// compare correct. 58// 59// - we don't use the epilogue counter to exit the loop but we need to set 60// it to zero beforehand. 61// 62// - after the loop we must test for Nat values because neither the 63// czx nor cmp instruction raise a NaT consumption fault. We must be 64// careful not to look too far for a Nat for which we don't care. 65// For instance we don't need to look at a NaT in val2 if the zero byte 66// was in val1. 67// 68// - Clearly performance tuning is required. 69// 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) 84 .prologue 85 .save ar.pfs, saved_pfs 86 alloc saved_pfs=ar.pfs,11,0,0,8 // rotating must be multiple of 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 v[1]=[src],8 // must not speculate: can fail here 100 shl tmp=tmp,3 // multiply by 8bits/byte 101 mov mask=-1 // our mask 102 ;; 103 ld8.s w[1]=[src],8 // speculatively load next 104 cmp.eq p6,p0=r0,r0 // sets p6 to true for 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 or v[1]=v[1],mask // now we have a safe initial byte pattern 112 ;; 1131: 114 ld8.s v[0]=[src],8 // speculatively load next 115 czx1.r val1=v[1] // search 0 byte from right 116 czx1.r val2=w[1] // search 0 byte from right following 8bytes 117 ;; 118 ld8.s w[0]=[src],8 // speculatively load next to next 119 cmp.eq.and p6,p0=8,val1 // p6 = p6 and val1==8 120 cmp.eq.and p6,p0=8,val2 // p6 = p6 and mask==8 121(p6) br.wtop.dptk 1b // loop until p6 == 0 122 ;; 123 // 124 // We must return try the recovery code iff 125 // val1_is_nat || (val1==8 && val2_is_nat) 126 // 127 // XXX Fixme 128 // - there must be a better way of doing the test 129 // 130 cmp.eq p8,p9=8,val1 // p6 = val1 had zero (disambiguate) 131 tnat.nz p6,p7=val1 // test NaT on val1 132(p6) br.cond.spnt .recover // jump to recovery if val1 is NaT 133 ;; 134 // 135 // if we come here p7 is true, i.e., initialized for // cmp 136 // 137 cmp.eq.and p7,p0=8,val1// val1==8? 138 tnat.nz.and p7,p0=val2 // test NaT if val2 139(p7) br.cond.spnt .recover // jump to recovery if val2 is NaT 140 ;; 141(p8) mov val1=val2 // the other test got us out of the loop 142(p8) adds src=-16,src // correct position when 3 ahead 143(p9) adds src=-24,src // correct position when 4 ahead 144 ;; 145 sub ret0=src,orig // distance from base 146 sub tmp=8,val1 // which byte in word 147 mov pr=saved_pr,0xffffffffffff0000 148 ;; 149 sub ret0=ret0,tmp // adjust 150 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 151 br.ret.sptk.many rp // end of normal execution 152 153 // 154 // Outlined recovery code when speculation failed 155 // 156 // This time we don't use speculation and rely on the normal exception 157 // mechanism. that's why the loop is not as good as the previous one 158 // because read ahead is not possible 159 // 160 // IMPORTANT: 161 // Please note that in the case of strlen() as opposed to strlen_user() 162 // we don't use the exception mechanism, as this function is not 163 // supposed to fail. If that happens it means we have a bug and the 164 // code will cause of kernel fault. 165 // 166 // XXX Fixme 167 // - today we restart from the beginning of the string instead 168 // of trying to continue where we left off. 169 // 170.recover: 171 ld8 val=[base],8 // will fail if unrecoverable fault 172 ;; 173 or val=val,mask // remask first bytes 174 cmp.eq p0,p6=r0,r0 // nullify first ld8 in loop 175 ;; 176 // 177 // ar.ec is still zero here 178 // 1792: 180(p6) ld8 val=[base],8 // will fail if unrecoverable fault 181 ;; 182 czx1.r val1=val // search 0 byte from right 183 ;; 184 cmp.eq p6,p0=8,val1 // val1==8 ? 185(p6) br.wtop.dptk 2b // loop until p6 == 0 186 ;; // (avoid WAW on p63) 187 sub ret0=base,orig // distance from base 188 sub tmp=8,val1 189 mov pr=saved_pr,0xffffffffffff0000 190 ;; 191 sub ret0=ret0,tmp // length=now - back -1 192 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what 193 br.ret.sptk.many rp // end of successful recovery code 194END(strlen) 195EXPORT_SYMBOL(strlen) 196